Bernard W. Evans's research while affiliated with University of Washington Seattle and other places

Publications (35)

Article
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Low-temperature hydrous alteration of FeTi oxide-rich ferroan peridotite, Laramie anorthosite complex, Wyoming, produced silician ferrihydrite, cronstedtite, greenalite, hisingerite, and talc. Ferrihydrite occurs as nanocrystals in ~50 nm diameter granules that form monomineralic masses up to 300 μm across. It is inferred to have formed by the repl...
Article
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We present microanalyses of secondary phyllosilicates in altered ferroan metaperidotite, containing approximately equal amounts of end-members serpentine ((Mg,Fe2+)3Si2O5(OH)4) and hisingerite (□Fe3+2Si2O5(OH)4·nH2O). These analyses suggest that all intermediate compositions can exist stably, a proposal that was heretofore impossible because phyllo...
Article
Serpentine formed from Fe-rich igneous forsterite (Fa38-43) in two samples of peridotite from the Layered Series of the Duluth igneous complex, Minnesota, is enriched to at least 30% of the Fe endmember. In addition to lizardite, chrysotile, and clinochlore, electron microprobe and transmission electron microscope study reveals the presence of meta...
Article
Despite claims to the contrary, the compositions of magnetite and ilmenite in the Bishop Tuff correctly record the changing conditions of T and fO2 in the magma reservoir. In relatively reduced (ΔNNO < 1) siliceous magmas (e.g., Bishop Tuff, Taupo units), Ti behaves compatibly (DTi ≈ 2–3.5), leading to a decrease in TiO2 activity in the melt with c...
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We describe a sample of Proterozoic banded iron-formation from the inner contact aureole of the Duluth complex near Babbitt, Minnesota, that contains a compositionally unique Fe-rich monoclinic ferromagnesian amphibole. Its content of TAl (1.3 atoms per 23 anhydrous oxygens) and ANa (0.55) places it in a compositional field (with the label clinofer...
Article
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Outcrops of spinifex-textured metaperidotite hi the Cerro del Almirez ultramafic massif, S. Spain, are located a few meters upgrade from a unique eclogite-facies (1.6-1.9 GPa, approximate to 680 degrees C), antigorite-out (orthopyroxene-in) isograd. Olivine in the spinifex rock is pleochroic brown hi thin section, and microprobe analysis (carefully...
Article
Microprobe analyses of antigorite show that (Al+Cr) and inferred Fe3+ correlate inversely with Si apfu in a Tschermaks substitution. This observation suggests that the uptake of Fe3+ is not simply related to fO2. For Si = 1.95 apfu estimated Fe3+ = 0.032 apfu (or 0.95 wt% Fe2O3). Such estimates of Fe3+ require high analytical accuracy and precision...
Article
In two samples of antigorite serpentinite from Sasaguri, Kyushu, Japan, we have found trace amounts of a NiFe-rich silicate whose atomic proportions in terms of the ratio of M to T cations match those of the fibrous MgFe-silicate hydrate balangeroite. Balangeroite in serpentinite at the type locality in Italy is not reported to contain Ni, whereas...
Article
The serpentinization of peridotite operates according to one or the other, or a combination, of two end-member mechanisms. In low-temperature environments (50-300 degrees C), where lizardite is the predominant serpentine mineral, olivine is consumed by reaction with H2O but its composition (Mg#) remains unchanged. Mg-rich lizardite, magnetite, and...
Article
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Nearly 30 years have elapsed since Kretz (1983) provided the mineralogical community with a systematized list of abbre-viations for rock-forming minerals and mineral components. Its logic and simplicity have led to broad acceptance among authors and editors who were eager to adopt a widely recognized set of mineral symbols to save space in text, ta...
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A model for the thermodynamic properties of rhombohedral oxide solid solutions in the system Fe 2 O 3 -FeTiO 3 -MgTiO 3 -MnTiO 3 (containing minor amounts of Al 2 O 3) is presented. The model accounts for temperature and compositionally dependent long-range cation-order and the related high to low symmetry structural phase transition. The model is...
Article
It is argued that the high-Mg content (mg-number = 95 ± 3) of the serpentine minerals in serpentinized peridotite is a consequence of the environmental Fe2+Mg −1 exchange potential imposed on the system by the abundance of olivine and orthopyroxene. Mass balance in the serpentinization reaction then requires the precipitation of an iron-rich minera...
Article
Reliable characterization of the properties of amphibole end-members is a prerequisite for many important things in mineralogy and petrology, especially for the calculation of phase equilibria that involve amphiboles. Laboratory measurements of the thermochemical and thermophysical properties of amphibole end-members, as well as the experimental re...
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A synthetic, low-melting rhyolite composition containing TiO2 and iron oxide, with further separate additions of MgO, MnO, and MgO+MnO, was used in hydrothermal experiments to crystallize Ilm-Hem and Usp-Mt solid solutions at 800 and 900°C under redox conditions slightly below nickel–nickel oxide (NNO) to » 3log10 f\textO2\approx 3\,\log_{10} f_{{...
Article
A thermodynamic model has been developed and calibrated for monoclinic and orthorhombic amphiboles compositionally contained within the Ca-Mg-Fe2+ amphibole quadrilateral. The model incorporates the energetic consequences of cation ordering of Fe2+ and Mg over nonequivalent sites in the crystal structure, and accounts for the temperature, pressure,...
Article
A transmission electron microscopy study reveals the microstructure of polysynthetically twinned, near end-member grunerite from Rockport, Massachusetts. A small percentage of Pnma ferroanthophyllite of similar composition is present as thin slabs intergrown parallel to (100) of grunerite and as individual μ-size crystals. HRTEM imaging shows that...
Article
Ten new single-crystal X-ray structure refinements of unheated and heat-treated anthophyllite, new measurements of the optical indicatrix of anthophyllite, and previously published data from Mössbauer spectroscopy of heated anthophyllite, show that temperature-dependent long-range order of Fe2+ and Mg on the M-sites of cummingtonite-grunerite and a...
Article
Microprobe analysis of tremolite from 'St. Gotthard,' Switzerland, shows that it is close to end-member in composition (Ca = 1.97 apfu), and not hypercalcic (Ca = 2.16 apfu), as a previous wet-chemical analysis suggested. The latter formula, and a corresponding thermodynamic activity (0.67), were used in the experimental database for the extraction...
Article
The superimposition of Barrovian-type metamorphism on low-pressure contact aureoles in magmatic arcs is subject to the kinetic constraints of temperature and influx of H2O and may be cryptically developed or not at all. The spatial distribution of detectable overprint will reflect these kinetic factors rather than the geometry of the imposed load....
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Crystallization experiments were carried out on a representative sample of the crystal-rich dacite ejected during the 15 June 1991 eruption of Mt Pinatubo, to define the pre-eruption conditions of this major volcanic event. Experiments were performed in the temperature and pressure range of 750–900°C and 220–390 MPa, respectively. Redox conditions...
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Explosive eruptions involve mainly silicic magmas in which sulfur solubility and diffusivity are low. This inhibits sulfur exsolution during magma uprise as compared to more mafic magmas such as basalts. Silicic magmas can nevertheless liberate large quantities of sulfur as shown by the monitoring of SO2 in recent explosive silicic eruptions in arc...
Article
The crystal structures and M-site populations of a series of unheated and heat-treated, near-binary members of the actinolite series were determined from single-crystal X-ray diffraction data. Small, systematic differences in crystal structure (mean and individual bond lengths, octahedral distortions, and Mg-Fe site preferences) between the actinol...
Article
At the Overlook gold mine in northeastern Washington, a fayalite-bearing assemblage of iron silicates, magnetite, and pyrrhotite, plus quartz and calcite, formed during alteration of a Permian volcanogenic, carbonate-rich, massive magnetite-sulfide seafloor deposit. The temperature of fluid inclusion homogenization, oxygen isotope ratios, and phase...
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The oxidation state of the 1991 Pinatubo dacite and many other oxidized silicic arc volcanic rocks has been overestimated. We reproduce the compositions of magnetite and ferrian ilmenite found in Pinatubo dacite by doing crystallization experiments at 780 degrees C, 2.2 kbar, and NNO + 1.7 (+ or -0.2) log units. The extra components in igneous ilme...
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Garnet-bearing ultramafic rocks (GBU) enclosed in high-grade gneisses are known from several parts of the Bohemian Massif. One of these is the high-pressure(HP)-unit 1 in the Erzgebirge Crystalline Complex, which is the subject of the present study. Hitherto, two different models have been put forward to explain the stabilisation of garnet in mantl...
Article
A near-end-member natural tremolite, Na0.01Ca1.97Mg4.98Fe0.03Al 0.01Si8.00O22(OH)2 was studied by single-crystal X-ray diffraction at 140 and 295 K to seek a possible crystal-chemical explanation for the typically low Ca/Σ M ratios, relative to the ideal ratio of 2/5, observed in both natural and synthetic tremolite samples. Difference-Fourier maps...
Article
The field of ultra-high pressure metamorphism (UHPM) emerged only 12 years ago when microscopic relics of coesite were found in crustal rocks that had been metamorphosed during subduction related to continental collision. Coesite-bearing metamorphic rocks were soon found elsewhere, and then metamorphic microdiamonds were discovered. Suddenly, the p...
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Natural cummingtonite encompasses virtually the entire compositional range from Mg to Fe end-member. It occurs in several metamorphic rock types, but predominantly in amphibolites and metaironstones. Most examples were produced under amphibolite-facies conditions. Cummingtonite is also a critical indicator of T, aH2O, and fO2 in silicic volcanic ro...
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A thermodynamic solution model for the ferromagnesian amphiboles is developed. The model accounts explicitly for intersite nonconvergent cation ordering of FeH and Mg between octahedral Ml, M2, M3, and M4 sites and intrasite interaction energies arising from size-mismatch of unlike cations. The model is formulated with 15 parameters: two standard-s...
Article
An amphibole-bearing gneiss from the Seward Peninsula, Alaska, underwent local dehydration to two-pyroxene gneiss. Dehydration was driven isothermally and isobarically, close to the metamorphic maximum, by a small amount of CO2-rich fluid evolved from an underlying impure marble layer. Stable isotope evidence indicates that the CO2 diffused 85 cm i...
Article
Stable-isotope profiles show that flat-lying marble units acted as impermeable barriers to upward fluid flow in transitional amphibolite-granulite grade rocks of the Kigluaik Mountains, Seward Peninsula, Alaska. The degree of permeability is related to the composition of the marble. The margin of a thick pure dolomite marble chemically reacted with...
Article
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Epidote phenocrysts (Ps19-24) in Laramide-age, porphyritic dacite dikes in the Front Range of Colorado prove that epidote can crystallize directly from a magma. The dikes are high-K, calc-alkaline dacites and rhyodacites (SiO2 = 64%-70%). Petrographic and microprobe data distinguish three types of magmatic epidote phenocrysts, with different petrog...
Article
Phase diagrams have been computed for minerals in the system NCMASH, using the optimised thermodynamic data-base of Berman (1988) augmented by data on glaucophane and pumpellyite, to illustrate the relationships between rocks containing epidote and sodic amphibole and those containing assemblages characteristic of neighbouring metamorphic facies. T...

Citations

... Experimental results suggested the critical temperature for a solid-state transformation was increased with an elevated mole ratio of Si/Fe in ferrihydrite (Campbell et al., 2002). When the Si/Fe mole ratio ranged from 0.07 to 0.24, as revealed mostly in natural settings (Evans et al., 2021), the transformed temperature ranged from 550 • C to 700 • C (Campbell et al., 2002). This temperature range is much higher than the formation temperature of hematite in this study (∼250 • C; Fig. 5a). ...
... Mineral analysis showed that serpentine can incorporate both ferrous and ferric iron into its crystal structure (Andreani et al., 2013;Mayhew & Ellison, 2020;O'Hanley & Dyar, 1993). Thus, more recent studies often incorporate mixed-valence serpentine (i.e., Fe-and/or Mg-cronstedtite) into thermodynamic models Klein et al., 2013;Lazar, 2020;Leong & Shock, 2020;Tutolo et al., 2019Tutolo et al., , 2020Zolotov, 2014). Oxidation of ferrous iron hosted in MgFe 2+ -brucite and MgFe 2+ -serpentine into ferric iron in cronstedtite (±hisingirite) is dependent on the SiO 2 activity constrained by these mineral assemblages. ...
... Microprobe totals are comprised between 82.97 and 89.49 wt%, falling in some cases below ideal for serpentine as also previously reported by Evans et al., [50] in serpentine from peridotite of the Duluth complex, USA. ...
... We propose that these high CO 2 concentrations are caused by the interaction of the Bandelier magma with CO 2 -rich vapours, which exsolved from fresh, volatile-saturated magma that was injected into the magma chamber shortly before the Bandelier eruptions and during the early VTM, after which the CO 2 -rich vapours percolated upwards through the crystal mush. This process, often described as CO 2 fluxing or flushing, has been recognised and modelled in arc magmas (Wallace 2005;Spilliaert et al. 2006;Collins et al. 2009;Blundy et al. 2010;Caricchi et al. 2018) and has also been suggested in large silicic magma bodies, such as the Bishop Tuff (Evans et al. 2016), even though these are usually assumed to follow simple open or closed degassing trends (Blundy et al. 2010). ...
... But, for simplicity and illustrative purposes, it is instructive to assume no ordering of Fe 31 or Al 31 between T2a and T2b sites (s = t = 0), particularly as Akasaka and Ohashi (1985) and Hamada and Akasaka (2013) do not report evidence of splitting of T2 sites into T2a and T2b subsites in their Mössbauer spectra. We may also assume that a Taylor expansion of second degree in composition variables and the non-convergent ordering variable r is sufficient to describe the vibrational Gibbs energy, as is the case for spinels, fahlores, and quadrilateral amphiboles (for example, Sack and Ghiorso, 1991;Ghiorso and Evans, 2002;Sack, 2014Sack, , 2017. ...
... Spectrum d f has been recorded on a fiber bundle extracted from sample d. The bands, for instance, the Si-O stretching and bending modes at~960 (1015 and 1080) and 620 cm −1 (Figure 3), are characteristic of asbestos and similar compounds (amphiboles and pyroxenes) [14][15][16][17][18][19][20][21]. From this first analysis, it can be seen that the samples b and d on the one hand and the samples a and c on the other hand are rather similar, but sample d appears to be more heterogeneous. ...
... The compositions of coexisting cummingtonite-grunerite and orthoamphibole (Al-anthophyllite and gedrite) clearly demonstrate the much greater capacity of orthorhombic amphiboles to take up Al (Robinson et al. 1982). Monoclinic MgFe-amphiboles encompass almost the entire compositional range from 90% Mg-endmember to 100% Fe-endmember, whereas, with rare exceptions (Ferré 1989, Bozhilov & Evans 2001, anthophyllite seems to be limited generally to no more than 50% Fe-endmember. On the other hand, several occurrences of ferrogedrite have been reported (Deer et al. 1997). ...
... Thermal approximations derived from chlorite-based geothermometers 38,39 provide T estimates of 202 ± 10 °C 38 and 206 ± 10 °C 39 (Supplementary Table S9). This temperature is in agreement with the occurrence of lizardite as the only serpentine phase, being lizardite more stable than chrysotile and antigorite between 200 and 320 °C 40 , having as upper limit the lizardite-antigorite boundary 5,40,41 . The association tremolite-chloritelizardite is stable in the zeolite/pumpellyite facies 42 thus suggesting that the chlorite thermometric estimates may represent the minimum crystallization temperatures for tremolite. ...
... This is the first published structure refinement of fibrous anthophyllite, as literature reports only structure models of non-fibrous orthorhombic anthophyllites (Warren and Modell, 1930;Lindemann, 1964;Finger, 1970;Walitzi et al., 1989;Evans et al., 2001;Schindler et al., 2008;Hawthorne et al., 2008). The results of our structure refinement ( Table 2, 5, and 6) will be compared to those of the fully characterized samples by Walitzi et al. (1989), Schindler et al. (2008), and Hawthorne et al. (2008). ...
... (Ghiorso & Evans, 2002). (b) Section of P-T phase diagram of stability field for the assemblage orthopyroxene + cummingtonite + quartz (Evans & Ghiorso, 1995). The thin-lines are isopleths of X Fe in cummingtonite. ...