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Moissanite (SiC) from kimberlites: Polytypes, trace elements, inclusions and speculations on origin

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Abstract

An extensive collection of moissanite (SiC) grains from the Mir, Aikhal and Udachnaya kimberlite pipes of Yakutia has been characterized in terms of structural perfection, defects and the major- and trace-element chemistry of SiC and its included phases. The natural grains are clearly distinct from synthetic SiC produced by various methods. Most of the natural SiC grains are 6H and 15R polytypes. Some of the grains (< 10%) show extremely complex Raman spectra indicating strongly disordered structures. Some grains also show zoning in impurities, C-isotope composition and cathodoluminescence brightness.

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... SRPs have been reported in widespread geological settings. For example, moissanite has been identified in mantle-derived magmatic rocks, such as kimberlites [1,[3][4][5]; volcanic breccias [6,7]; and as inclusions in kimberlite diamonds [8][9][10]. More enigmatic occurrences include those in metamorphic rocks, limestones, peralkaline syenite, pegmatites, and chromitite pods within ophiolites [11][12][13][14][15][16][17]. ...
... More enigmatic occurrences include those in metamorphic rocks, limestones, peralkaline syenite, pegmatites, and chromitite pods within ophiolites [11][12][13][14][15][16][17]. Various silicides (e.g., FeSi, FeSi 2 , and Fe 3 Si 7 ) and native metals (e.g., Si 0 and Fe 0 ) have been reported as inclusions in some of the aforementioned SiC [1,4,5,7,11,12,14,15,[17][18][19][20]. Moreover, diverse silicides have been successively identified from the heavy mineral separation of chromitites from the Luobusa ophiolite in Tibet, which generally hosts native Si 0 inclusions [20][21][22][23][24]. Additionally, NiFe-silicides, Si 0 , and interstellar SiC have been found in meteorites and cosmic dust [25][26][27][28][29] while SiC has also been reported from rocks impacted by meteorites [30]. ...
... Moreover, diverse silicides have been successively identified from the heavy mineral separation of chromitites from the Luobusa ophiolite in Tibet, which generally hosts native Si 0 inclusions [20][21][22][23][24]. Additionally, NiFe-silicides, Si 0 , and interstellar SiC have been found in meteorites and cosmic dust [25][26][27][28][29] while SiC has also been reported from rocks impacted by meteorites [30]. The widespread presence of SRPs in natural rocks raises several questions, such as, how do SRPs crystallise in such a wide range of environments, the majority of which do not provide extremely reducing conditions reported by experimental studies as crucial for SRP stability [5]? A comprehensive survey of the SRP literature revealed mineralogical evidence that definitively indicates the natural origin of SRPs in various terrestrial rocks limited to moissanite inclusions enclosed in diamonds and other encapsulating minerals and rock matrix [10,18,19,[31][32][33][34][35]. ...
Article
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Super-reduced phases (SRPs), such as silicon carbide (SiC) and metal silicides, have increasingly been reported in various geological environments. However, their origin remains controversial. SRP inclusions (e.g., metal silicides and metallic silicon (Si0)) within SiC are commonly believed to indicate a natural origin. Here, we identified an unusual SRP assemblage (SiC, (Fe,Ni)Si2, and Si0) in situ in an H5-type Jingshan ordinary chondrite. Simultaneously, our analysis showed that the SiC abrasives contain (Fe,Ni)Si2 and Si0 inclusions. Other inclusions in the artificial SiC were similar to those in natural SiC (moissanite) reported in reference data, including diverse metal silicides (e.g., FeSi, FeSi2, Fe3Si7, and Fe5Si3), as well as a light rare earth element-enriched SiO phase and Fe-Mn-Cr alloys. These inclusions were produced by the in situ reduction of silica and the interaction between Si-containing coke and hot metals during the synthesis of the SiC abrasives. The results demonstrate that the SRP assemblage in the Jingshan chondrite originates from abrasive contamination and that the SRP inclusions (with a low content of Ca, Al, Ti, and Zr) cannot be used as a conclusive indicator for natural SiC. Additionally, the morphologies, biaxiality, and polytypes (determined by Raman spectroscopy) of SiC abrasives bear resemblance to those reported for natural SiC, and caution must be exercised when identifying the origin of SRP in samples processed by conventional methods using SiC abrasives. At the end of this paper, we propose more direct and reliable methods for distinguishing between natural and synthetic SiC.
... Moissanite (SiC) is a rare carbon-bearing mineral that provides extraordinary clues to the evolution and behaviour of carbon in Earth. SiC has been found in a variety of mantle-derived rocks, ranging from kimberlite, lamproite, basalt, ophiolitic chromitite to peridotite, and in crustal rocks including granulite, gneiss, syenite, carbonatite and even limestone [2][3][4][5][6]. Some previous reports of SiC grains have been ascribed to contamination during sampling and/or sample preparation, as synthetic SiC is used in some drill bits and abrasives. ...
... Where SiC has crystallized within oxidized mantle or crust, reduced phases such as carbon and/or hydrogen must be involved to locally buffer the oxygen fugacity [14,15]; reduced gases such as H 2 and CH 4 may play a critical role [16,17]. Electrochemical processes offer another potential mechanism to form reduced phases in oxidized environments [3]. However, when SiC is formed by Chemical Vapor Deposition (CVD) processes such as in space [18] or is formed by impact [19], it is not in equilibrium with the surrounding oxidized minerals. ...
... The moissanite samples studied in this paper are from two distinct tectonic and magmatic contexts: cretaceous off-craton basaltic eruptions in northern Israel, and Devonian diamondiferous kimberlites in Yakutia, Siberian craton, Russia (Fig. 1). Some SiC grains from each setting were studied before by Dobrzhinetskaya et al. [23] and Shiryaev et al. [3], respectively. ...
Article
The occurrence of moissanite (SiC), as xenocrysts in mantle-derived basaltic and kimberlitic rocks sheds light on the interplay between carbon, hydrogen and oxygen in the lithospheric and sublithospheric mantle. SiC is stable only at ƒO2 <ΔIW-6, while the lithospheric mantle and related melts commonly are considered to be much more oxidized. SiC grains from both basaltic volcanoclastic rocks and kimberlites contain metallic inclusions whose shapes suggest they were entrapped as melts. The inclusions consist of Si⁰+ Fe3Si7± FeSi2Ti± CaSi2Al2± FeSi2Al3± CaSi2, and some of the phases show euhedral shapes toward Si⁰. Crystallographically-oriented cavities are common in SiC, suggesting the former presence of volatile phase(s), and the volatiles extracted from crushed SiC grains contain H2+CH4±CO2±CO. Our observations suggest that SiC crystalized from metallic melts (Si-Fe-Ti-C±Al±Ca), with dissolved H2+CH4±CO2±CO derived from the sublithospheric mantle and concentrated around interfaces such as the lithosphere-asthenosphere and crust-mantle boundaries. When mafic/ultramafic magmas are continuously fluxed with H2+CH4 they can be progressively reduced, to a point where silicide melts become immiscible, and crystallize phases such as SiC. The occurrence of SiC in explosive volcanic rocks from different tectonic settings indicates that the delivery of H2+CH4 from depth may commonly accompany explosive volcanism and modify the redox condition of some lithospheric mantle volumes. The heterogeneity of redox states further influences geochemical reactions such as melting and geophysical properties such as seismic velocity and the viscosity of mantle rocks.
... Since its discovery in 1893, moissanite (SiC) was recovered in an increasing number of terrestrial tectonic settings: as inclusions in diamonds (Klein-Bendavid et al., 2007), in kimberlites (Leung et al., 1990;Marshintsev et al., 1967;Mathez et al., 1995;Shiryaev et al., 2011), in ophiolites (Trumbull et al., 2009), in granulites and metamorphic rocks (Janák et al., 2015;Machev et al., 2018;Perraki and Faryad, 2014), and in volcanic breccias (Di Pierro et al., 2003;Dobrzhinetskaya et al., 2018). However, despite the increased findings from continental crust sources, moissanite has never been reported from present-day oceanic geodynamic settings. ...
... The presence of moissanite in such different rocks opened an intense debate to explain its origin (Di Pierro et al., 2003;Dobrzhinetskaya et al., 2018;Griffin et al., 2016;Machev et al., 2018;Shiryaev et al., 2011), mainly because it only forms at extremely reducing conditions, i.e. fO 2 5-7 log units below Iron-Wustite (IW) for mantle conditions (P = 2-10 GPa T = 1000-1700°C; Ulmer et al., 1998;Schmidt et al., 2014). How a silicide (like moissanite) or native elements (like Si 0 or Fe 0 ) could coexist with silicates and oxides is still far to be explained. ...
... The 3C polytype was found in shocked meteorites and only rarely in terrestrial rocks (Machev et al., 2018); on the other hand, 6H, 4H and 15R were found in kimberlitic, ophiolitic and volcanic rocks (i.e. Di Pierro et al., 2003;Dobrzhinetskaya et al., 2018;Shiryaev et al., 2011). ...
Article
Our discovery of moissanite grains in a peralkaline syenite from the Água de Pau Volcano (São Miguel, Azores Islands, Portugal) represents the first report of this mineral in present day oceanic geodynamic settings. Raman spectroscopy and single-crystal X-ray diffraction show the presence of both the 6H and 4H polytypes with the predominance of the first one. The distribution of trace elements is homogeneous, except for Al and V. Azorean moissanite often hosts rounded inclusions of metallic Si and other not yet identified metallic alloys. A process involving a flushing of CH4-H2 ultra-reducing fluids in the alkaline melts might be considered as a possible mechanism leading to the formation of natural SiC, thus calling for strongly reducing conditions that were locally met in the crust-mantle beneath the São Miguel Island.
... High-Ti corundum also occurs as inclusions in diamonds in the latter two mines. SiC is also relatively common in heavy-mineral concentrates from Siberian kimberlites (Shiryaev et al. 2011;up to 1 wt%, V. Malkovets pers. comm. ...
... The Ti-corundum+SiC association is also reported from Devonian kimberlitic rocks in the Donetsk region on the edge of the Ukrainian shield (Tatarintsev et al. 1987) and in diamondiferous olivine-pyroxene xenoliths from the Karashoho shoshonite complex in Uzbekistan (Golovko and Kaminsky 2010;Kaminsky 2017). The most common inclusion in SiC from all localities examined by us is silicon metal, enclosing Fe-silicides and related phases (Shiryaev et al. 2011). These are high-temperature phases (1300-1500°C; Weitzer et al. 2008), and their morphology suggests that they represent trapped volumes of the melts from which the SiC grew. ...
... The universal occurrence in SiC of silicon (Si 0 ) + Fe-silicides inclusions (Shiryaev et al. 2011) strongly suggests that the SiC grew from immiscible silicide melts, generated at very low fO 2 . Similarly, the ophiolitic diamonds contain inclusions of Ni-Mn-Co alloys, and their growth from immiscible metallic melts therefore is probable (Howell et al. 2015). ...
Article
Full-text available
Oxygen fugacity (ƒO2) is a key parameter of Earth’s mantle, because it controls the speciation of the fluids migrating at depth; a major question is whether the sublithospheric mantle is metal-saturated, keeping ƒO2 near the Iron-Wustite (IW) buffer reaction. Cretaceous basaltic pyroclastic rocks on Mt. Carmel, Israel erupted in an intraplate environment with a thin, hot lithosphere. They contain abundant aggregates of hopper-shaped crystals of Ti-rich corundum, which have trapped melts with phenocryst assemblages (Ti2O3, SiC, TiC, silicides, native V) requiring extremely low ƒO2. These assemblages are interpreted to reflect interaction between basaltic melts and mantle-derived fluids dominated by CH4 + H2. Similar highly reduced assemblages are found associated with volcanism in a range of tectonic situations including subduction zones, major continental collisions, intraplate settings, craton margins and the cratons sampled by kimberlites. This distribution, and the worldwide similarity of δ13C in mantle-derived SiC and associated diamonds, suggest a widespread process, involving similar sources and independent of tectonic setting. We suggest that the common factor is the ascent of abiotic (CH4 + H2) fluids from the sublithospheric mantle; this would imply that much of the mantle is metal-saturated, consistent with observations of metallic inclusions in sublithospheric diamonds (e.g. Smith et al. 2016). Such fluids, perhaps carried in rapidly ascending deep-seated magmas, could penetrate high up into a depleted cratonic root, establishing the observed trend of decreasing ƒO2 with depth (e.g. Yaxley et al. in Lithos 140:142–151, 2012). However, repeated metasomatism (associated with the intrusion of silicate melts) will raise the FeO content near the base of the craton over time, developing a carapace of oxidizing material that would prevent the rise of CH4-rich fluids into higher levels of the subcontinental lithospheric mantle (SCLM). Oxidation of these fluids would release CO2 and H2O to drive metasomatism and low-degree melting both in the carapace and higher in the SCLM. This model can explain the genesis of cratonic diamonds from both reduced and oxidized fluids, the existence of SiC as inclusions in diamonds, and the abundance of SiC in some kimberlites. It should encourage further study of the fine fractions of heavy-mineral concentrates from all types of explosive volcanism.
... The Mt Carmel occurrences are notable for their abundance of SiC, which occurs as crystals up to 4.14 mm long; this is much larger than moissanite reported from kimberlites, ophiolites (typically 200 mm) or xenoliths (Di Pierro et al., Moissanite has been observed together with TiC, TiSi 2 and other phases in two melt pockets in the NGC, providing a physical link between these highly reduced mineral assemblages. The occurrence of Ti-V alloys and Ca-Al-Fe silicides as inclusions in the Mt Carmel SiC links them to the Type-DF assemblage described above (Table S2), and distinguishes them from the moissanite from kimberlites (Shiryaev et al., 2011) and ophiolites. ...
... Moissanite is generally regarded as rare in nature, but it may be more abundant than commonly realized. It has been separated from kimberlites and lamproites (Shiryaev et al., 2011) and from kimberlite-borne eclogite xenoliths (our unpublished data), and reported as inclusions in diamonds (e.g., Moore & Gurney, 1989;Leung et al., 1990). A summary of the literature is provided by Di Pierro & Gnos (2016). ...
... Composite energy-dispersive X-ray element map of DFtype melt pocket in corundum, showing resorbed corundum with hibonite, native V, grossite, spinel, fluorite, and unknown Zr-Pierro & Gnos, 2016;Trumbull et al., 2009;Shiryaev et al., 2011;Liu et al., 2015;Xu et al., 2015). ...
Article
Ultrahigh-pressure (UHP) materials (e.g., diamond, high-pressure polymorph of chromite) and super-reduced (SuR) phases (e.g., carbides, nitrides, silicides and native metals) have been identified in chromitites and peridotites of the Tibetan and Polar-Urals ophiolites. These unusual assemblages suggest previously unrecognized fluid- or melt-related processes in the Earth’s mantle. However, the origin of the SuR phases, and in particular their relationships with the UHP materials in the ophiolites, are still enigmatic. Studies of a recently recognized SuR mineral system from Cretaceous volcanics on Mt Carmel, Israel, suggest an alternative genesis for the ophiolitic SuR phases. The Mt Carmel SuR mineral system (associated with Ti-rich corundum xenocrysts) appears to reflect the local interaction of mantle-derived CH4 ± H2 fluids with basaltic magmas in the shallow lithosphere (depths of ∼30–100 km). These interactions produced desilication of the magma, supersaturation in Al2O3 leading to rapid growth of corundum, and phase assemblages requiring local oxygen fugacity (fO2) gradually dropping to ∼11 log units below the iron–wüstite (IW) buffer. The strong similarities between this system and the SuR phases and associated Ti-rich corundum in the Tibetan and Polar-Urals ophiolites suggest that the ophiolitic SuR suite probably formed by local influx of CH4 ± H2 fluids within previously subducted peridotites (and included chromitites) during their rapid exhumation from the deep upper mantle to lithospheric levels. In the final stages of their ascent, the recycled peridotites and chromitites were overprinted by a shallow magmatic system similar to that observed at Mt Carmel, producing most of the SuR phases and eventually preserving them within the Tibetan and Polar-Urals ophiolites.
... introDuCtion Natural silicon carbide (SiC) was first reported in the Canyon Diablo meteorite by French chemist Moissan, after whom the material was named as moissanite in 1904. Since then, SiC was discovered in various rocks, including kimberlites (Mathez et al. 1995;Shiryaev et al. 2011), dunite (Liang et al. 2014), tuff (Mukhin et al. 2015), volcanic breccias (Di Pierro et al. 2003), chromitite pods within ophiolites (Trumbull et al. 2009;Xu et al. 2015;Yang et al. 2015), metasedimentary crustal rocks (Janák et al. 2015), and serpentinite in the ultrahigh-pressure (UHP) metamorphic belt (Xu et al. 2008). However, the natural occurrence of SiC has been challenged scientifically for a long time because contamination issues arise from carborundum, which is widely used in the geological lab as cutting and abrasion material (Milton and Vitaliano 1985). ...
... Natural SiC always occurs alongside with other reduced phases. SiC can have Si (Di Pierro et al. 2003;Robinson et al. 2004;Shiryaev et al. 2011;Trumbull et al. 2009;Xu et al. 2008), FeSi 2 (Shiryaev et al. 2011), Fe 3 Si 7 (Di Pierro et al. 2003;Mathez et al. 1995;Robinson et al. 2004) as inclusions and can be accompanied by native Fe and Ni (Fang et al. 2013;Xu et al. 2015) in terrestrial rock. These phases usually require highly reduced conditions, implying that SiC is also formed under highly reduced environmental conditions. ...
... Natural SiC always occurs alongside with other reduced phases. SiC can have Si (Di Pierro et al. 2003;Robinson et al. 2004;Shiryaev et al. 2011;Trumbull et al. 2009;Xu et al. 2008), FeSi 2 (Shiryaev et al. 2011), Fe 3 Si 7 (Di Pierro et al. 2003;Mathez et al. 1995;Robinson et al. 2004) as inclusions and can be accompanied by native Fe and Ni (Fang et al. 2013;Xu et al. 2015) in terrestrial rock. These phases usually require highly reduced conditions, implying that SiC is also formed under highly reduced environmental conditions. ...
Article
Sic and associated ultra-reduced minerals were reported in various geological settings, however, their genesis and preservation mechanism are poorly understood. Here, we reported a Sic-dominated ultra-reduced mineral assemblage, including Sic, Tic, native metals (Si, Fe, and Ni) and iron silicide, from carbonatitic xenoliths in Dalihu, Inner Mongolia. All minerals were identified in situ in polished/thin sections. Sic is 20-50 μm in size, blue to colorless in color, and usually identified in the micro-cavities within the carbonatitic xenolith. Four types of Sic polytypes were identified, which are dominated by β-SiC (3C polytype) and 4H polytype followed by 15R and 6H. These Sic are featured by ¹³C-depleted isotopic compositions (δ¹³C = -13.2 to -22.8%, average = -17.7%) with obvious spatial variation. We provided a numerical modeling method to prove that the C isotopic composition of the Dalihu SiC can be well-yielded by degassing. Our modeling results showed that degassing reaction between graphite and silicate can readily produce the low δ¹³C value of SiC, and the spatial variations in C isotopic composition could have been formed in the progressive growth process of SiC. The detailed in situ occurring information is beneficial for our understanding of the preservation mechanism of the Dalihu ultra-reduced phase. The predominant occurrence of SiC in micro-cavities implies that exsolution and filling of CO2 and/or CO in the micro-cavities during the diapir rising process of carbonatitic melt could have buffered the reducing environment and separated SiC from the surrounding oxidizing phases. The fast cooling of host rock, which would leave insufficient time for the complete elimination of SiC, could have also contributed to the preservation of SiC.
... Finally, from the Luobusa ophiolite, Tibet, Robinson et al. (2015) and Liang et al. (2014) reported moissanite in olivine from peridotite, and in Cr-spinel from dunite, respectively. Euhedral, unbroken crystals, the second criterion, have been reported from Fuxian , Turkey (Di Pierro et al. 2003), and Yakutia (Shiryaev et al. 2011), while abundant silicon and Fe-silicides, systematically reported as inclusions in terrestrial moissanite and considered to represent former melt-inclusions (Marschintsev 1990;Pankov and Spetsius 1990;Mathez et al. 1995;Bai et al. 1993Bai et al. , 2000Bai et al. , 2003Di Pierro et al. 2003;Robinson et al. 2004), is the third criterion to distinguish synthetic from natural moissanite. Besides the above-mentioned Chinese findings, freshly broken rocks showing abundant enclosed SiC, the fourth criterion, have been reported by Bauer et al. (1963), Leung (1988), andDi Pierro et al. (2003). ...
... By analogy with similar findings from kimberlitic (Pankov and Spetsius 1990;Mathez et al. 1995;Shiryaev et al. 2011) and ophiolitic environments (Bai et al. 2000;Robinson et al. 2004;Trumbull et al. 2009;Yang et al. 2011), a natural origin of the Turkish moissanite was proposed, mainly based on presence of silicon and Fe-silicide inclusions (Di Pierro et al. 2003). A subsequent carbon isotope study confirmed that the moissanites have d 13 C values typical of other occurrences from the deep mantle (Trumbull et al. 2009). ...
... The Si isotope distribution between moissanite, silicon metal and silicides, and of course CAS phases, would be helpful to discern between a common origin or not for this association, and REE pattern of the CaAl-silicates could be helpful to support such a scenario (Shiryaev et al. 2011). implicationS Hundred-micrometer-sized grains consisting of crystalline Ca-Al-silicates and Al-silicates have been found as inclusions in moissanite that has lower mantle d 13 C isotopic signature (Trumbull et al. 2009;Horita and Polyakov 2015), providing an additional criterion to distinguish natural moissanite from synthetic SiC. ...
Article
Hundred-micrometer-sized calcium-aluminum-silicates (CAS) inclusions occur in moissanite-4H, moissanite-15R, and moissanite-6H from Turkey. These inclusions commonly consist of tabular exsolution lamellae of two different minerals. By combined electron microprobe and Raman spectroscopy analysis, at least eight different, essentially Mg- and Fe-free Ca-Al-silicate or Al-silicate phases have been discerned. The most common phase is dmisteinbergite, a hexagonal modification of CaAl2Si2O8, occurring in association with lamellae of Cax(Al,Si)1−xO3 or Ca1−x(Al,Si)2+xO5 compositions. All three phases contain significant amounts of BaO (up to 2 mol% of celsiane component in dmisteinbergite), SrO, SO3, and light rare earth elements (LREE). In particular, Ca1−x(Al,Si)2+xO5 contains up to 2.1 wt% of LREE, 3.9 wt% of F, and significant traces of Cl, while it is also associated to osbornite (TiN). Pure ghelenite, Ca2Al2SiO7, and three additional compositions, namely CaAl4−xSixO7, Ca1−x(Al,Si)3+xO6, and Ca3−x(Al,Si)6+xO14 have been found, either occurring as single grains or forming exsolution lamellae. They also contain significant amounts of BaO, SrO, SO3, and LREE. One last intriguing phase is composed in average of 65.9 wt% SiO2, 17.4% Al2O3, 3.0% alkalis, 6.0% BaO, 2.0% CaO+MgO, 0.9% ZrO2, and up to 0.5% LREE. Dmisteinbergite and ghelenite show Raman peaks in very good agreement with literature data, Cax(Al,Si)1−xO3 shows main Raman modes at 416 and 1009 cm−1, Ca1−x(Al,Si)3+xO6 at 531 and 1579 cm−1 while Ca3−x(Al,Si)6+xO14 has a strong peak at 553 cm−1. CaAl4−xSixO7 shows a weak Raman pattern, while Ca1−x(Al,Si)2+xO5 has no detectable Raman modes. Since the association moissanite-CAS is thermodynamically not stable at ambient pressure and moissanite crystals hosting the CAS phases have δ13C values typical of deep-mantle origin, we interpret the CAS inclusions as partially retrogressed HP minerals. Striking analogies exist between observed CAS compositions and experimentally obtained HP-HT mineralogy. For instance, Cax(Al,Si)1−xO3 contains up to 25 mol% of Al2O3, which is considered as the upper limit of alumina solubility in Ca-perovskite. The study confirms that CAS phases are an important mantle depository for large ion lithophile elements (LILE) and LREE.
... At the same time, there is a wealth of evidence for SiC which is likewise found together with carbonates [Gnoevaja and Grozdanov, 1965;Miyano et al., 1982;Klein-BenDavid et al., 2007;Shiryaev et al., 2008] but may result from another redox process: electrochemical precipi-tation in relatively oxidized carbonate-silicate melts [Shiryaev et al., 2011]. Thus, carbonated fluids are important metasomatic agents in deep lithosphere where they participate in diamond formation [Shiryaev et al., 2011]. ...
... At the same time, there is a wealth of evidence for SiC which is likewise found together with carbonates [Gnoevaja and Grozdanov, 1965;Miyano et al., 1982;Klein-BenDavid et al., 2007;Shiryaev et al., 2008] but may result from another redox process: electrochemical precipi-tation in relatively oxidized carbonate-silicate melts [Shiryaev et al., 2011]. Thus, carbonated fluids are important metasomatic agents in deep lithosphere where they participate in diamond formation [Shiryaev et al., 2011]. The interaction of carbonatitic melts with other mantle phases may be a plausible mechanism, considering different values of electrical conductivity [Weiss et al., 2010]. ...
Article
The article presents new data on multiphase inclusions in two diamonds sampled from placers in the northeastern Siberian craton. Diamond HLS-4 is a round variety V crystal containing a multiphase mineral inclusion with more or less strongly oxidized iron carbides and moissanite in one part and calcite and iron oxides in another part. Another sample, HI-180, is a partly dissolved yellow cuboid that hosts a multiphase inclusion and numerous submicrometer inclusions delineating the crystal zoning. Sample HI-180 is deformed, with cracks and cavities up to 200 μm in size exposed in a polished section. The submicrometer inclusions plot in the field of microinclusions in fibrous diamonds from the world database, mostly near the silicic corner. They must have a hydrous composition, judging by the water-carbonate ratio of H2O/(H2O + CO2) = 0.80–0.82 estimated from FTIR data. The multiphase inclusion consists of quartz, Fe-armalcolite, anatase, and diamond grains in an amorphous matrix, as well as moissanite and calcite detected by SEM-EDS, FTIR and Raman spectroscopy. One diamond grain in the inclusion, in turn, encloses moissanite. The coexistence of calcite and moissanite in multiphase inclusions is evidence that cracks were fully healed up under extreme redox variations, possibly, during diamond growth in a subduction setting.
... SiC is commonly found in the Solar System in the form of stardust and chondrite meteorites (e.g., Alexander 1993 and references therein), and has been proposed to potentially constitute a significant volume of planetary interiors in carbon-rich star systems (Madhusudhan et al. 2012). SiC is known as the rare mineral moissanite, which has been reported in bulk kimberlitic chemistry and as inclusions in kimberlitic diamonds (Leung et al. 1990;Shiryaev et al. 2011). Several moissanite grains recovered from ophiolites and kimberlites have been reported to contain inclusions of metallic Si and Fe-silicides (Di Pierro et al. 2003;Trumbull et al. 2009;Shiryaev et al. 2011), and silicides were also found in meteorites (Ross et al. 2019) Because magnesium and silicon are both cosmochemically abundant elements, they are expected to be major components of rocky planets. ...
... SiC is known as the rare mineral moissanite, which has been reported in bulk kimberlitic chemistry and as inclusions in kimberlitic diamonds (Leung et al. 1990;Shiryaev et al. 2011). Several moissanite grains recovered from ophiolites and kimberlites have been reported to contain inclusions of metallic Si and Fe-silicides (Di Pierro et al. 2003;Trumbull et al. 2009;Shiryaev et al. 2011), and silicides were also found in meteorites (Ross et al. 2019) Because magnesium and silicon are both cosmochemically abundant elements, they are expected to be major components of rocky planets. SiC and (Fe, Mg)silicide are, therefore, candidates to form Si-bearing minerals under reducing conditions in the Earth and planetary interiors. ...
Article
SiC and (Fe, Mg)-silicide are candidate phases forming under reducing conditions in the Earth and planetary interiors. However, structural studies of SiC and Mg₂Si at high pressure and their thermal stability are presently lacking. In this work, we applied single-crystal X-ray diffraction in a diamond anvil cell at high pressure and determined the equations of state of α-SiC (6H) and βʹ-Mg₂Si₁.₁ up to 60 and 40 GPa, respectively, yielding bulk moduli of 226.0(4) and 56(1) GPa. We also report the formation of a novel orthorhombic Mg₂Si₇ phase upon laser heating βʹ-Mg₂Si₁.₁ at ~ 45 GPa and 2000 °C [Pbam, a = 7.16(1) Å, b = 12.490(3) Å, c = 2.6545(3) Å, V = 237.5(3) Å3]. The structure of this compound contains layers formed by irregular 12-member silicon rings, which are arranged in channels filled with both Mg and Si atoms. No signs of the Mg₂Si₇ phase were detected upon releasing the pressure in the DAC, which suggests that this phase is unstable under ambient conditions.
... SiC is commonly found in the Solar System in the form of stardust and chondrite meteorites (e.g., Alexander 1993 and references therein), and has been proposed to potentially constitute a significant volume of planetary interiors in carbon-rich star systems (Madhusudhan et al. 2012). SiC is known as the rare mineral moissanite, which has been reported in bulk kimberlitic chemistry and as inclusions in kimberlitic diamonds (Leung et al. 1990;Shiryaev et al. 2011). Several moissanite grains recovered from ophiolites and kimberlites have been reported to contain inclusions of metallic Si and Fe-silicides (Di Pierro et al. 2003;Trumbull et al. 2009;Shiryaev et al. 2011), and silicides were also found in meteorites (Ross et al. 2019) Because magnesium and silicon are both cosmochemically abundant elements, they are expected to be major components of rocky planets. ...
... SiC is known as the rare mineral moissanite, which has been reported in bulk kimberlitic chemistry and as inclusions in kimberlitic diamonds (Leung et al. 1990;Shiryaev et al. 2011). Several moissanite grains recovered from ophiolites and kimberlites have been reported to contain inclusions of metallic Si and Fe-silicides (Di Pierro et al. 2003;Trumbull et al. 2009;Shiryaev et al. 2011), and silicides were also found in meteorites (Ross et al. 2019) Because magnesium and silicon are both cosmochemically abundant elements, they are expected to be major components of rocky planets. SiC and (Fe, Mg)silicide are, therefore, candidates to form Si-bearing minerals under reducing conditions in the Earth and planetary interiors. ...
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SiC and (Fe, Mg)-silicide are candidate phases forming under reducing conditions in the Earth and planetary interiors. However, structural studies of SiC and Mg2Si at high pressure and their thermal stability are presently lacking. In this work, we applied single-crystal X-ray diffraction in a diamond anvil cell at high pressure and determined the equations of state of α-SiC (6H) and βʹ-Mg2Si1.1 up to 60 and 40 GPa, respectively, yielding bulk moduli of 226.0(4) and 56(1) GPa. We also report the formation of a novel orthorhombic Mg2Si7 phase upon laser heating βʹ-Mg2Si1.1 at ~ 45 GPa and 2000 °C [Pbam, a = 7.16(1) Å, b = 12.490(3) Å, c = 2.6545(3) Å, V = 237.5(3) ų]. The structure of this compound contains layers formed by irregular 12-member silicon rings, which are arranged in channels filled with both Mg and Si atoms. No signs of the Mg2Si7 phase were detected upon releasing the pressure in the DAC, which suggests that this phase is unstable under ambient conditions.
... Many synthetic routes have been reported (see review by Abderrazak and Hmida [11]), with variations in the resulting crystal properties, but none truly mimic geological conditions, which range from relatively low pressure (upper crust) to high pressure (mid to lower mantle), high temperature, and very low oxygen fugacity. For more than a century, natural occurrences of moissanite have been reported in the literature and these reports show that SiC origin may be broken up into three categories: (i) high-, ultra-high pressure environments, such as upper-and lower mantle and mantle transition zone, and even core-mantle boundary; (ii) ambient/low-pressure environments, such as metamorphic, magmatic rocks and hydrothermal processes recorded in both continental and oceanic crust; or (iii) formation of SiC and native metals during lightning strikes in the ophiolitic rocks exposed on the Earth's surface [12][13][14][15][16][17][18][19][20]. All require temperature ranged from 700-800 °C to 2500 °C, and extremely low oxygen fugacity, e.g. ...
... ~8 to <12 log-bar units [21], or 6 to 8 log-bar units [22] below the iron-wüstite (IW) buffer. In the last two decades many more well-documented, including in-situ, finds of natural moissanite have been reported [12][13][14][15][16][17][18]. These reports suggest that moissanite may be a much more common accessory mineral than previously thought. ...
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Moissanite, SiC, is an uncommon accessory mineral that forms under low oxygen fugacity. Here, we analyze natural SiC from a Miocene tuff-sandstone using synchrotron Laue microdiffraction and Raman spectroscopy, in order to better understand the SiC phases and formation physics. The studied crystals of SiC consist of 4H-and 6H-SiC domains, formed from either, continuous growth or, in one case, intergrown, together with native Si. The native Si is polycrystalline, with a large crystal size relative to the analytical beam dimensions (>1-2 μm). We find that the intergrown region shows low distortion or dislocation density in SiC, but these features are comparatively high in Si. The distortion/deformation observed in Si may have been caused by a mismatch in the coefficients of thermal expansion of the two materials. Raman spectroscopic measurements are discussed in combination with our Laue microdiffraction results. Our results suggest that these SiC grains likely grew from an igneous melt.
... Although rare and volumetrically trivial as reservoirs of carbon in the crust, they may represent a significant volume of carbon in Earth's deep interior, and thus may provide insight to the deep carbon cycle (Dasgupta 2013;Wood et al. 2013). Since its discovery in 1893 by Henri Moissan in mineral residues from the Canyon Diablo meteor crater in Arizona (Moissan 1904b), natural moissanite has been found in dozens of localities, including meteorites, serpentinites, chromitites, ophiolite complexes, and in close association with diamond in kimberlites and eclogites (Lyakhovich 1980;Leung et al. 1990;Alexander 1990Alexander , 1993Di Pierro et al. 2003;Lee et al. 2006;Qi et al. 2007;Xu et al. 2008;Trumbull et al. 2009;Shiryaev et al 2011;see also http://MinDat.org). While some occurrences of moissanite are apparently of near-surface origin, including sites of forest fires and contact metamorphism of silicate magmas with coal beds (Sameshima and Rodgers 1990), silicon carbide also represents one of the deepest mantle minerals known to reach the surface. ...
... Hazen, Downs, Jones,Kah Carbon Mineralogy & Crystal Chemistry 19 been characterized (Krishna and Verma 1965;Lee et al. 2006;Capitani et al. 2007;Shiryaev et al. 2011; see also http://img.chem.ucl.ac.uk/www/kelly/LITERATURESICWEB.HTM#4 and http://MinDat.org). Such complex polytypes have inspired a variety of models related to possible growth models, including screw dislocation or spiral growth mechanism (e.g., Frank 1949Frank , 1951Verma and Krishna 1966), the faulted matrix model (Pandey and Krishna 1975a, 1975b, one-dimensional disorder theory (Jagodzinski 1954a(Jagodzinski , 1954b, and the axial nearest neighbor Ising model (Price and Yeomans 1984). ...
... The Raman spectra, with peaks at 162, 769, 786, 797 and 968 cm −1 , confirms that the mineral is moissanite. Single crystal X-ray microdiffraction (Figure 7f) shows that SiC has a hexagonal structure that corresponds to the 6H polytype (Figure 7g), the most common polytype of moissanite [57]. ...
... Similar phases have been reported in other ophiolitic chromitites (Table 2) and interpreted as part of a super-reducing (SuR) assemblage [3,4,[6][7][8]10,14,15,17]. Although only recently reported in ophiolitic chromitites [58,77], moissanite has already been found in a variety of terrestrial and extraterrestrial rocks, including meteorites [78] and references therein, kimberlites and associated diamond [57,[79][80][81][82], volcanic breccias and tuff [72,83,84], high-and low-grade metamorphic rocks [59,85], and limestones [86,87]. According to experimental and empirical data, the formation of SiC only occurs at a fugacity of oxygen 6.5-7 log units below the iron-wüstite buffer (IW) [88,89]. ...
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The origin of the assemblage of ultra-high pressure (UHP), super-reduced (SuR) and several crustally derived phases in ophiolitic chromitites is still hotly debated. In this paper, we report, for the first time, this assemblage of phases in ophiolitic chromitites of the Caribbean. We studied the Mercedita chromitite deposit in the eastern Cuban ophiolitic complexes. The mineral phases were characterized using microRaman spectroscopy, energy-dispersive spectroscopy with a scanning electron microscope (SEM-EDS), X-ray microdiffraction and electron microprobe analyses. Mineral concentrates were prepared using hydroseparation techniques. We have identified oriented clinopyroxene lamellae in chromite, oriented rutile lamellae in chromite, moissanite hosted in the altered matrix of the chromitite, graphite-like amorphous carbon, corundum and SiO 2 hosted in healed fractures in chromite grains, and native Cu and Fe-Mn alloy recovered in heavy-mineral concentrates obtained by hydroseparation. This assemblage may correspond to UHP-SuR conditions, implying recycling of chromitite in the mantle or formation of the chromite grains at deep mantle depths, followed by emplacement at a shallow level in the mantle. However, the chromitite bodies contain gabbro sills oriented parallel to the elongation of the chromitite lenses, and these show no evidence of HP/UHP metamorphism. Therefore, the identified "exotic" phases may not be indicative of UHP. They formed independently as oriented clinopyroxene lamellae in chromite during cooling (clinopyroxene and rutile), in super-reduced microenvironments during the serpentinization processes, and by transference of subducted crustal material to the mantle wedge via cold plumes.
... Natural moissanite also occurs in various extraterrestrial and terrestrial rocks, including meteorites (Alexander 1993); kimberlites (Leung et al. 1990;Mathez et al. 1995;Shiryaev et al. 2011); serpentinites (Xu et al. 2008); peridotites and related podiform chromitites (Trumbull et al. 2009;Yang et al. 2015). Based on the thermodynamic calculation, Mathez et al. (1995) concluded that moissanite is only stable in the upper mantle with the oxygen fugacity five to six log units below the IW buffer. ...
... Golubkova et al. (2016) computed phase diagrams sections for the alloys, carbides, and Fe-silicides and concluded that moissanite can only occur at oxygen fugacities 6.5-7.5 log units below the IW buffer. Metallic Si is a very common inclusion in moissanite both from kimberlites and podiform chromitite (Shiryaev et al. 2011;Trumbull et al. 2009), which has also been observed in moissanite recovered from the PKO chromitites. Experiments indicate that the formation of metallic Si require the environment to have oxygen fugacity 3-5 log units below that of SiC-forming reaction (Golubkova et al. 2016;Schmidt et al. 2014). ...
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The Pozanti-Karsanti ophiolite situated in the eastern Tauride belt, southern Turkey, is a well-preserved oceanic lithosphere remnant comprising, in ascending order, mantle peridotite, ultramafic and mafic cumulates, isotropic gabbros, sheeted dikes, and basaltic pillow lavas. Two types of chro-mitites are observed in the Pozanti-Karsanti ophiolite. One type of chromitites occurs in the cumulate dunites around the Moho, and the other type of chromitites is hosted by the mantle harzburgites below the Moho. The second type of chromitites has massive, nodular, and disseminated textures. We have conducted the mineral separation work on the podiform chromitites hosted by harzburgites. So far, more than 100 grains of microdiamond and moissanite (SiC) have been recovered from the podiform chromitite. The diamonds and moissanite are accompanied by large amounts of rutile. Besides zircon, monazite and sulfide are also very common phases within the separated minerals. The discovery of diamond, moissanite, and the other unusual minerals from podiform chromitite of the Pozanti-Karsanti ophiolite provides new evidences for the common occurrences of these unusual minerals in ophiolitic peridotites and chromitites. This discovery also suggests that deep mantle processes and materials have been involved in the formation of podiform chromitite.
... Highpressure experiments of Wade and Wood (2005) simulating formation of Earth's core, using graphite crucibles, recorded a significant amount of carbon (~11 wt%) in the metal end products, but were unable to obtain accurate measurements. Fe-Si and Si 0 also have been reported in ophiolites (Bird and Weathers 1975), kimberlites (Shiryaev et al. 2011), and chromitites Yang et al. 2015) and interpreted as samples of the mantle/core boundary brought to the surface. Fe-Si and Si 0 have also been reported as detrital grains in limestone from Russia (Novoselova and Lyul' 1986;Novgorodova et al. 1989), and inferences were made to a possible extraterrestrial source. ...
... Fe-Si and Si 0 have also been reported as detrital grains in limestone from Russia (Novoselova and Lyul' 1986;Novgorodova et al. 1989), and inferences were made to a possible extraterrestrial source. It is interesting to note that Fe-Si and Si 0 minerals in terrestrial rocks are often found as inclusions within moissanite (SiC) or associated with diamond (Di Pierro et al. 2003;Shiryaev et al. 2011;Yang et al. 2015) consistent with the hypothesis that carbon could be a reductant that allows Fe-Si and Si 0 to form in some high temperature-high pressure settings. ...
Article
Atom Probe Tomography of Reduced Phases in Apollo 16 Regolith Sample 61501,22 - Volume 23 Issue S1 - P. Gopon, M. Spicuzza, T.F. Kelly, Reinhard, T.J. Prosa, D.J. Larson, J. Fournelle
... Highpressure experiments of Wade and Wood (2005) simulating formation of Earth's core, using graphite crucibles, recorded a significant amount of carbon (~11 wt%) in the metal end products, but were unable to obtain accurate measurements. Fe-Si and Si 0 also have been reported in ophiolites (Bird and Weathers 1975), kimberlites (Shiryaev et al. 2011), and chromitites Yang et al. 2015) and interpreted as samples of the mantle/core boundary brought to the surface. Fe-Si and Si 0 have also been reported as detrital grains in limestone from Russia (Novoselova and Lyul' 1986;Novgorodova et al. 1989), and inferences were made to a possible extraterrestrial source. ...
... Fe-Si and Si 0 have also been reported as detrital grains in limestone from Russia (Novoselova and Lyul' 1986;Novgorodova et al. 1989), and inferences were made to a possible extraterrestrial source. It is interesting to note that Fe-Si and Si 0 minerals in terrestrial rocks are often found as inclusions within moissanite (SiC) or associated with diamond (Di Pierro et al. 2003;Shiryaev et al. 2011;Yang et al. 2015) consistent with the hypothesis that carbon could be a reductant that allows Fe-Si and Si 0 to form in some high temperature-high pressure settings. ...
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The lunar regolith contains a variety of chemically reduced phases of interest to planetary scientists and the most common, metallic iron, is generally ascribed to space weathering processes (Lucey et al. 2006). Reports of silicon metal and iron silicides, phases indicative of extremely reducing conditions, in lunar samples are rare (Anand et al. 2004; Spicuzza et al. 2011). Additional examples of Fe-silicides have been identified in a survey of particles from Apollo 16 sample 61501,22. Herein is demonstrated the utility of low keV electron probe microanalysis (EPMA), using the Fe Ll X-ray line, to analyze these submicron phases, and the necessity of accounting for carbon contamination. We document four Fe-Si and Si0 minerals in lunar regolith return material. The new Fe-Si samples have a composition close to (Fe,Ni)3Si, whereas those associated with Si0 are close to FeSi2 and Fe3Si7. Atom probe tomography of (Fe,Ni)3Si shows trace levels of C (60 ppma and nanodomains enriched in C, Ni, P, Cr, and Sr). These reduced minerals require orders of magnitude lower oxygen fugacity and more reducing conditions than required to form Fe0. Documenting the similarities and differences in these samples is important to constrain their formation processes. These phases potentially formed at high temperatures resulting from a meteorite impact. Whether carbon played a role in achieving the lower oxygen fugacities—and there is evidence of nearby carbonaceous chondritic material—it remains to be proven that carbon was the necessary component for the unique existence of these Si0 and iron silicide minerals.
... The results of these calculations are in good agreement with the phases observed as inclusions in or attached to natural moissanite. These phases are mostly native Si, Fe-Si alloys, or iron silicides (Di Pierro et al. 2003;Shiryaev et al. 2011;Trumbull et al. 2009). In the Luobusa ophiolite, PGE alloys with variable amounts of Fe, Fe-Ni-Cr alloys, Cr-carbide, and iron silicide are observed (Bai et al. 2000). ...
... SiC is also described from kimberlites and other volcanic rocks (Shiryaev et al. 2011;Kaminsky et al. 2016). Such SiC could simply be picked up upon magma ascent; alternatively, fluid-mediated SiC formation and eruption could be linked. ...
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Natural moissanite (SiC) is reported from mantle-derived samples ranging from lithospheric mantle keel diamonds to serpentinites to podiform chromitites in ophiolites related to suprasubduction zone settings (Luobusa, Dongqiao, Semail, and Ray-Iz). To simulate ultra-reducing conditions and the formation of moissanite, we compiled thermodynamic data for alloys (Fe–Si–C and Fe–Cr), carbides (Fe3C, Fe7C3, SiC), and Fe-silicides; these data were augmented by commonly used thermodynamic data for silicates and oxides. Computed phase diagram sections then constrain the P–T–fO2 conditions of SiC stability in the upper mantle. Our results demonstrate that: Moissanite only occurs at oxygen fugacities 6.5–7.5 log units below the iron–wustite buffer; moissanite and chromite cannot stably coexist; increasing pressure does not lead to the stability of this mineral pair; and silicates that coexist with moissanite have X Mg > 0.99. At upper mantle conditions, chromite reduces to Fe–Cr alloy at fO2 values 3.7–5.3 log units above the moissanite-olivine-(ortho)pyroxene-carbon (graphite or diamond) buffer (MOOC). The occurrence of SiC in chromitites and the absence of domains with almost Fe-free silicates suggest that ultra-reducing conditions allowing for SiC are confined to grain scale microenvironments. In contrast to previous ultra-high-pressure and/or temperature hypotheses for SiC origin, we postulate a low to moderate temperature mechanism, which operates via ultra-reducing fluids. In this model, graphite-/diamond-saturated moderately reducing fluids evolve in chemical isolation from the bulk rock to ultra-reducing methane-dominated fluids by sequestering H2O into hydrous phases (serpentine, brucite, phase A). Carbon isotope compositions of moissanite are consistent with an origin of such fluids from sediments originally rich in organic compounds. Findings of SiC within rocks mostly comprised by hydrous phases (serpentine + brucite) support this model. Both the hydrous phases and the limited diffusive equilibration of SiC with most minerals in the rocks indicate temperatures below 700–800 °C. Moissanite from mantle environments is hence a mineral that does not inform on pressure but on a low to moderate temperature environment involving ultra-reduced fluids. Any mineral in equilibrium with SiC could only contain traces of Fe2+ or Cr3+.
... In the past, it was sometimes questioned, whether the moissanite in mantle samples is actually a natural phase or a synthetic contamination. However, SiC has now been discovered in situ in a wide range of geological settings, including kimberlites and lamproites (Kaminsky 2012;Shiryaev et al. 2011), ophiolites (Golubkova et al. 2016;Pujol-Solà et al. 2018;Trumbull et al. 2009), alkaline basalt tuffs (Dobrzhinetskaya et al. 2018), and syenites (Nazzareni et al. 2019). In such super-reducing mineral assemblages, moissanite is commonly associated with native iron, silicon, various alloys, wüstite, phosphides and nitrides. ...
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Immiscibility between water and hydrogen-rich fluids may be responsible for the formation of super-reduced mineral assemblages and for the early oxidation of Earth´s upper mantle. In the current study, we present new data on the critical curve in the H2-H2O system to 1400 ℃ and 4 GPa. We utilized a synthetic fluid inclusion method to trap fluids at high P–T conditions within quartz and olivine crystals. Experiments were performed in a piston-cylinder type apparatus, employing a double-capsule technique. The inner capsule contained the crystal and fluids of interest, while the outer served as oxygen fugacity buffer, maintaining f(O2) at the iron-wüstite (Fe-FeO) equilibrium. Our results suggest that below ~ 2.5 GPa, the critical curve has a mostly linear slope of 200 ℃/GPa, while at more elevated pressure it becomes significantly steeper (400 ℃/GPa). This implies that in most of the modern, reduced upper mantle, water and hydrogen are immiscible, while localized heating events, such as rising plumes, may close the miscibility gap. The steep increase of the critical curve at high pressure observed in this study implies that even for very hot geotherms in the early Archean or the late Hadean, H2-H2O immiscibility likely occurred in the deeper parts of the upper mantle, thus making a plausible case for rapid H2 loss as a mechanism of early mantle oxidation. To constrain the geochemical fingerprint of this process, we performed a series of element partitioning experiments to unravel how the H2-H2O unmixing may affect element transfer. Noble gases such as Xe as well as methane are preferentially incorporated in the hydrogen-rich phase, with a XeH2O/XeH2 ratio of ~ 8. This partitioning pattern may, for example, explain the underabundance of Xe isotopes produced by fission of Pu in the mantle. These Xe isotopes may have been removed by a primordial H2-H2O unmixing event during the early stages of planetary evolution.
... Polytypes, defined as structural modifications of a compound arising from different arrangements of the same module (consequently, not resulting in significant chemical variations), are not generally given separate species names (Nickel and Grice 1998). Thus, for example, IMA-CNMNC protocols do not define species based on varied stacking arrangements of layered minerals, such as the 3R and 4H polytypes of graphite (C; Trubkin and Novgorodova 1996), the 6H and 15R polytypes of moissanite (SiC; Shiryaev et al. 2011), and the 1M and 3T polytypes of mica group minerals (Fleet 2003). A similar situation related to orientations of adjacent glycolate molecules was observed by Yang et al. (2021) in lazaraskeite [Cu(C 2 H 3 O 3 ) 2 ], which occurs in two topologically equivalent polytypes-slightly different structural variants designated M 1 and M 2 but not separate species. ...
Article
How does one best subdivide nature into kinds? All classification systems require rules for lumping similar objects into the same category, while splitting differing objects into separate categories. Mineralogical classification systems are no exception. Our work in placing mineral species within their evolutionary contexts necessitates this lumping and splitting because we classify “mineral natural kinds” based on unique combinations of formational environments and continuous temperature-pressure-composition phase space. Consequently, we lump two minerals into a single natural kind only if they: (1) are part of a continuous solid solution; (2) are isostructural or members of a homologous series; and (3) form by the same process. A systematic survey based on these criteria suggests that 2310 (~41%) of 5659 IMA-approved mineral species can be lumped with one or more other mineral species, corresponding to 667 “root mineral kinds,” of which 353 lump pairs of mineral species, while 129 lump three species. Eight mineral groups, including cancrinite, eudialyte, hornblende, jahnsite, labuntsovite, satorite, tetradymite, and tourmaline, are represented by 20 or more lumped IMA-approved mineral species. A list of 5659 IMA-approved mineral species corresponds to 4016 root mineral kinds according to these lumping criteria. The evolutionary system of mineral classification assigns an IMA-approved mineral species to two or more mineral natural kinds under either of two splitting criteria: (1) if it forms in two or more distinct paragenetic environments, or (2) if cluster analysis of the attributes of numerous specimens reveals more than one discrete combination of chemical and physical attributes. A total of 2310 IMA-approved species are known to form by two or more paragenetic processes and thus correspond to multiple mineral natural kinds; however, adequate data resources are not yet in hand to perform cluster analysis on more than a handful of mineral species. We find that 1623 IMA-approved species (~29%) correspond exactly to mineral natural kinds; i.e., they are known from only one paragenetic environment and are not lumped with another species in our evolutionary classification. Greater complexity is associated with 587 IMA-approved species that are both lumped with one or more other species and occur in two or more paragenetic environments. In these instances, identification of mineral natural kinds may involve both lumping and splitting of the corresponding IMA-approved species on the basis of multiple criteria. Based on the numbers of root mineral kinds, their known varied modes of formation, and predictions of minerals that occur on Earth but are as yet undiscovered and described, we estimate that Earth holds more than 10 000 mineral natural kinds.
... There are indications that the sublithospheric upper mantle, and the lower mantle, are saturated in Fe metal or Fe-Ni-S melts, constraining f O2 to near the IW buffer (Frost and McCammon 2008;Zhang et al. 2016). However, there also is evidence that some volumes of the upper mantle have experienced much more reducing conditions, defined by minerals such as moissanite (SiC), which commonly occurs in kimberlites (Huang et al. 2020;Shiryaev et al. 2011) and requires f O2 at least 6 log units below the IW buffer (ΔIW-6) at lithospheric pressure (P) and temperature (T) (Ulmer et al. 1998). Another example is the super-reduced mineral association (native elements, carbides, silicides) described from chromitites and peridotites in the ophiolites of the Yarlung-Zangbo suture of southern Tibet, and similar bodies in the Polar Urals (Griffin et al. 2016b;Yang et al. 2015). ...
Article
Xenolithic corundum aggregates in Cretaceous mafic pyroclastics from Mount Carmel contain pockets of silicate melts with mineral assemblages [SiC (moissanite), TiC, Ti2O3 (tistarite), Fe-Ti-Zr silicides/phosphides] indicative of magmatic temperatures and oxygen fugacity (fO2) at least 6 log units below the iron-wüstite buffer (ΔIW ≤ –6). Microstructural evidence indicates that immiscible, carbon-rich metallic (Fe-Ti-Zr-Si-P) melts separated during the crystallization of the silicate melts. The further evolution of these metallic melts was driven by the crystallization of two main ternary phases (FeTiSi and FeTiSi2) and several near-binary phases, as well as the separation of more evolved immiscible melts. Reconstructed melt compositions fall close to cotectic curves in the Fe-Ti-Si system, consistent with trapping as metallic liquids. Temperatures estimated from comparisons with experimental work range from ≥1500 °C to ca. 1150 °C; these probably are maximum values due to the solution of C, H, P, and Zr. With decreasing temperature (T), the Si, Fe, and P contents of the Fe-Ti-Si melts increased, while contents of Ti and C decreased. The increase in Si with declining T implies a corresponding decrease in fO2, probably to ca. ΔIW-9. The solubility of P in the metallic melts declined with T and fO2, leading to immiscibility between Fe-Ti-Si melts and (Ti,Zr)-(P,Si) melts. Decreasing T and fO2 also reduced the solubility of C in the liquid metal, driving the continuous crystallization of TiC and SiC during cooling. The lower-T metallic melts are richer in Cr, and to some extent V, as predicted by experimental studies showing that Cr and V become more siderophile with decreasing fO2. These observations emphasize the importance of melt-melt immiscibility for the evolution of magmas under reducing conditions. The low fO2 and the abundance of carbon in the Mt. Carmel system are consistent with a model in which differentiating melts were fluxed by fluids that were dominated by CH4+H2, probably derived from a metal-saturated sublithospheric mantle. A compilation of other occur-rences suggests that these phenomena may commonly accompany several types of explosive volcanism.
... This is consistent with iron phosphides being dredged from the continental 'keels' of continental cratons on Earth [60] as xenoliths. However, such xenoliths represent milligrams of material in kilograms of rock, not the bulk melt [52,[61][62][63][64][65][66] (hence the name xenolith-"Foreign rock"). At lower mantle depths ( Figure 3A), several phosphides are strongly favored over phosphate, consistent with such minerals being found as inclusions in mantle diamonds. ...
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The initial reports of the presence of phosphine in the cloud decks of Venus have led to the suggestion that volcanism is the source of phosphine, through volcanic phosphides ejected into the clouds. Here, we examine the idea that mantle plume volcanism, bringing material from the deep mantle to the surface, could generate observed amounts of phosphine through the interaction of explosively erupted phosphide with sulfuric acid clouds. The direct eruption of deep mantle phosphide is unphysical, but a shallower material could contain traces of phosphide, and could be erupted to the surface. The explosive eruption that efficiently transports material to the clouds would require ocean:magma interactions or the subduction of a hydrated oceanic crust, neither of which occur on modern Venus. The transport of the erupted material to altitudes coinciding with the observations of phosphine is consequently very inefficient. Using the model proposed by Truong and Lunine as a base case, we estimate that an eruption volume of at least 21,600 km3/year would be required to explain the presence of 1 ppb phosphine in the clouds. This is greater than any historical terrestrial eruption rate, and would have several detectable consequences for remote and in situ observations to confirm. More realistic lithospheric mineralogy, volcano mechanics or atmospheric photochemistry require even more volcanism.
... The Luobusha ophiolite group occurs at a plate suture between the Eurasian Plate and the Indo-Australian Plate [63]. Furthermore, iron silicides have been found throughout the literature as an unusual material that may co-occur with moissanite (SiC) [62,[64][65][66][67]. Silicides are occasionally attributed to impact, but it is plausible that lightning may be a source of some of these silicides [28]. ...
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Iron silicide minerals (Fe-Si group) are found in terrestrial and solar system samples. These minerals tend to be more common in extraterrestrial rocks such as meteorites, and their existence in terrestrial rocks is limited due to a requirement of extremely reducing conditions to promote their formation. Such extremely reducing conditions can be found in fulgurites, which are glasses formed as cloud-to-ground lightning heats and fuses sand, soil, or rock. The objective of this paper is to review reports of iron silicides in fulgurites, note any similarities between separate fulgurite observations, and to explain the core connection between geological environments wherein these minerals are found. In addition, we also compare iron silicides in fulgurites to those in extraterrestrial samples.
... This is consistent with iron phosphides being dredged from the continental 'keels' of continental cratons [60] as xenoliths, which is observed on Earth. However such xenoliths represent milligrams of material in kilograms of rock, not the bulk melt [52,[61][62][63][64][65][66] (hence the name xenolith -"Foreign rock"). At lower mantle depths ( Figure 3A) several phosphides are strongly favoured over phosphate, consistent with such minerals being found as inclusions in mantle diamonds. ...
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The initial reports of the presence of phosphine in the cloud decks of Venus has led to the suggestion that volcanism was the source of phosphine, through volcanic phosphides ejected into the clouds. Here we examine the idea that mantle plume volcanism, bringing material from the deep mantle to the surface, could generate observed amounts of phosphine through interaction of explosively erupted phosphide with sulfuric acid clouds. Direct eruption of deep mantle phosphide is unphysical, but shallower material could contain traces of phosphide, and could be erupted to the surface. Explosive eruption that efficiently transported material to the clouds would require ocean:magma interactions or subduction of hydrated oceanic crust, neither of which occur on modern Venus. The transport of erupted material to altitudes coinciding with the observations of phosphine is consequently very inefficient. Using the model proposed by Truong and Lunine as a base case, we estimate that an eruption volume of at least 21,600 km3/year would be required to explain the presence of 1 ppb phosphine in the clouds. This is greater than any historical terrestrial eruption rate, and would have several detectable consequences for remote and in situ observations to confirm. More realistic lithospheric chemistry or atmospheric photochemistry require even more volcanism.
... Based on one of the hypotheses proposed by Ross et al. (2019) for the formation of natural suessite, carletonmooreite could also have formed in ureilite in a very reduced (Shiryaev et al., 2011); (c) in the alluvial rocks near Mt. Carmel (Israel) (Griffin et al., 2016); (d) in mantle xenoliths from Kamchatka volcanics (Ishimaru et al., 2009); and (e) in fulgurites (Stefano et al., 2020). ...
Article
The occurrence of shock-induced diamonds in ureilite meteorites is common and is used to constrain the history of the ureilite parent bodies. We have investigated a fragment of the Kenna ureilite by micro-X-ray diffraction, micro-Raman spectroscopy and scanning electron microscopy to characterize its carbon phases. In addition to olivine and pigeonite, within the carbon-bearing areas, we identified microdiamonds (up to about 10 μm in size), nanographite and magnetite. The shock features observed in the silicate minerals and the presence of microdiamonds and nanographite indicate that Kenna underwent a shock event with a peak pressure of at least 15 GPa. Temperatures estimated using a graphite geothermometer are close to 1180 °C. Thus, Kenna is a medium-shocked ureilite, yet it contains microdiamonds, which are typically found in highly shocked carbon-bearing meteorites, instead of the more common nanodiamonds. This can be explained by a relatively long shock event duration (in the order of 4-5 seconds) and/or by the catalytic effect of Fe-Ni alloys known to favour the crystallization of diamonds. For the first time in a ureilite, carletonmooreite with formula Ni3Si and grain size near 4-7 nm, was found. The presence of nanocrystalline carletonmooreite provides further evidence to support the hypothesis of the catalytic involvement of Fe-Ni bearing phases into the growth process of diamond from graphite during shock events in the ureilite parent body, enabling the formation of micrometer-sized diamond crystals.
... However, several lines of evidence indicate that volumes with much lower fO 2 exist locally within the mantle. One is the common occurrence of moissanite (SiC) in kimberlites 8,9 , which implies fO 2 6-8 orders of magnitude below the IW buffer 10 . SiC and other phases indicative of very low fO 2 (nitrides, silicides, carbides) have been reported from peridotites and chromitites in ophiolites from Tibet, Myanmar, and the Polar Urals 11-13 . ...
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Aggregates of corundum crystals with skeletal to hopper morphology occur in pyroclastic rocks erupted from Cretaceous basaltic volcanoes on Mt Carmel, N. Israel. The rapid growth of the crystals trapped volumes of the parental Al 2 O 3 -supersaturated melt; phenocrysts of tistarite (Ti 2 O 3 ) in the trapped melts indicate crystallization at oxygen fugacities 6–7 log units below the Iron-Wüstite buffer ( f O 2 = ΔIW − 6 to − 7), induced by fluxes of mantle-derived CH 4 -H 2 fluids. Cathodoluminescence images reveal growth zoning within the individual crystals of the aggregates, related to the substitution of Ti ³⁺ in the corundum structure. Ti contents are < 0.3 wt% initially, then increase first linearly, then exponentially, toward adjacent melt pockets to reach values > 2 wt%. Numerical modelling indicates that the first skeletal crystals grew in an open system, from a moving magma. The subsequent linear increase in Ti reflects growth in a partially closed system, with decreasing porosity; the exponential increase in Ti close to melt pockets reflects closed-system growth, leading to dramatic increases in incompatible-element concentrations in the residual melts. We suggest that the corundum aggregates grew in melt/fluid conduits; diffusion modelling implies timescales of days to years before crystallization was terminated by explosive eruption. These processes probably operate in explosive volcanic systems in several tectonic settings.
... Moissanite (SiC) occurs in kimberlites, alkaline basalts, podiform chromitites in ophiolites and inclusions in diamonds [1][2][3]. Thermodynamic data suggest formation at high pressures and temperatures in an extremely reducing environment, 5-8 log units below the iron-wüstite buffer, which is the lowest level of fO2 activity in the lithospheric mantle [4]. This, together with common association with diamond, abundance of native metal and metal carbide inclusions and 13 C-depleted isotope composition, led several authors to propose lower mantle or transitional zone origin for natural moissanite [3,5]. ...
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A magnesian low-Ti shoshonite dike intruding Archean Norwegian Terrane includes numerous grains of black, blue and gray moissanite (SiC). Moissanite contains inclusions of native Si, Fe and Al, Fe-Cr carbides, Fe-Cr-Mn alloys and diamond. The range of observed δ ¹³ C values (-24.8 to -29.6‰) is similar to ophiolite-hosted SiC, lower mantle diamonds and slab-derived biogenic carbon. Norwegian moissanites may have been formed during interaction of Earth’s mantle with carbonaceous slab fluids under extremely reducing conditions as suggested by native metal and carbide inclusions. ¹³ C-depleted moissanite can be used as a new exploration tool for sub-lithospheric diamonds in Archean to Phanerozoic accreted terranes and mobile belts.
... In this scenario, it is necessary to allow for low oxygen fugacity in the region of the lithospheric mantle where diamonds containing iron carbide inclusions are formed. Moissanite found in kimberlites and the super-reduced minerals occurring in the peridotites of Tibet and the Polar Urals suggest that very low oxygen fugacity occurs in certain mantle regions (Griffin et al., 2016;Shiryaev et al., 2011;Trumbull et al., 2009;Xu et al., 2015;Yang et al., 2015). ...
Article
We demonstrate for the first time the presence of iron carbides in placer diamonds from the northeastern region of the Siberian craton. It was found that the inclusions are polycrystalline aggregates, and iron carbides filling the fissures in the diamonds, thus providing clear evidence that the iron melts were captured first. Iron carbides were identified in diamonds containing mineral inclusions of eclogitic (Kfsp, sulfide) and peridotitc (olivine) paragenesis Iron carbides with minor amounts of admixed nickel were detected in 58a diamond sample containing an olivine inclusion (0.3 wt% Ni), indicating that the iron melt was not in equilibrium with the mantle peridotite.The low nickel contents of the iron carbides provide the best evidence that the subducted crust is 58a likely source of the iron melt. Diamonds containing carbide inclusions are characterised by 58a relatively low nitrogen aggregation state (5–35%), which is not consistent with the high temperature of the transition zone. Therefore, we have reason to assume that the studied diamonds are from the lower regions of the lithosphere.Considering all factors, the model for the interaction of the ascending asthenospheric mantle with the subducting slab seems to be more realistic.
... The Raman spectra on the worn surface of M A composite under 10 N load are shown in Figure 6. The Raman spectra of the spherical shape particles in the worn surface ( Figure 4A) is shown in Figure 6A, where the SiC is identified as the main phase, 35 which confirm the nonchemical change of the later even under 10 N load. Figure 6B exhibit the Raman spectra of two analysis carried out on the light area on the worn track of M A composite pair under 10 N (marked as B2 in Figure 4A) where the TiC, and Ti 3 SiC 2 was clearly identified. ...
Article
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In this study, wear and friction behavior of two based‐composites from the Ti‐Si‐C system, (40 wt% TiC; 28 wt% Ti5Si3; 17 wt% Ti3SiC2) and (18 wt% TiC; 26 wt% Ti5Si3; 41 wt% Ti3SiC2) reinforced by 15 wt% of large size SiC (100‐150 µm) particles were investigated. The four‐phase composites exhibited approximatively the same friction coefficient (µ ~ 0.9) under high loads (10 N and 7 N). The composite with high Ti3SiC2 showed higher wear rate values by one order of magnitude. However, under 1 N, the composite with high TiC content showed a higher running‐in period and a lower steady state µ value (0.37 after 1000 m sliding distance). Scanning electron microscopy, Energy Dispersive X‐Ray and Raman spectroscopy analysis of the worn surfaces of the two composites revealed that oxidation was the dominant wear mechanism. The oxidation process and the removal kinetics of the oxides during sliding controlled the tribological behavior of the composites. The influence of processing variables on microstructures development and wear mechanisms of the composites is discussed.
... However, the stability of SiC inside a planetary body in the presence of other elements and materials has not been extensively explored, and the conditions of its formation on Earth are still a topic of active research (Dobrzhinetskaya et al., 2017;Lyakhovich, 1980;Schmidt et al., 2014;Shiryaev et al., 2011). As SiC has been proposed to be a component in planetary systems, experimentally determining the stability of SiC in the presence of other planetary materials is essential. ...
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We present results from high‐pressure and high‐temperature experiments on mixtures of SiC and SiO2 to explore the stability of SiC in the presence of oxygen‐rich silicates at planetary mantle conditions. We observe no evidence of the ambient pressure predicted oxidation products, CO or SiO, resulting from oxidation reactions between SiC and SiO2 at pressures up to ~40 GPa and temperatures up to ~2500 K. We observe the decomposition of SiC through releasing C, resulting in vacancies in the SiC lattice and consequently the contracted SiC ambient volume V0 observed in the heated regions of sample. The decomposition is further supported by the observations of diamond formation and the expanded SiO2 V0 in the heated regions of samples indicating the incorporation of C into SiO2 stishovite. We provide a new interpretation of SiC decomposition on laboratory timescales, in which kinetics prevent the reaction from reaching equilibrium. We consider how the equilibrium decomposition reaction of SiC will influence the differentiation of a SiC‐containing body on planetary timescales and find that the decomposition products may become isolated during early planetary differentiation. The resulting presence of elemental Si and C within a planetary body may have important consequences for the compositions of the mantles and atmospheres of such planets.
... Worldwide occurrence of moissanite, which is another SR phase in many ophiolitic rocks and other magmatic sequences requires special attention and will be considered in one of our next reports. Here we briefly notice that moissanite is typically found in mineral concentrates from kimberlite, ophiolitic peridotite and chromitite and other rocks (Gromilov et al., 2018;Mathez et al., 1995;Shiryaev et al., 2011;Xu et al., 2015;Yang et al., 2015aYang et al., , 2015b. This moissanite should be compared with synthetic SiC more carefully. ...
Article
Recently a number of reports claimed enigmatic appearance of high-pressure and super-reduced minerals in ophiolitic chromitite and peridotite. Diamond, moissanite, various metal alloys, and native metals, carbides and nitrides were found in mineral separates from bulk rock probes of chromitite and peridotite from Tibet, Polar Ural and other localities. Similar findings of super-reduced phases were reported for pyroclastic rocks and alluvial deposits of Mt. Carmel in Israel. We performed the study of microinclusions in corundum grains from abrasive materials produced industrially in an electric arc furnace and found that they are very similar to microinclusions in corundum grains from natural samples. The key similar phases are Ti ³⁺4 Al 2 (Zr,Ti ⁴⁺ ) 4 O 11 carmeltazite, Ti 2 O 3 tistarite, TiN 1-x , TiC 1-x , Fe–Si and Fe-Si-Ti alloys, hibonite, grossite, anorthite, and residual feldspatic glass. Although some differences between abrasive corundum and corundum from Tibet and Mt. Carmel are obvious, the morphology of melt pockets and amount of mineralogical similarities is more than critical to suggest that they can be of the same origin. The additional possible source of corundum grains with super-reduced inclusions is various Al-bearing slags after aluminothermic reactions during steelmaking. Moreover, taking into account the spectacular similarity of diamonds from ophiolite with synthetic diamonds, we claim for a thorough reconsideration of ultrahigh-pressure and super-reduced phases in natural rocks and argue that we need to find criteria for discrimination between natural and artificial samples. One of the strongest criteria would be textural and structural features that clearly demonstrate the indigenous character of the host minerals.
... The IW 42 buffer is generally thought to define the minimum fO 2 of both the lithospheric mantle 43 and the deeper mantle (Frost and McCammon 2008). However, SiC with high-44 temperature metallic inclusions is a relatively common trace phase in heavy-mineral 45 concentrates from kimberlites, lamproites and similar deep-seated volcanic rocks, and 46 occurs as an inclusion in diamonds (Shiryaev et al. 2011;Griffin et al. 2018Griffin et al. , 2019a. It 47 has also been found in the peridotites and chromitites of many ophiolitic complexes, 48 particularly in the Tethyan belt across Tibet into Turkey, and in the Polar Urals. ...
Article
Although hydrogen is the most abundant element in the solar system, the mechanisms of exchange of this element between the deep interior and surface of Earth are still uncertain. Hydrogen has profound effects on properties and processes on microscopic-to-global scales. Here we report the discovery of the first hydride (VH 2) ever reported in nature. This phase has been found in the ejecta of Cretaceous pyroclastic volcanoes on Mt Carmel, N. Israel, which include abundant xenoliths containing highly reduced mineral assemblages. These xenoliths were sampled by their host magmas at different stages of their evolution but are not genetically related to them. The xenoliths are interpreted as the products of extended interaction between originally mafic magmas and CH 4 +H 2 fluids, derived from a deeper, metal-saturated mantle. The last stages of melt evolution are recorded by coarse-grained aggregates of hibonite (CaAl 12 O 19) + grossite (CaAl 4 O 7) + V-rich spinels ± spheroidal to dendritic inclusions of metallic vanadium (V 0), apparently trapped as immiscible metallic melts. The presence of V 0 implies low oxygen fugacities and suggests crystallization of the aggregates in a hydrogen-rich atmosphere. The presence of such reducing conditions in the upper mantle has major implications for the transport of carbon, hydrogen and other volatile species from the deep mantle to the surface.
... Natural iron silicide materials have been found in various, generally extremely reducing, environments, e.g., in lightning-induced fulgurites (Essene & Fisher, 1986), in micro-meteoritic impactformed lunar regolith (Gopon et al., 2013), in rocks formed deep in the Earth's mantle (Shiryaev et al., 2011), and in Project Stardust samples (Rietmeijer et al., 2008). Their study is of great importance to understand the formation mechanism of the materials in which they occur. ...
Article
The recent availability of Schottky-type field emission electron microprobes provides incentive to consider analyzing micrometer-sized features. Yet, to quantify sub-micrometer-sized features, the electron interaction volume must be reduced by decreasing accelerating voltage. However, the K lines of the transition elements (e.g., Fe) then cannot be excited, and the L lines must be used. The Fe L α1,2 line is the most intense of the L series but bonding effects change its atomic parameters because it involves a valence band electron transition. For successful traditional electron probe microanalysis, the mass absorption coefficient (MAC) must be accurately known, but the MAC of Fe L α1,2 radiation by Fe atoms varies from one Fe-compound to another and is not well known. We show that the conventional method of measuring the MAC by an electron probe cannot be used in close proximity to absorption edges, making its accurate determination impossible. Fortunately, we demonstrate, using a set of Fe–silicide compounds, that it is possible to derive an accurate calibration curve, for a given accelerating voltage and takeoff angle, which can be used to quantify Fe in Fe–silicide compounds. The calibration curve can be applied to any spectrometer without calibration and gives accurate quantification results.
... So, we termed suite of studied mantle-derived particles as high-reduced mantle mineral association (HRMMA). In order to create classification of mantle-derived particles an attempt has been made to draw together published information [7][8][9][10][11][12][13][14][15][16] and our own observations (table 1). Current version of the classification is shown in fig. 1. ...
... Their stoichiometry varies widely, from Me/Si = 3 to 0.42. Most of them are identified in meteorites and cosmic dust, such as suessite and hapkeite (Anand et al. 2004;Rietmeijer et al. 2008), as well as the manganese silicide, brownleeite MnSi (Nakamura-Messenger et al. 2010); some are known from terrestrial formations, frequently as inclusions in moissanite along with native Si 0 (Shiryaev et al. 2011). ...
Article
A series of polycrystalline diamond grains were found within the Valizhgen Peninsula in Koryakia, northern Kamchatka, Russia. A grain from the Aynyn River area is studied in detail with TEM. Diamond crystallites, 2-40 μm in size are twinned and have high dislocation density. They are cemented with tilleyite Ca 5 (Si 2 O 7)(CO 3) 2 , SiC, Fe-Ni-Mn-Cr silicides, native silicon, graphite, calcite, and amorphous material. Among SiC grains, three polymorphs were discriminated: hexagonal 4H and 6H and cubic C3 (β-SiC). Silicides have variable stoichiometry with (Fe,Ni,Mn,Cr)/Si = 0.505-1.925. Native silicon is an open-framework allotrope of silicon S 24 , which has been observed, to date, as a synthetic phase only; this is a new natural mineral phase. Three types of amorphous material were distinguished: a Ca-Si-CO material, similar in composition to tilleyite; amorphous carbon in contact with diamond, which includes particles of crystalline graphite; and amorphous SiO 2. No regularity in the distribution of the amorphous material was observed. In the studied aggregate, diamond crystallites and moissanite are intensively twinned, which is characteristic for these minerals formed by gas phase condensation or chemical vapor deposition (CVD) processes. The synthetic analogs of all other cementing compounds (β-SiC, silicides, and native silicon) are typical products of CVD processes. This confirms the earlier suggested CVD mechanism for the formation of Avacha diamond aggregates. Both Avacha and Aynyn diamond aggregates are related not to "classic" diamond locations within stable cratons, but to areas of active and Holocene volcanic belts. The studied diamond aggregates from Aynyn and Avacha, by their mineralogical features and by their origin during the course of volcanic eruptions via a gas phase condensation or CVD mechanism, may be considered a new variety of polycrystalline diamond and may be called "kamchatite." Kamchatite extends the number of unusual diamond localities. It increases the potential sources of diamond and indicates the polygenetic character of diamond.
... Most synthetic SiC crystals have 2H, 3C, 4H, or 6H structure, as these are the most important SiC polytypes for the semi-conductor and ceramic industries. The most common polytypes known from natural terrestrial environments (e.g., kimberlites, chromitites) are the 6H and the 15R, but the 4H, 8H, and 3C polytypes and their mixtures also occur (e.g., Shiryaev et al. 2011). In contrast, the overwhelming majority (~80%) of presolar SiC has been found to be of the 3C polytype. ...
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We analyzed seven presolar SiC grains of supernova origin (average diameter: 1-2 μm) with transmission electron microscopy. Five grains are polycrystalline, whereas two grains are single crystals. Individual crystal domains of polycrystalline grains are in epitaxial relationship, with two grains consisting almost entirely of twinned crystal domains. Most grains are free of inclusions (only one TiC inclusion and one iron- and nickel-rich inclusion were found in two separate grains). Almost all crystals have cubic symmetry (3C polytype), but we found hexagonal SiC (6H polytype) in two grains. The large range of crystal domain sizes (average diameter: 50-970 nm), as well as the larger fraction of noncubic SiC polytypes in supernova grains relative to SiC grains that crystallized in the winds of asymptotic giant branch (AGB) stars, suggest that SiC condensation in supernova ejecta occurs at a larger range of chemical and physical conditions, including supersaturation, than in the winds of AGB stars. Modeling condensation of SiC struggles to produce SiC grains as large as, or bigger than, observed here, if condensation of large (i.e., several μm in diameter) graphite grains is to precede that of SiC, which is suggested by the presolar grain record and published equilibrium condensation models. We propose that future models of graphite and SiC condensation in SN ejecta explore higher ejecta densities than before, as well as gas compositions that are more silicon- and carbon-rich. Furthermore, we infer that some supernova SiC grains may have formed without prior condensation of graphite from their parent gas. © 2018. The American Astronomical Society. All rights reserved.
... It was found in variety of terrestrial rocks, first of all in some kimberlites, magmatic and volcanic rocks ( [97] op. cit., [98][99][100][101][102]), metasedimentary crustal rocks [103] and others geological environments [104]. It should be underlined that the presence of moissanite in evaporates formation is extremely rare [105]. ...
... In order to address the issue of natural versus synthetic SiC, Shiryaev et al. (2011) proposed to create a list of symptomatic features. They assumed that systematics of the rare earth elements (REE) and high field strength elements (HFSE) might serve as a great foundation for distinguishing between natural and synthetic SiC, with deep negative anomalies in Eu and Sm indicating a strongly reducing environment. ...
Article
Here, we present studies of natural SiC that occurs in situ in tuff related to the Miocene alkaline basalt formation deposited in northern part of Israel. Raman spectroscopy, SEM and FIB-assisted TEM studies revealed that SiC is primarily hexagonal polytypes 4H-SiC and 6H-SiC, and that the 4H-SiC polytype is the predominant phase. Both SiC polytypes contain crystalline inclusions of silicon (Sio) and inclusions of metal-silicide with varying compositions (e.g. Si58V25Ti12Cr3Fe2, Si41Fe24Ti20Ni7V5Zr3, and Si43Fe40Ni17). The silicides crystal structure parameters match Si2TiV5 (Pm-3m space group, cubic), FeSi2Ti (Pbam space group, orthorhombic), and FeSi2 (Cmca space group, orthorhombic) respectively. We hypothesize that SiC was formed in a local ultra-reduced environment at respectively shallow depths (60-100 km), through a “desilification” reaction of SiO2 with highly reducing fluids (H2O-CH4–H2–C2H6) arisen from the mantle “hot spot” and passing through alkaline basalt magma reservoir. SiO2 (melt) interacting with the fluids may originate from the walls of the crustal rocks surrounding this magmatic reservoir. The “desilification” process led to the formation of SiC and the reduction of metal-oxides to native metals, alloys, and silicides. The latter were trapped by SiC during its growth. Hence, interplate “hot spot” alkali basalt volcanism can now be included as a geological environment where SiC, silicon, and silicides can be found.
... However, several lines of evidence indicate that volumes with much lower f O 2 exist locally within the mantle. One is the common occurrence of moissanite (SiC) xenocrysts in kimberlites (Shiryaev et al., 2011;Trumbull et al., 2009), which implies f O 2 6-8 orders of magnitude below the IW buffer (Ulmer et al., 1998). SiC and other phases indicative of very low f O 2 (nitrides, silicides, carbides) have been reported from peridotites and chromitites in ophiolites from Tibet, Myanmar, and the Polar Urals, Russia Yang et al., 2015). ...
Article
The minimum oxygen fugacity (f(O2'')) of Earth's upper mantle probably is controlled by metal saturation, as defined by the iron-wustite (IW) buffer reaction (FeO -> Fe + O). However, the widespread occurrence of moissanite (SiC) in kimberlites, and a suite of super-reduced minerals (SiC, alloys, native elements) in peridotites in Tibet and the Polar Urals (Russia), suggest that more reducing conditions (f(O2) = 6-8 log units below IW) must occur locally in the mantle. We describe pockets of melt trapped in aggregates of corundum crystals ejected from Cretaceous volcanoes in northern Israel which contain high-temperature mineral assemblages requiring extremely low f(O2) (IW < -10). One abundant phase is tistarite (Ti2O3), previously known as a single grain in the Allende carbonaceous chondrite (Mexico) and believed to have formed during the early evolution of the solar nebula. It is associated with other reduced phases usually found in meteorites. The development of super-reducing conditions in Earth's upper mantle may reflect the introduction of CH4 + H-2 fluids from the deep mantle, specifically related to deep-seated volcanic plumbing systems at plate boundaries.
... Some of the grains contain small inclusions of Fe-Si and native Si, indicating much reduced conditions of formation. The common occurrence of native Si, SiC and FeSi 2 in the chromitites requires fO 2 4 to 7 orders of magnitude below the IW buffer (ΔFMQ = − 9 to − 12); the Si-SiO 2 buffer lies around ΔIW = − 8 (Trumbull et al., 2009;Shiryaev et al., 2011). Moissanite inclusions discovered in diamonds of the Rio São Luiz placer deposit are believed to have formed in the lower mantle (Wilding et al., 1991;Kaminsky, 2012). ...
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Podiform chromitites and their host peridotites in the Kangjinla mining district of the Luobusa ophiolite contain similar collections of ultrahigh pressure (UHP), highly reduced and crustal-type minerals. Abundant diamonds have been recovered from both lithologies and these are associated with a wide range of base metal alloys, native elements, carbides, oxides, silicates and others. The presence of UHP and highly reduced minerals in these rocks indicates that at least some of the chromite must have crystallized deep within the mantle as well as in a shallow mantle wedge in a suprasubduction zone (SSZ) environment. The unusual minerals were encapsulated in chromite grains and carried upward by mantle convection. The peridotite of Luobusa was trapped in the mantle wedge where it was modified by SSZ fluids and melts. Partial melting and mobilization of the chromite grains allowed them to be carried to shallow levels in melt channels and eventually deposited as chromitites near the crust mantle boundary. The unusual minerals were preserved during this process because they were encapsulated in chromite grains, either during crystallization or by later fluid fluxing.
... In addition to being dissolved in silicate or carbonate melts, oxidized species could also be present as crystalline carbonate or in a fluid. Reduced carbon could be present in a melt, a fluid, or under very reduced conditions in phases such as moissanite (SiC; see Di Pierro et al., 2003;Shiryaev et al., 2011;Ulmer et al., 1998). Fluids at mantle conditions may contain oxidized carbon-bearing species such as CO 2 or reduced species such as CH 4 , depending on the oxidation state. ...
Article
Geothermobarometric calculations for a worldwide database of inclusions in diamond indicate that formation of the dominant harzburgitic diamond association occurred predominantly (90%) under subsolidus conditions. Diamonds in eclogitic and lherzolitic lithologies grew in the presence of a melt, unless their formation is related to strongly reducing CHO fluids that would increase the solidus temperature or occurred at pressure-temperature conditions below about 5 GPa and 1050 °C. Three quarters of peridotitic garnet inclusions in diamond classify as "depleted" due to their low Y and Zr contents but, based on LREEN-HREEN ratios invariably near or greater than one, nevertheless reflect re-enrichment through either highly fractionated fluids or small amounts of melt. The trace element signatures of harzburgitic and lherzolitic garnet inclusions are broadly consistent with formation under subsolidus and supersolidus conditions, respectively. Diamond formation may be followed by cooling in the range of ~ 60-180 °C as a consequence of slow thermal relaxation or, in the case of the Kimberley area in South Africa, possibly uplift due to extension in the lithospheric mantle. In other cases, diamond formation and final residence took place at comparable temperatures or even associated with small temperature increases over time.
Article
A large range of minerals, native, intermetallic, amorphous compounds containing K, Na, Fe, Mn, Ca, Ba, Sr, Cu, Pb, Co, Ni, Sn, Zn, Al, Ce, Nd, La, Pr, Sm, Y, Yb, Nb, Hf, W, Mo, Zr, Cr, V, Ag, Ti, Si, has been found in the Bobruisk buried ledge of the Belarusian crystalline massif in the western part of the East European craton. As, P, Bi, O, H, F, Cl, S, Se, C, B, N F. Among them there is moissanite. It forms xenomorphic and subidiomorphic separations up to 1.5 mm in size and is represented by a hexagonal polytype modification 6H. Inclusions inherent in meteorites were found in one of its grains – sinoite (Si2N2O), xifengite (Si5Fe3) and awaruite (Ni3Fe). Moissanite, the native, intermetallic, amorphous compounds associated with it, are developed in the form of scattered, poor, multi-grained inclusions in the rocks of three structural-material complexes of the centuries. This indicates the superimposed nature of mineralization. The whole set of crystalline and amorphous secretions observed with moissanite is proposed to be designated as Bobruiskites. Mineral formation, with a high degree of probability, is due to the impact interaction of the meteoroid with rocks of the East European craton.
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This review systematically presents all finds of geogenic, impact-induced, and extraterrestrial iron silicide minerals known at the end of 2021. The respective morphological characteristics, composition, proven or reasonably suspected genesis, and possible correlations of different geneses are listed and supported by the available literature (2021). Artificially produced iron silicides are only dealt with insofar as the question of differentiation from natural minerals is concerned, especially regarding dating to pre-industrial and pretechnogenic times.
Article
Presolar silicon carbide (SiC) grains in meteoritic samples can help constrain circumstellar condensation processes and conditions in C-rich stars and core-collapse supernovae (CCSNe). This study presents our findings on eight presolar SiC grains from asymptotic giant branch (AGB) stars (four mainstream and one Y grain) and CCSNe (three X grains), chosen on the basis of μ-Raman spectral features that were indicative of their having unusual non- 3C polytypes and/or high degrees of crystal disorder. Analytical transmission electron microscopy (TEM), which provides elemental compositional and structural information, shows evidence for complex histories for the grains. Our TEM results confirm the presence of non-3C,2H crystal domains. Minor-element heterogeneities and/or subgrains were observed in all grains analyzed for their compositions. The C/O ratios inferred for the parent stars varied from 0.98 to </=1.03. Our data show that SiC condensation can occur under a wide range of conditions, in which environmental factors other than temperature (e.g., pressure, gas composition, heterogeneous nucleation on precondensed phases) play a significant role. Based on previous μ-Raman studies, ∼10% of SiC grains may have infrared (IR) spectral features that are influenced by crystal defects, porosity, and/or subgrains. Future subdiffraction- limited IR measurements of complex SiC grains might shed further light on the relative contributions of each of these features to the shape and position of the characteristic IR 11 μm SiC feature and thus improve the interpretation of IR spectra of AGB stars like those that produced the presolar SiC grains.
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Different polytypes of SiC are described and predicted in literature. Here, we report the first occurrence of an orthorhombic 6O-SiC polytype as rock-forming mineral in the nickel laterite mine of Tiebaghi (New Caledonia). This new class of SiC crystallizes in the space group Cmc21 with 12 atoms per unit cell [a = 3.0778(6) Å, b = 5.335(2) Å, c = 15.1219(6) Å, α = 90°, β = 90°, γ = 120°]. The density of 6O-SiC is about 3.22 g/cm3 and the calculated indirect bandgap at room temperature of 3.56 eV is identical to 6HSiC. Our results suggest that 6O-SiC is the intermediate state in the wurtzite to rocksalt transformation of 6H-SiC.
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Moissanites were found in tagamites of the Popigai meteorite crater along with impact diamonds. We have studied 55 samples including 49 individual polytypes and six intergrowths. The numbers of 6H, 15R, 4H, 6H/15R, and 6H/4H polytypes are 82, 7, 5, 4, and 2%, respectively. By the assemblage of polytypes, the moissanites of the Popigai astrobleme are distinct from kimberlite moissanites, as well as from synthetic SiC, which is characterized by the absence of the 4H polytype and the presence of more diverse inclusions (including Fe-bearing). The Popigai astrobleme is one of few objects with reliable natural moissanite. Technogenic contamination is excluded, since any researcher can find this mineral in tagamites.
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Moissanite (SiC) has been accepted as naturally occurring in some mantle and meteorite samples. The thermochemical data bases have allowed calculations of the stability of SiC, moissanite, at 1 bar in such reactions as: SiC + O-2 = C + SiO2 (MsCQ) and SiC + Mg2SiO4 + O-2 = C + 2MgSiO(3) (MsOCP) The fO(2) values of these reactions at igneous petrogenetic temperatures and 1 bar are as much as six to eight orders of magnitude too reduced for Earth upper mantle models that place the log fO(2) value between Delta(FMQ) = +2 and Delta(FMQ) = -3. Utilizing a ZrO2 membrane technique, in both piston-cylinder and in multi-anvil experiments, the fO(2) of the (MsOCP) buffer was compared with various metal-metal oxide buffers at 15 kbar and at 90 kbar at 1773 K. Results can be summarized as: [GRAPHICS] This order of redox potentials does not appear to change from 1 bar to 90 kbar. While we cannot measure the absolute values of these buffers at high pressure, we have demonstrated that high pressure does not stabilize SiC anywhere near the value of the (IW) buffer. The oxygen fugacity of the MsCOP assemblage is similar to 1-2 log units lower than the (Mn-MnO) buffer on the geotherm, at least to depths equivalent to 90 kbar at 1773 K. The thermodynamic calculations are thus closely verified by the results of the experiments. The implications for the state of the mantle are discussed.
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Silicon carbide is a material of significant interest for various fundamental and applied fields: from high-power electronics to crystallography and the oxidation state of Earth's mantle. Since the stability of SiC requires extremely low fO 2 values, some geologists consider moissanite as a ''geological aberration''. However, a growing body of data confirms the rather widespread existence of small SiC grains in various geological settings. Moissanite has been found not only in mantle derived rocks (e.g., kimberlites, diamonds), but also in high-and low-grade metamorphic rocks, limestones and pegmatites (for a review see Derkachenko et al., 1972, Lyakhovich, 1980, Marshintsev, 1990). SiC is also reported in meteorites and interstellar dust. Interestingly, isotopic analyses show that SiC is always enriched in 12 C (Marshintsev, 1990, Mathez et al., 1995). The genesis of natural SiC remains a matter of debate. In this work we report results of a detailed investigation by complementary techniques of natural SiC grains recovered from two contrasting geological settings: 1) from the heavy fraction of the Mir kimberlite pipe (Yakutia), and 2) from Triassic limestones in Bulgaria. Structural perfection, major and trace element chemistry of natural SiC grains and description of inclusions of other phases are presented. Samples and methods Issues of contamination were carefully addressed during extraction of kimberlitic SiC and the contamination is completely excluded in the case of Bulgarian samples. The rocks containing the latter set of SiC grains were collected at the geographical location and from strata described as moissanite-bearing by Gnoevaja and Grozdanov (1965). They were recovered from the heavy fraction of the acid-resistant residue of these rocks, containing well-preserved Triassic fossils. Note, that the SiC grains are genetically unrelated to the host limestones. The kimberlitic moissanite grains (n=106) are up to 1 mm across and the Bulgarian grains (n=22) are less than 100 microns. The grains are usually transparent and show various colors, but usually are bluish-green. Similar to earlier observations (Derkachenko et al., 1972, Marshintsev, 1990) most grains are fractured, but some sides preserve well-formed crystallographic faces. Raman microspectroscopy was employed to determine the polytypes, to assess degree of crystalline perfection, and to identify of some inclusions. Major and trace elements in SiC bulk and in inclusions exposed by polishing were measured using Electron Microprobe (EMP) and by LAM-ICP-MS. For comparative purposes we studied synthetic SiC samples produced by very different methods: the Acheson and the modified Lely (sublimation) methods. The SiC grains were cast in epoxy and polished prior to analyses. Transmission Electron Microscopy (TEM) was used for direct investigation of submicroscopic inclusions. Loose grains were mechanically crushed and suspension brought to the Cu grid.
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We report the first occurrence of moissanite (SiC) as a rock-forming mineral (8.4 vol%) in one unique specimen of a terrestrial rock. The sample has a homogeneous, porphyritic texture, and was found as a beach pebble thought to be derived from a Tertiary volcanic province of the Aegean Sea region. The matrix is bluish-colored and consists of very fine-grained brucite, calcite, and magnesite, in which macrocrysts of quartz (25.3 vol%) and moissanite are found. Other accessory phases are phlogopite-3T, magnesiochromite, an Fe-rich phase, Cl-bearing brucite, Al-rich orthopyroxene, and unidentified MgFe-silicates (4 vol%). The bulk-rock composition shows a "kimberlitic" chemistry (55.8 wt% SiO2, 28.5 wt% MgO, 1.4 wt% CaO, 18.1 wt% LOI). Colorless gemmy, and blue or black moissanite crystals are subhedral and display characteristic hexagonal symmetry (6H polytype). Most moissanite grains contain metallic Si and Fe-silicide (Fe3Si7) inclusions, and more rarely, other Fe-silicides with varying amounts of Al (≤24.5 wt%), Ca (≤8.0 wt%), Mn (≤6.8 wt%), Ti (≤16.3 wt% , and Ni (≤2.6 wt%). The δ13C value of the moissanite is -28.1‰. According to available data, the fo2 stability field of SiC is five to six log units below the iron-wüstite (IW) buffer curve. Therefore, the observed Fe-bearing silicates cannot have been equilibrated with SiC under ambient pressure. Instead, our finding indicates that the rock most likely formed at the ultrahigh-pressure conditions of the upper mantle or transition zone.
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The composition of the subcontinental lithospheric mantle (SCLM) is broadly related to the tectonothermal age of the overlying crust, suggesting a secular change in SCLM-forming processes. Most estimated compositions of Archean SCLM, based on well-studied suites of xenoliths and xenocrysts, are depleted garnet lherzolites with high orthopyroxene/olivine. However, these compositions make it difficult to account for the high shear-wave velocities measured in the cores of large cratons, and predict deeper geoid anomalies and higher elevations than are observed in most cratons. Global and regional seismic tomography indicates that most cratonic xenolith suites represent material from the lower-velocity margins of lithospheric blocks. This implies that previous compositional estimates are strongly biased toward metasomatized material. We suggest that most Archean SCLM originally consisted of highly depleted dunites/harzburgites, similar to the Archean orogenic massifs of western Norway. Incorporation of such rocks in the cold upper parts of the cratonic SCLM satisfies the seismic and gravity data, suggesting that large volumes of these rocks are preserved in the cores of cratons, but are poorly sampled by volcanic rocks. The roots of most Proterozoic shields probably consist of refertilized Archean SCLM; the juvenile SCLM beneath Proterozoic and Phanerozoic mobile belts reflects only moderate depletion of Primitive Mantle compositions. Rather than a gradual evolution in SCLM-forming processes, we suggest a sharp dichotomy between Archean and younger tectonic regimes. The differences in buoyancy and viscosity between these two types of SCLM have played a major role in the construction, preservation and recycling of continental crust. If originally Archean SCLM is more widespread than currently recognized, models of crustal growth rates and recycling may need to be revised.
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Polarization microscopy is a suitable tool for studying strain in appropriately cut SiC single crystals. The outline shape examined by electron microscopy and the induced interference pattern observed by polarization microscopy were both used to study the variation and character of macro-defects present in SiC wafers. While voids are usually in a relaxed state, hollow-core dislocations are characterized by large interference halos up to ∼100 µm in diameter. Conoscopy, i.e. evaluation of the interference pattern created by inserting an Amici Bertrand lens, is used to examine these optical phenomena in more detail and gain additional knowledge on the inclination of wafers cut towards the c -axis. The discrepancy between the simulated and observed interference patterns for (0001)-SiC strongly indicates that pipes are not pure screw dislocations, as commonly thought, but have an edge component.
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Laser ablation microprobe ICP-MS has been used to determine quantitatively the trace-element composition of diamond. Experiments with different synthetic multi-element carbon-based standards, various lasers and a range of instrument conditions have shown that a 266 nm UV laser at 10 Hz provided the best sensitivity, and synthetic oil and a doped cellulose proved most suitable as external standards; 13C was used as the internal standard. The precision and accuracy of the method, and the homogeneity of the cellulose multi-element standard, were tested by multiple analyses. Artefacts resulting from polyatomic interferences were quantified by analysis of a pure synthetic diamond. Concentrations of 41 elements were determined for two fibrous diamonds from Jwaneng in Botswana (JWA 110 and JWA 115), which have been analysed previously by instrumental neutron-activation analysis (INAA) and proton microprobe (PIXE). A comparison of these three analytical techniques shows that the use of the cellulose standard produces accurate and precise data for most elements. Typical detection limits for the rare earth elements are 5–20 ppb, and for transition elements <500 ppb. Sodium and Fe have higher detection limits (2–3 ppm). The precision (expressed as % rsd) ranges through 10% for concentrations between 1–100 ppm, 15% for values between 0.1–1 ppm, 30% for 0.01–0.1 ppm and 25% for values <0.01 ppm, with the accuracy lying in the same range. The trace-element patterns obtained by this technique may be used for the characterisation of diamond in genetic studies. Further analyses are required to test whether reliable identification of the source locality of the diamonds is possible; if so this may have important forensic applications.
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Research on natural intra- and extraterrestrially produced electromagnetic waves with periods ranging from 0.2 to 600 s is reviewed. The way in which the energy of rock movements transforms into the energy of an alternating magnetic field is analyzed. Methods for detecting seismomagnetic signals against a strong background are described. In discussing the physics of ultra-low-frequency waves in the magnetosphere, the 11-year activity modulation of 1-Hz waves and ponderomotive forces affecting plasma distribution are emphasized.
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The electrochemical behavior of K 2 SiF 6 in chloro-fluoride melts and that of SiO 2 in carbonate melts has been studied. Silicon, titanium silicides, boron silicide and ternary compounds Ti-Si-B have been deposited from chloro-fluoride melts. Only SiC was deposited from carbonate-silica melts under carbon dioxide atmosphere (that is, excessive pressure of CO 2).
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A study is performed on the phase composition and morphology of surface films on an aluminum alloy SI100 (10.5% Si - 0.2% Fe) in a molten eutectic mixture of lithium, sodium and potassium carbonates in the CO2+0.5O2 atmosphere at 773 and 823 K during anodic treatment and with no current. It is demonstrated that oxide films formed on silumine with no current or with slight cathode polarization show better protective properties as compared to those formed under anodic polarization. In the latter case silicate phase is formed which is corrosion-unresistant.
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