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Rock-forming moissanite (Natural α-silicon carbide)

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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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American Mineralogist, Volume 88, pages 1817–1821, 2003
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INTRODUCTION
The terrestrial origin of silicon carbide (SiC = moissanite)
samples has been the subject of controversial debates during
the last century. Since the discovery of SiC crystals as inclu-
sions in natural diamonds in kimberlites (Moore et al. 1986;
Moore and Gurney 1989; Otter and Gurney 1989; Leung 1990)
and in lamproites (Jaques et al. 1989), however, the natural
occurrence of moissanite in terrestrial rocks has been widely
accepted.
The first discovery of naturally formed SiC dates back to
1904, when Moissan reported its occurrence from the Canyon
Diablo Fe meteorite. However, Moissan’s finding was thought
to be an artifact (Mason 1967) from SiC-bearing cutting tools
used to prepare the meteorite samples. Since then, reports of
new occurrences of natural SiC (e.g., Regis and Sand 1958;
Bobrievich et al. 1957; Bauer et al. 1963; Marshintsev et al.
1967; Kaminskiy et al. 1969; Moskvitin et al. 1978; He 1984;
Jaques et al. 1986; Marshintsev 1990; Filippidis 1993) have
been debated vigorously and many geologists considered a
natural terrestrial origin as highly improbable (e.g., Milton and
Vitaliano 1984; Woermann and Rosenhauer 1985, p. 316). The
* Present address: UMR CNRS 5570, Laboratoire de Sciences
de la Terre, ENS Lyon, France. E-mail: simonpietro.di.pierro@
ens-lyon.fr
LETTERS
Rock-forming moissanite (natural
aa
aa
a-silicon carbide)
SIMONPIETRO DI PIERRO,
1,
* EDWIN GNOS,
2
BERNARD H. GROBETY,
1
THOMAS ARMBRUSTER,
3
STEFANO M. BERNASCONI,
4
AND PETER ULMER
4
1
Department of Geosciences, Mineralogy and Petrography, University of Fribourg, Switzerland
2
Institute for Geology, Baltzerstrasse 1-3, University of Bern, Switzerland
3
Laboratory of Chemical and Mineralogical Crystallography, University of Bern, Switzerland.
4
Department of Earth Sciences, ETH Zürich, Switzerland
ABSTRACT
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 magnes-
ite, 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” chem-
istry (55.8 wt% SiO
2
, 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 (Fe
3
Si
7
) 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 d
13
C value of the moissanite is –28.1‰. According to available data,
the f
O
2
stability field of SiC is five to six log units below the iron-wüstite (IW) buffer curve. There-
fore, the observed Fe-bearing silicates cannot have been equilibrated with SiC under ambient pres-
sure. Instead, our finding indicates that the rock most likely formed at the ultrahigh-pressure conditions
of the upper mantle or transition zone.
issue of a possible contamination with synthetic SiC from abra-
sives could have been, instead, ruled out in more recent reports
of SiC found in meteorites, as measured isotopic anomalies in
many trapped elements could definitely be identified as presolar
in origin (e.g., Bernatowicz et al. 1987; Tang et al. 1989; Lewis
et al. 1990; Daulton et al. 2002).
Typically, moissanite has been found as a constituent of
kimberlitic pipes or in associated volcanic rocks. Recently, well-
ordered a-SiC and b-SiC crystals were found as inclusions
within diamonds (Leung 1990) and dispersed in the matrix of
kimberlites from Fuxian, China (Leung et al. 1990). Leung et
al. (1996) discussed the heteroepitaxial intergrowth between
b-SiC and diamond along the (110) plane as a possible mecha-
nism for the genesis of natural diamonds. Mathez et al. (1995)
investigated the chemical composition of metallic Si and Fe-
silicide inclusions in moissanite grains extracted from
kimberlites from Yakutia, Russia. They also presented C-iso-
tope data of moissanite grains, which were clearly distinguish-
able from interstellar SiC (e.g., Stone et al. 1991; Zinner et al.
1989). According to Mathez et al. (1995, p. 781–782), chemi-
cal and mineralogical data obtained from the SiC, the inclu-
sions, and the associated minerals “…leaves little doubt that
SiC occurs naturally and is present in the Earth’s mantle…”
and “…establish that SiC…is a widespread, albeit rare, phase
in diamond-bearing rocks.”
Moissanite, besides diamond, is a potential “window” into
DI PIERRO ET AL.: ROCK-FORMING MOISSANITE1818
the redox conditions of the Earth’s mantle. The current study
characterizes one unique specimen of a newly discovered rock
pebble containing 8.4 vol% SiC, thought to be associated with
calc-alkaline volcanic rocks. Moissanite is, to our knowledge,
reported for the first time in rock-forming quantities. In con-
trast to previous studies, where moissanite grains were obtained
from heavy mineral concentrates or as inclusions in diamonds,
the present occurrence allows the study of textural relation-
ships between SiC and the other phases using conventional
polished thin sections.
SAMPLE AND METHODS
One unique specimen was found at a beach along the Turkish coast of the
Mediterranean Sea, around 150 km NW from Izmir, and is most likely derived
from Tertiary volcanic rocks outcropping in the area. The source outcrop, how-
ever, has not yet been located. The sample displays an unusual bluish color (Fig.
1a). It is macroscopically homogeneous and texturally isotropic. It was collected by
Mr. Salvatore Musacchia and given to the first author as a “curiosity.”
The sample has been analyzed by optical microscopy, powder X-ray dif-
fraction (XRD, Philips PW 1800, CuKa radiation, 2–65 2q), X-ray fluores-
cence (XRF, Philips PW 2400 spectrometer), scanning electron microscopy with
energy dispersive system (FEI SEM-EDS XL30 Sirion, operating conditions 20
kV), and a Multiphase Carbon Determinator (Leco RC 412). Single-crystal data
were obtained on an ENRAF NONIUS CAD4 X-ray diffractometer, using a
graphite monochromator and MoKa X-radiation at room temperature (293 K).
Electron-microprobe analyses of silicates and oxides were obtained with a wave-
length-dispersive system (Cameca SX-50: operated at 15 kV accelerating po-
tential with a 20 nA beam current, counting times were 20 s on peak and
background for major elements and up to 40 s for trace elements); a synthetic
SiC standard was used for moissanite, and Si and Fe metal standards were used
for metallic inclusions (10 nA beam current). Physical conditions of
cathodoluminescence analyses were 25 kV and 90 mA. The C-isotope ratio of
the SiC was determined by flash combustion using a Carlo Erba CNS elemental
analyzer coupled in continuous flow to a Micromass Optima mass spectrom-
eter. Carbon- and O-isotope ratios of the carbonate were determined by reaction
at 90 C with 100% phosphoric acid on an automated carbonate device con-
nected to a VG-PRISM mass spectrometer. Mineral modes were determined by
point counting (2400 points). Moissanite crystals were separated from the sample
by etching away the matrix in a 5% HCl solution. XRD powder, XRF, SEM and
Leco analyses were performed at the University of Fribourg. XRD powder, XRF,
SEM and Leco analyses were performed at the University of Fribourg. XRD
powder, XRF, SEM, and Leco analyses were performed at the University of
Fribourg. XRD single crystal, EMPA, and CL analyses were performed at the
University of Bern. Isotopic analyses were performed at the ETH-Zürich.
RESULTS
The bulk-chemical composition obtained by XRF (Table 1)
shows that SiO
2
, MgO, and CaO are the only oxides present in
significant quantities, all the other analyzed major and trace
elements show very low concentrations. The volatiles (H
2
O and
CO
2
) are also present in significant quantities. The principal
phases are moissanite (8.4 vol%) and undeformed,
xenomorphic, mm-sized and inclusion-free quartz grains (25.3
vol%), dispersed in a brucite-dominated matrix (58.2 vol%)
also containing calcite, magnesite, phlogopite-3T, Cl-bearing
brucite, magnesiochromite, Al-rich orthopyroxene, two uniden-
tified MgFe-silicates, and an unidentified Fe-rich phase. Quartz
and moissanite are never observed in direct contact with each
other. The MgFe-silicates are reddish and yellowish in color,
and have compositions intermediate between olivine and
orthopyroxene. Spherical structures or “globules,” ranging in
diameter from a few hundred micrometers to a millimeter, are
dispersed throughout the groundmass. These globules have the
same mineral assemblage as the bulk but a smaller grain size,
FIGURE 1. (a) Photo of the SiC-rich specimen. Black spots are
moissanite crystals. Scale bar is 2 cm. (b) Gemmy, platy crystal of
moissanite shown with largest (001) face. The arrow indicates a drop-
shaped, black metallic inclusion. Scale bar is 300 mm. (c) Back-
scattered electron image of metallic inclusion in moissanite. The gray
shading in the BSE image consists mainly of pure metal Si (Table 2).
The brighter areas in the middle and along the rims of the inclusion
are composed of Fe-silicide. The very bright irregular white spots are
likely inclusions of a third phase with higher average atomic number.
Scale bar is 200 mm.
and account for 6.2 vol% of the bulk sample. Crystals of light-
blue Cl-bearing brucite (1.9 vol%), a few micrometers in size,
are homogeneously scattered throughout the matrix and are re-
sponsible for the bluish color of the pebble. All unidentified
DI PIERRO ET AL.: ROCK-FORMING MOISSANITE 1819
phases are microcrystalline and
subject of on-going studies.
Separated moissanite
crystals range between 0.2
and 1.5 mm in size, are blue
or black in color, and have a
metallic luster. Some com-
pletely transparent gemmy
crystals with brilliant sub-
adamantine luster are present
as well (Fig. 1b). In general,
the crystals show well-devel-
oped crystallographic faces,
but some are corroded or frac-
tured. They have a character-
istic platy or elongated,
pinacoidal, hexagonal shape
bounded by {100} faces (a-
SiC). The unit-cell dimensions obtained with the single-crys-
tal X-ray diffraction analysis on three grains are a = 3.080(1)
Å and c = 15.12(1) Å (6H polytype). In a standard thin section
(5 ¥ 2 cm), at least 341 crystals of moissanite have been counted.
Under transmitted light, moissanite ranges from colorless to
light blue, or dark blue to almost black. Greenish-yellow or
pinkish crystals also have been observed. Some moissanite
grains are pleochroic, others are probably twinned. The optical
character is always uniaxial positive. Moissanite grains are
commonly associated closely with the red MgFe-silicates,
which are commonly in contact with SiC crystal faces. A simi-
lar mineral paragenesis has been described by Mathez et al.
(1995) for natural SiC crystals from Yakutia. The moissanite
crystals are very homogeneous chemically, with none of the
analyzed elements other than Si present above the detection
limit (Table 2). A few moissanite grains show yellow, blue,
and red cathodoluminescence. Yellow luminescence is confined
to grain boundaries. Carbon-isotope data were obtained on
moissanite and on carbonates. The moissanite has a d
13
C value
of –28.1‰ relative to the Pee Dee belemnite standard (PDB),
and the bulk carbonate yielded a d
13
C value of –11.9‰ and a
d
18
O of –3.6‰. Carbon isotope analyses have been performed
also on a synthetic SiC sample and a value of –27.1‰ was
obtained. The C-isotope signature, therefore, is not well suited
to distinguish natural terrestrial SiC from synthetic SiC.
One third of the examined moissanite crystals contain me-
tallic inclusions with rounded shapes. The average size of the
inclusions is between 50 and 100 mm, with a maximum of 400
mm. Most of the inclusions occur inside the crystals. A few
metallic grains have been found along moissanite crystal bound-
aries or dispersed in the brucitic matrix. Crystals with more
than one inclusion are rare (max 10 inclusions). Pure Si is the
most common inclusion. Fe-silicides are present along the Si
metal-SiC boundaries (Fig. 1c), or form exsolution domains in
metallic Si. Fe
3
Si
7
is the most common Fe-silicide. This phase
is stable at ambient pressure above 937 C and decomposes to
Si and FeSi
2
at lower temperatures (Kubaschewski 1982). Man-
ganese and Ni substitute for Fe in various proportions in Fe
3
Si
7
(Table 2). Other silicide-containing phases have variable stoichi-
ometries and were classified as Si-Fe-Al-Ca and Si-Fe-Ti (possi-
bly alloys), and as Si
2
(Fe,Al,Ca)
3
and Si
3
(Fe,Al)
2
(probably sto-
ichiometric compounds) phases. Another metallic phase consists
of Si
2
Ca (tentatively stoichiometric compound) (Table 2).
ARTIFICIAL VS. NATURAL
An artificial origin of the beach pebble may be suspected
considering the large quantity of SiC crystals and the presence
of quartz in an ultramafic matrix. Moreover, the outcrop from
which the specimen may have originated has not been located
yet. There are, however, a number of indications that the mate-
rial is natural. The beach where the pebble was collected is in
an unpopulated region, around 40 km away from the closest
village and 150 km away from the closest industrial town. A
thorough investigation of the patent literature concerning SiC,
as well as inquiries with different synthetic SiC producers (e.g.,
Timcal AG. Ticino, Switzerland) using the Acheson method
(e.g., Knippenberg 1963) and their industrial customers (e.g.,
Smyris s.r.l. Milano, Italy), gave no indications that the pebble
as a whole or the silicon carbide per se could have been a syn-
thetic product.
In contrast, all analytical results are typical for natural
moissanite.
(1) In synthetic SiC processed either with or without addi-
tives, C constitutes the major impurity phase and has a gra-
phitic character (Backhaus-Ricoult et al. 1993, p. 2204). Other
reported inclusions, all always nanometer-sized, are B
4
C and
B
25
C, metallic Fe and Si, and FeSi and Ti
5
Si
3
precipitates (e.g.,
More et al. 1986; Backhaus-Ricoult et al. 1993; Munro 1997).
In the present sample, neither graphite nor any other form of
C-bearing inclusions have been found.
(2) The metallic inclusions are characteristic, although not
unequivocal (e.g., Lyakhovich 1980, p. 965), indicators of the
natural origin of the moissanite crystals. Many silicide and al-
loy compositions have been observed previously in natural SiC:
metallic Fe by Bauer et al. (1963); native Si and Fe by Leung
(1990) and Leung et al. (1990); native Si and ferrosilicide by
Marshintsev (1990); micrometer-sized metallic Si, ferrosilicide
(Fe
3
Si
7
), and light rare earth element (LREE) rich Fe-Ti-Zr si-
licides (up to 16 wt% Ce and up to 4 wt% Th) by Mathez et al.
(1995). Except for the LREE-rich metallic phases, all the above
TABLE 1. Bulk rock chemistry
XRF 1s d.l. XRF 1s d.l. Leco
SiO
2
55.84 0.25 0.01 Ba b.d. 20 12 CO
2
* 1.96
TiO
2
0.02 0.01 0.01 Cr 31 5 5 H
2
O
+
16.19
Al
2
O
3
0.41 0.08 0.01 Cu b.d. 4 4 H
2
O
3.61
FeO 0.09 0.05 0.01 Nb 4 2 2
MnO 0.01 0.01 Ni 35 3 3
MgO 28.49 0.02 0.01 Pb b.d. 4 7
CaO 1.43 0.05 0.01 Rb b.d. 3 3
Na
2
O 0.02 0.02 0.01 Sr 46 4 3
K
2
O 0.03 0.02 0.01 V b.d. 6 5
P
2
O
5
0.01 0.01 0.01 Y 51 3 3
LOI 18.13 Zn b.d. 4 3
Zr 13
Total 104.48
Note:
The SiO
2
value includes metallic Si and SiC, which brings the total above 100 wt%. *At
T
max
= 1000 C of
the Leco instrument SiC does not break down. The listed CO
2
content is therefore considered as CO
2
from the
carbonates only. Based on the modal mineralogy, the total CO
2
should be 8–9 wt% (SiC + carbonates), and
approximately 1 wt% Cl should be present too. 1s = relative error; d.l. = detection limit; b.d. = below detection.
Major and minor elements in wt%; trace elements in ppm; CO
2
and H
2
O also in wt%.
DI PIERRO ET AL.: ROCK-FORMING MOISSANITE1820
TABLE 4. EDS semiquantitative analyses
opx 1 opx 2 mg-cr
wt% wt% wt%
SiO
2
47.42 48.87
Al
2
O
3
10.54 7.13 26.35
Cr
2
O
3
1.31 42.57
FeO 13.38 18.96 19.66
NiO 1.86 1.56
MgO 25.49 23.48 11.43
Total 100.00 100.00 100.01
Si 1.717 1.805
Al 0.450 0.310 0.951
Cr 0.038 1.031
Fe 0.405 0.586 0.504
Ni 0.054 0.046
Mg 1.376 1.293 0.522
S cations 4.039 4.040 3.008
Note:
opx = orthopyroxene; mg-cr = magnesiochromite
TABLE 3. Carbon isotopic data for SiC and associated minerals.
Mineral d
13
C Description Source
Moissanite –28.1‰ Bluish pebble this study
Carbonate –11.9‰ Bluish pebble this study
Moissanite –24‰ Fuxian kimberlite Leung et al. 1990
Diamond –2.9‰ to –4.8‰ Fuxian kimberlite Leung et al. 1990
Moissanite –22‰ to –29‰ Yakutia kimberlite Mathez et al. 1995
Diamond –31‰ Yakutia kimberlite Mathez et al. 1995
SiC 6 to 160‰ Interstellar SiC Zinner et al. 1989
SiC 150 to 5200‰ Interstellar SiC Stone et al. 1991
SiC –27.1‰ Synthetic this study
Note:
Uncertainty of the measurements for this study: 0.1‰.
compounds, always at the micrometer scale, have been found
in our sample (Table 2).
(3) Two unidentified MgFe-silicates, with FeO contents
ranging between 3 and 8 wt%, are found throughout the matrix
and in contact with SiC. According to Mathez et al. (1995), Fe-
bearing silicate phases cannot be in equilibrium with SiC un-
der conditions characteristic of the industrial process. At
ambient pressure, the f
O
2
stability field of moissanite is five to
six log units below the IW buffer. Under such conditions, the
Fe in Fe-bearing silicates is reduced to the metallic form and
expelled from the structure. Therefore, in the system Fe-Mg-
Si-O-C, the coexistence of SiC and Fe-bearing silicates is prob-
ably only possible at the ultra-high pressures conditions of the
upper mantle or greater depths.
(4) The moissanite d
13
C value of –28.1‰ is in agreement
with ranges published by Marshintsev (1990), e.g., lower than
–25‰, and Mathez et al. (1995), e.g., –22 to –29‰ for Rus-
sian moissanite, as well as those of Leung et al. (1990), e.g.,
–24‰, for Chinese natural SiC. In natural diamond genesis, a
strongly depleted
13
C isotope signature has been interpreted as
a primary mantle feature (e.g., Deines et al. 1993). Compared
to diamonds, the values for SiC exhibit a much narrower range
(Table 3). The d
13
C values of meteoritic interstellar SiC grains
range from 150 to 5200‰, and thus are far from terrestrial val-
ues (Stone et al. 1991, see Table 3).
(5) The presence of phlogopite (e.g., Dawson and Smith
1975) and brucite (e.g., Berg 1989), the high volatile content
and the presence of globules (e.g., Pell 1997), all point to a
“kimberlitic environment.” The high Mg content of chromite
(>11 wt% MgO, Table 4) is typical for primary mantle-derived
spinel (e.g., Shulze 2001). The high Al content of the
TABLE 2. Electron microprobe analyses of moissanite and metallic inclusions
moissanite silicon iron silicide
wt% SiC SiC 1s d.l. Si Fe
3
Si
7
Si-Fe-Al-Ca Si
2
(Fe,Al,Ca)
3
Si
2
(Fe,Al,Ca)
3
Si
3
(Fe,Al)
2
Si-Fe-Ti Si
2
Ca 1s d.l.
Si 69.67 70.23 0.11 0.03 100.82 52.72 37.48 32.39 33.84 47.13 41.16 56.38 0.39 0.20
Ti b.d. b.d. 0.03 b.d. 0.55 0.04 0.03 b.d. 0.15 16.27 b.d. 0.02 0.03
Cr b.d. b.d. 0.05 b.d. 0.08 b.d. b.d. 0.06 b.d. 0.08 b.d. 0.01 0.06
Al b.d. b.d. 0.01 0.02 0.12 20.99 24.55 22.64 5.51 2.35 0.42 0.01 0.01
Fe b.d. b.d. 0.06 b.d. 37.14 31.51 33.74 34.13 45.93 37.72 0.07 0.16 0.06
Mn b.d. b.d. 0.07 b.d. 6.76 0.50 0.50 0.65 1.28 1.65 b.d. 0.02 0.06
Mg b.d. b.d. 0.03 b.d. b.d. 0.06 0.12 b.d. b.d. 0.01 b.d. 0.003 0.01
Ni b.d. b.d. 0.08 b.d. 2.61 2.05 1.73 1.63 0.79 1.32 b.d. 0.02 0.10
Ca b.d. b.d. 0.02 b.d. b.d. 8.01 6.42 6.85 b.d. 0.08 40.52 0.01 0.03
Na b.d. b.d. 0.02
K b.d. b.d. 0.02
C 29.80* 30.04*
Total 99.49 100.32 100.90 99.98 100.63 99.49 99.82 100.85 100.65 97.53
Si 1.000 1.000 1.000 6.884 2.284 2.009 2.102 3.058 2.792 1.984
Ti 0.000 0.000 0.000 0.042 0.001 0.001 0.000 0.006 0.647 0.000
Cr 0.000 0.000 0.000 0.005 0.000 0.000 0.000 0.002 0.003 0.000
Al 0.000 0.000 0.000 0.016 1.330 1.584 1.463 0.372 0.166 0.016
Fe 0.000 0.000 0.000 2.439 0.966 1.052 1.066 1.498 1.287 0.001
Mn 0.000 0.000 0.000 0.451 0.015 0.016 0.021 0.042 0.057 0.000
Mg 0.000 0.000 0.000 0.000 0.004 0.008 0.000 0.000 0.000 0.000
Ni 0.000 0.000 0.000 0.163 0.059 0.051 0.048 0.024 0.043 0.000
Ca 0.000 0.000 0.000 0.000 0.342 0.279 0.298 0.000 0.004 0.999
Na 0.000 0.000
K 0.000 0.000
C 1.000 1.000
S cations 2.000 2.000 1.000 10.000 5.000 5.000 5.000 5.000 5.000 3.000
Note:
1s = relative error; d.l. = detection limit; b.d. = below detection.
* Calculated by stoichiometry.
DI PIERRO ET AL.: ROCK-FORMING MOISSANITE 1821
orthopyroxene (>10 wt% Al
2
O
3
, Table 4) points to high equili-
bration temperature (probably >1200 C) (e.g., Danckwerth and
Newton 1974).
The intimate association of SiC and Fe-bearing silicates,
along with the other above considerations, suggest an origin
from the mantle of the present sample. The fact that such an
assemblage is not reported experimentally shows the need for
more detailed ultra-high pressure studies in moissanite-bear-
ing systems.
ACKNOWLEDGMENTS
Salvatore Musacchia, the person who found the bluish pebble, is greatly
acknowledged for providing the material for research. Roberto Compagnoni,
Jürgen von Raumer, Vincent Serneels, and Gretchen Frueh-Green are thanked
for the fruitful discussions. We thank Cristophe Neururer, Cédric Metraux, Jes-
sica Chiaverini, Giulio Galetti, Odette Marbacher, and Paulo Bourqui for lab
assistance and thin sections preparation. Electron-microprobe analyses at the
University of Bern were supported by Schweizerischer Nationalfonds (credit
21-26579.89). We also thank Ed Mathez, Tyrone Daulton, and Robert F. Dymek
for their constructive reviews. Financial support from the Swiss National Sci-
ence Foundation Commission of the University of Fribourg (fellowship n.
PBFR2-101389 to SDP) is greatly acknowledged.
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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]. ...
... In contrast, the reliability of the above methods requires further validation when SiC grains possess a normal isotopic composition, i.e., isotopically similar to that of terrestrial contaminants. Notably, the presence of SRP inclusions such as metal silicides (e.g., FeSi, FeSi 2 , and Fe 3 Si 7 ) and native metals (e.g., Si 0 and Fe 0 ) is often mentioned as one of the important evidence for ruling out contamination issues during presumably SiCbearing samples and has been widely adopted as a consensus [1,5,7,14,17,19,46]. However, the evidence presented in this study challenges this prevailing viewpoint. ...
Article
Full-text available
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.
... The natural SiC is termed moissanite and it is not a common mineral, although there are rare findings on the Earth. [5][6][7][8][9][10][11][12][13][14] However, SiC may be the dominant phase in the deep interiors of carbon-rich extrasolar terrestrial planets, [15,16] and therefore control the structure and dynamics of these planets. Hence, the high-pressure investigation of SiC is important for planetary science. ...
Article
Full-text available
We have performed in situ X‐ray diffraction measurements of cubic silicon carbide (SiC) with a zinc‐blende crystal structure (B3) at high pressures and temperatures using multi‐anvil apparatus. The ambient volume inferred from the compression curves is smaller than that of the starting material. Using the 3rd‐order Birch‐Murnaghan equation of state and the Mie‐Grüneisen‐Debye model, we have determined the thermoelastic parameters of the B3‐SiC to be K0=228±3 GPa, K0’,=4.4±0.4, q=0.27±0.37, where K0, K0’ and q are the isothermal bulk modulus, its pressure derivative and logarithmic volume dependence of the Grüneisen parameter, respectively. Using the 3rd‐order Birch‐Murnaghan EOS with the thermal expansion coefficient, the thermoelastic parameters have been found as K0=221±3 GPa, K0’,=5.2±0.4, α0=0.90±0.02 ⋅ 10⁻⁵ ⋅ K⁻¹, where α0 is the thermal expansion coefficient at room pressure and temperature. We have determined that paired B3‐SiC – MgO calibrants can be used to estimate pressure and temperature simultaneously in ultrahigh‐pressure experiments up to 60 GPa.
... The attending silicon also reacts with Ti and carbon to form titanium disilicide (TiSi 2 ) (JCPDS: 01-085-0879) and silicon carbide (SiC). SiC is an exceptionally hard material [50], and the trace was found in XRD. Additionally, SiC can be identified by visually inspecting the microstructure as it often forms on the grain boundary and shows weaker bonding with visible gaps to the particle-matrix interface (Fig. 6 (e)) [51]. ...
Article
Full-text available
A 316L stainless steel (SS) alloy was developed with 1, 3, and 5 vol% titanium (Ti) reinforcement using the powder injection molding route, representing a low-cost option for biomedical implants. The investigation encompassed 1300 °C, 1350 °C, and 1380 °C sintering temperatures to ascertain the optimal physical and mechanical properties. Both sintering temperature and Ti influenced sintered density, and Ti mitigated the deleterious effects of residual carbon. At higher sintering temperatures, carbon and silicon tended to migrate and accumulate at the brink of Ti, leading to the formation of intermetallic compounds and increased brittleness. Dispersed Ti particles within the 316L matrix acted as nucleation sites and enhanced solid solubility with improved density. An astounding 96.11 % sintered density was achieved at 3 vol.% Ti sample sintered at 1380 °C. During the tensile test, 5 vol.% Ti at 1380 °C exhibited a low modulus of 58.9 GPa, which is highly desirable for orthopedic implant application. The XRD, SEM, tensile test, and nano-indentation results collectively provide evidence of beta-titanium formation during the sintering process. Conversely, the sample incorporating 3 vol.% titanium, sintered at 1380 °C, demonstrated a balanced performance, showcasing 432.94 ± 12.8 MPa ultimate tensile strength, 3.06 ± 0.17 % elongation, 74.2 GPa modulus, and 322 MPa and 423 MPa 0.2 % offset flexural and compressive yield strengths, respectively. Notably, an improvised wear resistance test underscored its aptitude for sliding wear resistance, solidifying its potential as a promising candidate for biomedical implants.
... 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). Others include unusual basalt-borne xenoliths (Liu et al. 2015) and a possibly kimberlitic beach pebble (Di Pierro et al. 2003) whose origins are unclear. ...
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.
... On a beach on the Turkish coast of the Mediterranean Sea, ca. 150 km north-west of Izmir, a special rock was found [119]. Though it was a pick-up find, analysis favors a natural source. ...
Article
Full-text available
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.
... 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. ...
Article
Full-text available
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]. ...
Article
Full-text available
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. ...
Preprint
Full-text available
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.
Article
More than 1 billion tons of bauxite was formed on the karstic surface of the Permian Maokou Formation limestone in the western Guangxi area of Youjiang Basin (YB). However, its formation mechanism is still a matter of debate. In this study, the Taiping bauxite deposit, a representative large deposit in this area, was selected to carry out detailed mineral microanalysis and pyrite sulfur isotope analysis to further understand the metallogenic process of bauxite in this region. Taiping bauxite contains two layers: the lower bauxite layer and the upper claystone layer. Bauxite ore was mainly composed of diaspore, pyrite, anatase, kaolinite, minor pyrophyllite, parisite, and bastnäsite. Small amounts of detrital moissanite, native boron, and zircon were also found in the ore. The claystone was dominated by kaolinite, diaspore, pyrite, and chlorite. The occurrence of detrital zircon, moissanite, and native boron suggests that they were most likely derived from volcanic ash or lava, further confirming the contribution of volcanoes. The mineral assemblages of diaspore, pyrite, anatase, parisite, and bastnäsite in the bauxite ore indicate alkaline and reducing depositional environments. The extensive development of ovoid microorganisms and the significant negative δ³⁴S values of pyrite (-42.80 ‰ to -12.54 ‰) indicate that microorganisms were involved in the formation of bauxite. This view was further confirmed by the wide development of fine pyrite (∼ 1 μm) cemented by cryptocrystalline diaspore in the ore. Extensive volcanism promotes the weathering of parent rocks, releasing Al³⁺, Fe³⁺, Ti⁴⁺, and REE³⁺ ions into the weathering system. Under strongly acidic and oxidizing conditions in the upper part of the weathering profile, these ions can migrate downward until they reach an alkaline and reducing environment on the carbonate surface. A large amount of diaspore, pyrite, anatase, parisite, and bastnäsite was formed near the karstic surface with the participation of microorganisms. Epigenetic modifications were also evident in the widespread development of kaolinite and pyrophyllite formed by the silicification of diaspore.
Article
Full-text available
Different techniques have been combined to identify the structure and the chemical composition of siliceous breccia from a drill core of nickel laterites in New Caledonia (Tiebaghi mine). XRD analyses show quartz as a major phase. Micro-Raman spectroscopy confirmed the presence of reddish microcrystalline quartz as a major phase with inclusion of microparticles of iron oxides and oxyhydroxide. Lithoclasts present in breccia are composed of lizardite, chrysotile, forsterite, heden-bergite and saponite. The veins cutting through the breccia are filled with Ni-bearing talc. Furthermore , for the first time, we discovered the presence of diamond microcrystals accompanied by mois-sanite polytypes (SiC), chromite (FeCr2O4) and uranophane crystals (Ca(UO2)2(SiO3OH)2.5(H2O)) and lonsdaleite (2H-[C-C]) in the porosities of the breccia. The origin of SiC and diamond polytypes are attributed to ultrahigh-pressure crystallization in the lower mantle. The SiC and diamond pol-ytypes are inherited from serpentinized peridotites having experienced interaction with a boninitic melt. Serpentinization, then weathering of the peridotites into saprolite, did not affect the resistant SiC polytypes, diamond and lonsdaleite. During karstification and brecciation, silica rich aqueous solutions partly digested the saprolite. Again, the SiC polymorph represent stable relicts from this dissolution process being deposited in breccia pores. Uranophane is a neoformed phase having crystallized from the silica rich aqueous solutions. Our study highlights the need of combining chemical and mineralogical analytical technologies to acquire the most comprehensive information on samples, as well as the value of Raman spectroscopy in characterizing structural properties of porous materials.
Article
Kimberlites have been found on the north China and Yangtze platforms, providing a source of diamond in China, and yielding samples of rocks and minerals for research into the physico- chemical properties of the upper mantle beneath the continental region of China. The geological background of kimberlites in China is discussed, as well as their minerals, textures and ages, the deep ultramafic xenoliths, models of magma ascent and physico-chemical properties of the upper mantle. -J.M.H.
Book
At the official dinner of a. meeting in May 1939, I was seated next to Max Hansen. When I congratulated him on the well deserved success of his "Aufbau der Zweistoff-Legierungen," he smiled: "yes, it was a struggle with the hydra, and so it has taken me seven years," meaning that whenever he had thought to have finished the phase diagram of a particular system, new evidence would turn up like the new heads of the Greek monster. There is no need to point out the importance of assessed phase diagrams to metallurgists or even anyone concerned with the technology and applica tion of metals and alloys. The information contained therein is fundamental to considerations concerning the chemical, physical and mechanical properties of alloys. Hansen's German monograph was followed by a revised English edition in 1958 with K. Anderko and the supplements by R.P. Elliott (1965) and F.A. Shunk (1969). All those who have made use of these volumes will admit that much diligent labour has gone into this work, necessary to cope with the ever increasing number of publications and the consequent improvements."
Chapter
Kimberlites have been found on the North China platform and Yangtze platform, providing a source of diamond in China, and yielding samples of rocks and minerals for research into the physico-chemical properties of the upper mantle beneath the continental region of China. The purpose of this paper is to discuss the geologic background of kimberlites in China, the mineral components, textures and age of the kimberlites, the deep ultramafic xenoliths, and to discuss models of magma ascent and physico-chemical properties of the upper mantle.