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Moissanite (SiC) from kimberlites: Polytypes, trace elements, inclusions andspeculations on origin
A.A. Shiryaev a,⁎, W.L. Griffin b, E. Stoyanov c
a Institute of Physical Chemistry and Electrochemistry RAS, Leninsky pr. 31, Moscow 119991, Russiab GEMOC ARC Key Centre, Macquarie University, NSW 2109, Australiac Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California at Davis, Davis, CA 95616, USA
An extensive collection of moissanite (SiC) grains from the Mir, Aikhal and Udachnaya kimberlite pipes ofYakutia has been characterized in terms of structural perfection, defects and the major- and trace-elementchemistry of SiC and its included phases. The natural grains are clearly distinct from synthetic SiC produced byvarious methods. Most of the natural SiC grains are 6H and 15R polytypes. Some of the grains (b10%) showextremely complex Raman spectra indicating strongly disordered structures. Some grains also show zoning inimpurities, C-isotope composition and cathodoluminescence brightness.Inclusions are heterogeneously distributed within the natural SiC; their size varies from a few nanometers tohundreds of microns. The most abundant inclusions in SiC are Si metal and iron silicide (FeSi2); a Si–C–Ophase with stoichiometry close to Si4(C,O)7 probably is related to the silicon oxycarbides. FeSi2 commonlyappears to have exsolved from Si metal; in some cases Ti metal then has exsolved from FeSi2 to formsymplectites. Trace elements are strongly concentrated in the inclusions of FeSi2 and Si4(C,O)7. The trace-element patterns of these phases are generally similar in the different kimberlites, but there are someconsistent minor differences between localities. The trace-element patterns of FeSi2 and Si4(C,O)7 are stronglyenriched in LREE/HREE and are broadly similar to the patterns of kimberlites, carbonatites and somediamond-forming fluids. However, extreme negative anomalies in Eu (and Sm) suggest highly reducingconditions. Yb also shows strong negative anomalies in FeSi2 from all three localities, and in Si4(C,O)7 fromAikhal and Mir, but not in those from Udachnaya. Trace-element chemistry and the nature of the inclusionsprovide a reliable basis for distinguishing natural and synthetic SiC. Textural and chemical features and thepresence of oxidation products (Si4(C,O)7 and SiO2) suggest that moissanite grew at high temperatures andelevated pressures and was subsequently partly oxidised, also at high T. Several important features ofmoissanite grains from kimberlites are consistent with the formation of natural SiC by electrochemicalprocesses in carbonate-silicate melts.
Recent research has brought an emerging recognition that thesubcontinental lithospheric mantle (SCLM), especially in the ancientcratonic roots, has undergone major compositional modificationthrough time. These studies suggest that the primary rocks of theArchean SCLM were magnesian dunites and harzburgites, highlydepleted by the removal of melts, and that these rocks have beenrepeatedly affected by metasomatic processes (Griffin et al., 2009 andreferences therein). These fluid-mediated processes have refertilisedthe barren SCLM, adding some of the components originally extractedduring large-scale partial melting. These studies illustrate the impor-tance of understanding fluid-related processes in the deep lithosphere.
Diamonds have played an important role in defining the types offluids that circulate in some parts of the SCLM, because some types ofdiamonds commonly contain visible fluid inclusions whose major-element, trace-element and isotopic compositions can be determinedby in-situ microanalytical techniques (e.g. Rege et al., 2010; Weisset al., 2009) or by solution techniques (e.g. McNeill et al., 2009). Theprecipitation of diamonds also underlines the importance of redoxreactions between fluids and their mantle wall-rocks, and the need todevelop better tools for studying redox processes. In this paper wepresent new data on natural silicon carbide (SiC), another highlyreduced phase from the deep lithosphere, and explore its implicationsfor redox processes in the SCLM.
SiC occurs in nature as the mineral moissanite, and there areseveral reviews of the extensive literature on its occurrence (e.g.Derkachenko et al., 1972; Kaminsky et al., 1968; Lyakhovich, 1979;Marshintsev, 1990). Moissanite has been found as inclusions indiamonds (Jaques et al., 1989; Klein-BenDavid et al., 2007; Leung
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et al., 1990; Moore and Gurney, 1989), and in mantle-derivedmagmatic rocks such as kimberlites (Bobrievich et al., 1957) andvolcanic breccias (Bauer et al., 1963; Di Pierro et al., 2003; Gorshkov etal., 1995). These finds suggest that SiC may be rare but ubiquitous in atleast the deeper parts of the subcontinental lithospheric mantle. Moreenigmatic occurrences include high- and low-grade metamorphicrocks, limestones, pegmatites and chromitite pods within ophiolites(Gnoevaja and Grozdanov, 1965; Lyakhovich, 1967, 1979; Marshint-sev, 1990 and references therein; Shiryaev et al., 2008a; Trumbull etal., 2009). These occurrences will not be dealt with here, but will bethe subject of future studies aimed at a general understanding ofredox processes in the lithosphere.
Grains of natural SiC often contain inclusions of highly reducedphases such as native Si (Marshintsev, 1990) and Fe-, Mg-, Ti-,Cr-silicides (Di Pierro et al., 2003; Marshintsev, 1990; Marshintsevet al., 1967; Mathez et al., 1995), which imply formation underextremely reducing conditions, well below the iron-wustite bufferthat is commonly regarded as a lower limit on the fO2 of the mantle. Ifthis implication is accepted, it requires a mechanism for drasticallylowering the fO2 of the subcontinental mantle, at least locally.Alternatively, other mechanisms must be proposed and tested. Ineither case, moissanite can potentially provide significant insights intoredox processes in the deep continental lithosphere.
In the past, most finds of moissanite in nature have been ascribedto industrial contamination during sample preparation. Discrimina-tion of natural vs synthetic SiC is indeed a problem; Bauer et al. (1963)noted that “the morphological, physical and chemical properties ofnatural moissanite and synthetic silicon carbide agree very closely”.Several criteria have been proposed for distinguishing naturalmoissanites from their synthetic analogues and the most commonlymentioned are the polytype abundance and inferred narrow compo-sitional range of “synthetic” inclusions. However, many of thesecriteria are unsuitable for such discrimination. The composition ofinclusions in synthetic SiC varies considerably between the samplesgrown by different methods. It is frequently mentioned that thenumber of polytypes present in synthetic SiC is very high (more than100), whereas moissanite is represented by very few polytypes.However, the vast majority of the polytypes in synthetic SiC exist onlyas unique samples and are not representative. Moreover, most of theso-called long-period polytypes are not, strictly speaking, indepen-dent species, since they commonly consist of stacks of “ordinary”polytypes such as 4H and 6H. This issue is discussed, for example, byvan Landuyt et al. (1983).
However, a growing body of data (see references above) confirmsthe widespread existence of small SiC grains in a variety of geologicalsettings. Petrologists are thus faced with the problem of defining themechanism(s) by which SiC might crystallise in such a wide range ofenvironments, most of which would not provide the extremelyreducing conditions implied by experimental studies of SiC stability.SiC is commonly reported from kimberlites (and diamonds), and theindustrial-scale separation of heavy minerals from kimberlites makesit feasible to recover trace mineral phases such as SiC. We thereforehave begun our investigations of natural SiC with a study ofmoissanite from kimberlites.
In this paper we report the results of a detailed investigation, using arange of techniques, of natural SiC grains recovered from the heavyfractions of the Mir, Aikhal and Udachnaya kimberlite pipes (Yakutia).Data are presented on the structural perfection and the major- andtrace-element chemistry of the natural SiC grains, aswell as descriptionsand analyses of syngenetic inclusions of other phases. Finally wepropose a tentative explanation for the formation of SiC in nature.
2. Samples and methods
The moissanite grains were hand-picked from the heavy fractionof crushed kimberlite samples. Issues of contamination were carefully
addressed by avoiding the use of SiC-containing machinery and byworking in clean rooms. The kimberlitic moissanite grains are up to1 mm across. Most of the grains are transparent and they showvarious colors, of which bluish-green is most common. As earlierobserved (Derkachenko et al., 1972; Marshintsev, 1990) most grainsare fractured, but some preserve well-formed crystallographic faces.
Raman microspectroscopy was employed to determine thepolytypes, to assess the degree of crystalline perfection, and toidentify some inclusions. Most analyses were performed using a514.4 nm laser in nearly back-scattering geometry either on polishedgrains placed in epoxy for chemical analysis or on loose grains. Thelaser spot was 5–20 μm in diameter and the explored wavelengthrange was between 90 and 3750 cm−1. This geometry allows rapididentification of polytypes, but the relative intensities of various peakscannot be compared for the polished samples since the exactorientation of the polished grains is sample-dependent. A siliconchip was employed for wavelength calibration. The influence of thelaser power on line positions is not pronounced for SiC; nevertheless,the power was kept low to reduce possible artefacts. Photolumines-cence was generally weak and posed no problems in data analysis.
The SiC grains were cast in epoxy and polished prior to analysis.Major elements were analysed at GEMOC, Macquarie University,using a CAMECA SX100 electron microprobe, fitted with 5 wave-length dispersive spectrometers, using an accelerating voltage of15 kV and a sample current of 20 nA. The diameter of the electronbeam was b5 μm. Standards included natural minerals and syntheticoxides; matrix corrections were done as described by Pouchou andPichoir (1984). Counting times were 10 s for peaks and 5 s forbackground on either side of the peak.
Trace-element contents of minerals were analysed at GEMOCusing an Agilent 7500cs ICPMS attached to a 266 nm Nd-YAG laseroperating at 10 Hz; the beam diameter was varied between 40 and150 μm depending on the type of material being analysed. The NIST610 glass was used as the external standard and EMP data for Si wereused as the internal standard. The NIST 610 glass was run twice at thebeginning and the end of each analytical session and the USGS BCR-2glass was run as an unknown to verify the quality of the data. The datawere processed using the software programme GLITTER 4.4, whichallows the recognition and exclusion of inclusions encountered duringthe time-resolved analysis (www.es.mq.edu.au/GEMOC; Griffin et al.,2008). Data were normalised to chondrites using values recom-mended by GERM (http://www-ep.es.llnl.gov/germ/). For compara-tive purposes synthetic SiC samples produced by two differentmethods, the Acheson and the modified Lely (sublimation) methods,were also studied.
Transmission Electron Microscopy (TEM) was used at BayerischesGeoinstitute (BGI) for direct investigation of submicroscopic inclu-sions. Loose grains were mechanically crushed in ethanol and thesuspension brought to the Cu grid. A Philips FEG CM20 instrumentoperating at 200 keV was used.
3. Results
3.1. Polytypes and SiC lattice perfection
From the mineralogical point of view one of the most interestingfeatures of SiC is its ability to form numerous polytypes, i.e. structuralmodifications, each of which may be regarded as built by stackinglayers of (nearly) identical structure and composition, where differentcrystallographic types differ only in their stacking sequence (Vermaand Krishna, 1966). In the Ramsdell notation used in this paper toindicate the polytype, the figure indicates the number of nonequiv-alent layers and the letter the type of lattice packing: cubic, hexagonalor rhombohedral. The most common polytypes in synthetic SiC arehexagonal 4H and 6H, rhombohedral 15R and cubic 3C.
Fig. 1. Raman spectra of moissanite grains. (A) TA and TO modes of SiC grains withvarious degrees of perfection (disorder increases from bottom to top); (B) LO modeillustrating various concentration of uncompensated impurities and zonation. Concen-tration of uncompensated dopants increases from top to bottom. The middle curveshows superposition of two domains.
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Although many physical properties of the different polytypes aresimilar, some important differences do exist, e.g., the band gap variesfrom 2.4 eV in 3C to 3.23 eV in 2H. Such variations in properties arepartly due to the different coordination of Si and C atoms in thepolytypes, which induces differences in the electronic structure. It isgenerally believed that in case of equilibrium growth the SiC polytypeis largely determined by pressure and temperature, althoughimpurities and kinetics may also play a role. The extensive modernliterature on polytypic transformations generally confirms the well-known SiC phase diagram of Knippenberg (1963), which shows thatthe hexagonal and rhombohedral SiC polytypes form at temperaturesN1300 °C at ambient pressure. Growth of synthetic SiC by variousmethods (vapour, sublimation, and melt) at low temperatures alwaysproduces the cubic (3C) variety.
One of the important observations for understanding the forma-tion of natural SiC is the remarkable predominance of the hexagonalover the cubic polytype in natural occurrences: only 3 naturaloccurrences of β-SiC (3C-SiC) have been reported (Leung et al.,1990; Marshintsev, 1990). Recent experiments on polytype intercon-version under HPHT-conditions (Sugiyama and Togaya, 2001) suggestthat these finds of the 3C-SiC could, in fact, reflect transformation ofthe original 6H to the 3C polytype during post-growth annealing.Temperatures in excess of 1300 °C are feasible in the deep lithosphere,where kimberlites and diamonds are generated, but are difficult toreconcile with reported occurrences of 6H SiC in limestones andsimilar rocks.
Due to differences in band gap the color of SiC is polytype-dependent. Therefore, already from optical examination the majorityof natural SiC grains were identified as the green- and blue-colored 6Hvariety. However, doping strongly influences SiC coloration and amore reliable method should be employed to identify the polytypes.Raman spectroscopy is a powerful tool for this purpose and forassessing the degree of lattice disorder (Nakashima and Harita, 1997).Typical spectra are shown in Fig. 1A. Grains of pure 6H SiC make up55% of the Mir sample set. The secondmost abundant pure polytype is15R: 8% of the Mir population. Some grains show the presence of 6H+15R mixtures in different proportions. In total the grains made ofpure or intermixed 6H and 15R polytypes make up 83% of the sample.Similar proportions were observed for the Aikhal and Udachnayasample suites. Several grains (~5%) contain mixtures of 6H and 4Hpolytypes. The rest of the studied populations consist of variousmixtures of 6H, 8H, 15R and 21R polytypes; such crystals may bethought of as containing long-period polytypes. Disorder in SiC is notcompletely random; disordered regions are mixtures of randomlydisordered simple polytype domains and those containing stackingfaults distributed periodically or near-periodically. Some of the grains(b10%) show extremely complex spectra, indicating strongly disor-dered structures (Fig. 1A).
The shape and position of the SiC Raman peak related to theLongitudinal Optic (or LO) phonon mode provide information aboutstresses (Falkovskii, 2004) and the concentrations of charge carriers(e.g., Nakashima and Harita, 1997). Spectra of some grains showstress-related features. However, the spectra of most grains show thepresence of various levels of uncompensated charge carriers (pre-sumably Al, B, perhaps N; see below), whose concentrations reachseveral hundred ppm in some grains. On average, the 15R grainscontain higher concentrations of uncompensated impurities than the6H ones. Many spectra showweak peaks around 640 cm−1, which aretentatively ascribed to the local vibrational mode of nitrogensubstituted on a Si site (Colwell and Klein, 1972).
The LO peak of some grains clearly reflects the superposition of atleast two components (Fig. 1B). This suggests the existence ofdomains with markedly different types and/or concentrations ofuncompensated dopants. Judging from the size of the laser spot thisheterogeneity is present at a scale of b10 μm. Cathodoluminescenceimages (Fig. 2) confirm that at least some of the grains show clear
growth zonation. A SIMS study (Shiryaev et al., 2008b) has shown thatsome grains from Mir are zoned in terms of C and Si isotopiccomposition; both elements are isotopically heavier in the 6Hpolytype than in the 15C.
Two grains from the Mir pipe show features in the OH vibrationregion (Fig. 3). However, the Raman spectra of these grains containlines not related to SiC, and the H-related bands may be due tosubmicroscopic H-rich inclusions rather than structural defects (C–Hcomplexes) in the SiC matrix.
Second-order Raman scattering is also clearly observed between1450 and 1950 cm−1. The spectra are sample- and polytype-dependent, but no attempt to extract quantitative information wasmade due to the uncertainty in the orientation of the polished grainsrelative to the laser beam.
Some rare features observed in our earlier TEM study (Shiryaev etal., 2008a) resemble extended defects called micropipes, which are avery special type of growth dislocation with a huge Burgers vector. Ifthe existence of micropipes in moissanite grains can be proved, thiswould indicate spiral growth, presumably on a substrate. Significant-ly, micropipes are not observed in SiC grown for abrasive purposes bythe Acheson process. Indirect support for the existence of micropipesin moissanite comes from the similarity of some defect-relatedconoscopic figures in natural SiC (Bauer et al., 1963) with calculationsby Presser et al. (2008). However, despite an extensive search, wefailed to observe such birefringence patterns in the grains from ourcollection. The birefringence of SiC grains from our collectiongenerally indicates the presence of various degrees of stress.
Another remarkable feature of the moissanite grains studied byTEM (this work, Shiryaev et al., 2009) is the surprisingly low densityof dislocations. Prolonged annealing at high temperatures may have
Fig. 2. Cathodoluminescence images. (A) SiC grain showing zoning and inclusions ofmetallic Si. Inclusions of FeSi2 are scattered through the Si0. The upper edge of the SiCcrystal is embayed by Si0, which runs between the main crystal and another mass of SiC.The bright spot on one margin of the main SiC grain is an irregular grain of the Si(C,O)phase, characterised by intense greenish CL. It is surrounded by a zone of apparentalteration, with variable contents of Si, O and Mg. (B) Euhedral crystal of SiC (Aikhal-2-35) showing apparent growth zoning. (C) Bright CL shows an aggregate of zoned grainsof the Si–C–O phase bordering SiC (grain Aikhal-2-86).
Fig. 3. H-related bands in the Raman spectrum of a Yakutian SiC grain.
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decreased the density of extended defects and led to exsolution ofimpurities as submicron inclusions.
3.2. Inclusion–host relationships
Several SiC grains from each locality contain inclusions of otherphases. The most common is silicon metal, followed by an iron silicide(FeSi2) and an oxygen-bearing Si(C,O) phase. The presence of Si metaland various silicides in natural SiC has been reported previously
(Di Pierro et al., 2003; Gnoevaja and Grozdanov, 1965; Lyakhovich,1979; Marshintsev, 1990; Mathez et al., 1995). However, the chemicalcomposition and crystal chemistry of previously reported compoundsdiffer from those reported here.
Typical BSE images of inclusion-bearing kimberlitic SiC grains areshown in Fig. 4. The most common inclusion is silicon metal (Si0),which typically is found in rounded (Fig. 4A) or negative-crystal(Fig. 4B) inclusions; these relationships suggest that the Si0 wastrapped as a liquid within the growing SiC crystal. Si0 also formsnetworks between grains of SiC (Fig. 4C). Many of the Si0 inclusionscontain globules or rims of FeSi2 (Fig. 4D; see below); the roundedinterfaces between Si0 and FeSi2 in these complex inclusions suggestthat the FeSi2 has exsolved from the molten Si0 as an immiscible melt.
A Si(C,O) phase (20–30 μm in size) is commonly found on contactsbetween Si lamellae and on the edges of SiC grains. This phase showsintense green (occasionally blue) CL and some grains show lamellarvariations in CL brightness. In some cases the SiC partly enclosesaggregates of Si(C,O) grains that show pronounced CL zoning(Fig. 2C); single homogeneous grains of the Si(C,O) phase also arefound enclosed in SiC (Fig. 2A).
Amore complex inclusion assemblage is illustrated in Figs. 4B and 5.A semi-euhedral crystal of SiC (150×350 μm; Fig. 4B) contains threeinclusionswith negative-crystal forms; each consists of Si0 with blebs ofFeSi2 nucleated along the contacts. At the top of the crystal, a largeirregular grain of FeSi2 is surrounded by a halo of cauliflower-shaped SiCgrains set in amatrix of SiO2. The SiO2 also forms botryoidal outgrowthsalong the edge of the SiC crystal. The large FeSi2 grain has a complexinternal structure, with anastomosing “worms” of Ti0 set in the FeSi2matrix (Fig. 5A). The microstructure strongly suggests the subsolidusexsolution of Ti0 fromthe FeSi2, and thepresence of subgrainswithin theFeSi2, defined by different orientation of the Ti0 “worms”. The Ti0
strongly concentrates Ni and Mn.The complex intergrowth of SiC and SiO2 around the FeSi2 grain
suggests a secondary replacement process, which may have oxidisedSi0 and to some extent SiC. The relationships between the phases inthese complex grains suggest that SiC coexisted with a melt phasedominated by Si0 and Fe0; this melt was trapped as inclusions in theSiC and also formed a grain-boundary phase. On cooling, FeSi2 appearsto have crystallised (exsolved?) from the Si0, typically along contactswith SiC.
The TEM study shows that submicroscopic inclusions of FeSi2 andthe Si(C,O) phases are also dispersed in the SiC matrix (Fig. 6).Electron diffraction confirms the identification of the silicide as FeSi2.Failure to detect the Raman signal of β-FeSi2 may be explained bydeviation of its composition from an ideal formula. Electrondiffraction indicates that the Si(C,O) phase is not amorphous; it ispoorly crystalline, but cannot be readily identified. Interestingly, thedistribution of the submicroscopic inclusions in the volume of SiC
Fig. 4. Typical BSE images of inclusion-bearing SiC grains from kimberlite (A, grain Mir-1-10; B, grain Mir-1-3; C, grain Aikhal-2-85; D, grain Aikhal-2-77). SiC is dark grey; Si metal,light grey; FeSi2, white. Triangular dark grey phase between two FeSi2 lamellae on top right of grain 1–3 is the Si–C–O phase described in the text. In B, note the apparent exsolutionof FeSi2 in a negative-crystal inclusion of Si0.
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grains is heterogeneous (Fig. 6A, B): inclusion-rich domains coexistwith rather perfect regions. The reason for such heterogeneity is notyet clear, since inclusions apparently are not associated with cracksand other defects. This relationship between SiC, Si0 and FeSi2 makestrace-element analysis of both Si0 and SiC problematic due to theprobability of contamination by submicroscopic silicide inclusions.
3.3. Compositions of phases
Averaged major-element compositions of SiC and included phasesare given in Table 1; trace-element data are summarized in Table 2.
3.3.1. MoissaniteMoissanite is typically quite a pure stoichiometric SiC, with mean
Si contents of 73–75 wt.%. Oxygen contents vary between 0.0–0.43 wt.%. The irregular grains around the FeSi2 in Fig. 4B are an exception,containing 0.25 wt.% Fe and nearly 3% O (Table 1). In the syntheticsamples only Si and C were present at levels detectable by EMP. Thesynthetic SiC also contains only a few trace elements (B, Al, Sc and Ba)at levels above the LAM-ICPMS detection limits (Table 2). Theseimpurities (as well as N) are typical of synthetic SiC. The levels of Al inthe synthetic SiC are higher than observed in natural samples fromMir and Aikhal, but lower than the mean values in SiC fromUdachnaya. Mg contents in the synthetic SiC are below detection(b0.4 ppm) but the natural samples typically contain 1–8 ppm Mg.The major- and trace-element data therefore define some cleardifferences between the synthetic SiC and the populations extractedfrom the kimberlites.
The presence of incompatible elements such as Ba in natural SiCwas noted by Gnoevaja and Grozdanov (1965), but detectable levels
of Ba (N0.05 ppm) were found in only a few point analyses in thepresent study. The most plausible explanation of chemical differencesbetween natural and synthetic materials lies in the abundance andcomposition of submicroscopic accessory phases. The SiC lattice issimilar to diamond in its extremely low capacity for hostingimpurities, except for some substitutional elements (Al, N, and B).Therefore, most trace elements analysed in natural SiC probablyreside in submicroscopic syngenetic inclusions and not in the SiCcrystalline lattice.
The contents of most trace elements in the natural SiC grains arelow (1–200 ppm), but several observations can be made. SiC grainsfrom Mir are generally lowest in trace elements, and those fromUdachnaya are highest. Contents of Al, Mg, Ti and Zr show wideranges, while B contents are more homogeneous. The low contents ofchalcophile elements (Cu, Zn, Mo, Pb, and Ni) could indicate that asulfide phase coexisted with the SiC but has not been trapped asinclusions. The REE patterns of natural SiC are poorly defined becausemost analyses are below the minimum detection limits (MDL; cfAcheson SiC, Table 2), but the available data suggest a flat or weaklynegative slope in a chondrite-normalised plot (not shown). OverallREE contents are higher in the samples from Aikhal and Udachnaya,whereas theMREE–HREE is below the detection limits in SiC fromMir.
There is no obvious correlation between the trace elementchemistry of the moissanite matrix and its polytype compositionand/or degree of lattice perfection as determined by Ramanspectroscopy. However, on the basis of limited statistics, a tentativeconclusion is that the 15R grains are somewhat richer in traceelements than the more abundant 6H. The 15R SiC also shows largerscatter in C and Si isotopic ratios than the 6H (Shiryaev et al., 2008b).These observations might suggest that impurities stabilize certain SiC
Fig. 5. Exsolution phenomena. (A)Map of TiKα distribution showing exsolution of Ti0 in FeSi2 grain attached to SiC (grain 1–10; see Fig. 4A). B–D: X-ray distributionmaps of O, Si andNi show intergrowths of SiC and SiO2 around the FeSi2 grain, and the concentration of Ni in the exsolved Ti0; E–F — maps of Mn and Fe distributions.
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polytypes (Tairov and Tsvetkov, 1983), but more complete data arerequired to address this question.
3.3.2. Metallic SiRaman spectroscopy and EMP analysis provide unambiguous
confirmation of the presence of metallic Si. A slight shift of theRaman peak of Si indicates a residual pressure of several kbar(Shiryaev and Kagi, unpublished). Metallic Si contains an average98.4–99.2 wt.% Si and 0.34 wt.% O. Most LAM-ICPMS analyses of theSi0 show measurable contents of a range of elements, but theseanalyses may be contaminated by minute inclusions of FeSi2. The
available analyses suggest that the intrinsic trace-element contents ofthe Si0 are extremely low.
3.3.3. Iron silicideThe iron silicide phase shows considerable compositional vari-
ability. In all but two analyses, Fe ranges from 43.1–46.4 wt.% (mean45.2–45.4 in Mir and Aikhal; 43.1–44.1 in Udachnaya); Si ranges from46.8–56.1 wt.% (mean from 50.8–62.0). Ti varies widely; many grainscontain 2.5–3.8 wt.% Ti, while others contain b0.5 ppm. X-ray mapssuggest that the pre-exsolution Ti content of the Fe–Si phase shown inFig. 6 was significantly higher. Ni (LAM-ICPMS) varies from b1 ppm toN800 ppm, andMn from 1.5 ppm to 1.3 wt.%. The calculated structural
Fig. 6. Bright-field TEM images of defects in natural SiC. (A) heterogeneous distribution of inclusions in SiC grains; dark horizontal line represents a micropipe-like defect; (B, C) — ironsilicide inclusions in SiC matrix; (D) SiC matrix with a silicide inclusion. At least two stacking faults (other SiC polytypes) are observed (marked by an arrow).
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formula for nearly all analyses is (Fe, Ti, Mn, Cr, and Ni) Si2. This isclearly different from the Fe3Si7 composition reported as inclusions inmoissanite by Di Pierro et al. (2003), which contains N5%Mn and littleTi. A second Fe–Si phase was found as an inclusion in one SiC grainfrom Udachnaya (Table 1); it has a stoichiometry close to (Fe,Ti)Si3.
The FeSi2 phase has high contents of many trace elements; itappears to concentrate most of these elements in the system, and thusmay give an indication of the nature of the environment in which theSiC and its associated phases formed.Most of the analysed FeSi2 grainsare smaller than the laser beam, and the analyses are contaminated tovarying degrees by Si0 or SiC; this is clearly seen as a deficit in Ferelative to the EMP analyses. Since the SiC and Si0 have very low trace-element contents, the overall patterns of the different analyses areparallel to one another, although they may differ in absoluteabundance by up to 3 orders of magnitude. The average patternsshown in Fig. 7A therefore were constructed by averaging the 3–5analyses with highest Fe contents in each locality, and normalising to45% Fe. These averages show enrichment of the LREE over the HREE,with unusual negative anomalies in Sm, Eu and Yb. Zr levels arestrikingly high; Zr/Hf ranges from subchondritic in some grains tosuprachondritic in others; on average both Zr/Hf and Nb/Ta are closeto the chondritic values.
In detail, the chondrite-normalised REE patterns of the FeSi2(Fig. 7A) show significant variations within each locality, and between
localities. All analyses of FeSi2 from Mir show a striking negative Euanomaly; Eu is below detection limits (0.02 ppm) in all analyses, eventhose with several hundred ppm of Nd and Gd. Negative anomaliesalso are apparent in Sm, Y and Yb. The patterns of the Aikhal FeSi2grains are similar, but the negative anomalies in Eu are less deep, andY shows minor negative or positive anomalies, or none. The FeSi2grains from Udachnaya show a range of negative Eu anomalies; onlyone grain has a negative Yb anomaly, and the same grain also has aweakly negative Y anomaly. Two grains (those with the least negativeEu anomalies) have small positive Ce anomalies. Within each locality,the depth of the Sm, Y and Yb anomalies varies from grain to grain, butthe average values for the anomalies in Sm and Yb are similar betweenlocalities (Fig. 7A).
3.3.4. Si–C–O phaseMost grains of the Si–C–Ophase contain 55.2–57.7% Si (mean 56.3)
and 8.3–20.8 wt.% O; we assume that the balance (12.0–32.4 wt.%) ismade up of C. The stoichiometry of this phase appears to be close to Si4(C,O)7, which would require a mixed valence (between 3 and 4) for Si.EMP analysis shows that parts characterised by darker CL (see above)contain minor amounts of Na (0.1–0.2 wt.%) and Al (0.2–0.3 wt.%).Qualitative EDX analysis in the TEM of microscopic inclusions ofcompositionally similar phases confirms the occasional presence of Aland K. Most likely, this phase belongs to the rich family of silicon
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oxycarbides (Si4C4−xO2-like compounds) commonly formed on thesurfaces of oxidised SiC (see recent review by Presser and Nickel,2008). In experiments on SiC oxidation such phases have beenobserved not only on surfaces, but also in the interiors of SiC grains(Lavrenko et al., 1981). The exact composition of the oxycarbideformed during such oxidation processes depends on the physio-
chemical conditions of oxidation and it is not surprising that naturaland synthetic compounds should differ from each other, nor that theanalyses should show a wide range of C/O.
The few Si4(C,O)7 grains that could be analysed by LAM-ICPMSshow high levels of trace elements and smooth REE-HFSE patterns(Fig. 7B) with pronounced negative Sr and Eu anomalies; one analysis
Table 2LAM-ICPMS Analyses of Trace Elements (ppm).
Acheson SiC Mir SiC Aikhal SiC Udachnaya SiC Mir FeSi2 Aikhal FeSi2 Udachnaya FeSi2 MirSi–C–O
Note: n=number of individual grains analysed; several were analysed in more than one spot. FeSi2 analyses normalised to 45% Fe to correct for beam overlap onto SiC or Si metal.
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shows a negative Yb anomaly. Zr/Hf and Nb/Ta are chondritic toslightly super-chondritic. The chalcophile elements are present inhigh but variable amounts. Given the aggregate nature of theindividual inclusions of the Si–C–O phase(s) (Fig. 2C) it is highlyprobable that the LAM-ICPMS analyses represent mixtures of two ormore phases.
4. Discussion
It is not surprising that the authors of most reports on natural SiChave faced serious problems explaining their findings. The formation ofnatural SiC is usually ascribed to hydrothermal activity (Lyakhovich,1979; Marshintsev, 1990), serpentinisation (Mathez et al., 1995), orunspecified HPHT processes (e.g., Trumbull et al., 2009). Some naturalmoissanite grains have been ascribed to disintegratedmeteorites, sinceminute SiC grains are present in some chondrites (Bernatowicz et al.,1987). However, meteoritic provenance cannot be very common evenfor non-kimberlitic SiC grains, because of contrasting C isotopecompositions, the large size of the terrestrial grains and the predom-inance of hexagonal polytypes in such grains.
The results of this study allow distinction of natural from syntheticmoissanites. The most obvious discrimination criterion is themarkedly different trace element chemistry, which reflects differ-ences in the composition of the growth media. As noted above, thetrace-element composition of the bulk SiC in natural samples reflectsthe composition and abundance of microinclusions of other phases,
especially the Fe-silicides, which concentrate trace elements. Severalfactors indicate that the moissanite grains from the kimberlites wereformed at high temperatures and elevated pressures: a) exsolution ofTi from FeSi2; b) precipitation of FeSi2 from metallic Si; c) clearlyobserved oxidation of SiC. In addition, the strongly negative Euanomalies require that, at least locally, the oxygen fugacity was low.
The FeSi2 phase clearly accepts highly charged small ions such asthe REE and HFSE. The REE patterns with deep negative anomalies inEu and Sm suggest a strongly reducing environment, in whichessentially all Eu, and much of the Sm, has been reduced to the 2+state and entered another phase. However, the great variability in thedegree of Eu-, Sm- and Yb depletion suggests that the redox state alsovaried even during the crystallisation of the SiC in the differentlocalities. The apparent lack of Eu and Sm anomalies in the moissanitefurther suggests that the SiC does not discriminate against Eu2+ orSm2+. Without samples of other coexisting phases, it is not clearwhere the Eu and Sm are concentrated in this environment.
If the SiC and FeSi2 precipitated from a fluid phase, the residualfluid might be left with a strong positive Eu anomaly, which mightimpose on other metasomatised domains, depending on the massbalance (fluid/rock ratios). Griffin and O'Reilly (2007) have noted thatmany Cr-garnets from cratonic mantle peridotites show positive (aswell as negative) Eu anomalies that cannot be reconciled with theformer presence of plagioclase, the usual explanation for Eu anomaliesin mantle-derived rocks. The inferred positive Eu anomaly of thefluids residual from the crystallisation of SiC (+ FeSi2) may offer an
A
B
Fig. 7. Chondrite-normalised patterns of trace elements. (A) mean compositions of FeSi2 populations, compared with the pattern of a typical fibrous diamond (sample JWA-1,Jwaneng; data from Rege et al., 2010); (B) mean values for Si(C,O)7 in the three kimberlites.
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explanation for Eu anomalies in deep mantle rocks, independent ofsubduction processes.
The overall trace-element patterns of the FeSi2 and the Si4(C,O)7phase are broadly similar to those of many fibrous diamonds (Fig. 7A;Araujo et al., 2008; Rege et al., 2005, 2010; Weiss et al., 2008). Thepatterns of such diamonds reflect the presence of inclusionsrepresenting a range of saline to hydrous to carbonatitic high-densityfluids, which may be derived by low-degree melting of carbonatedperidotites and eclogites (Weiss et al., 2008). The trace-element datasuggest that the formation of the kimberlitic moissanite and itsassociated inclusions could be related to similar high-density, low-volume fluids. However, thermodynamic calculations (Mathez et al.,1995) and experiments (Ulmer et al., 1998) show that in the Mg–Si–C–O system (a proxy for metasomatism in a mantle setting), SiCoccurs only under highly reducing conditions: 4–7 log units below theiron-wustite (IW) buffer.
Before discussing possible mechanisms of moissanite growth inthe lithosphere, we must briefly review the main methods ofsynthetic production of SiC. The most common (Acheson andsublimation) processes require very high temperatures (N2000 °C)and gas flow (Tairov and Tsvetkov, 1978). Chemical-vapour deposi-tion (CVD) is used to grow SiC films, most commonly by thedecomposition of SiCl4, but the only plausible niches where it could
operate in nature are in space and during impact-related events.Another synthetic process, liquid epitaxy, proceeds at geologicallyreasonable temperatures (N800 °C). Besides epitaxial growth on a SiC-substrate, spontaneous SiC crystallization has also been achieved.However, this process requires the presence of a Si-saturated metalmelt such as Fe–Al–Si or Al–Sn (e.g., Chaussende et al., 2001;Derkachenko et al., 1972; Yakimova et al., 1996). Usually suchprocesses lead to the formation of 3C-SiC, although early experimentsby Ellis (1960) allowed speculation that certain metals might stabilizethe hexagonal polytypes. The reaction of diamond with Si-rich meltcan lead to crystallisation of oriented SiC on the diamond (Varshavskiiand Shulpyakov, 1967). Similar processes might be operative incertain exotic mantle domains and could be responsible for 3C-SiCinclusions in diamonds, but are unlikely to be common. In addition,several papers report SiC formation from low-temperature solutionsin the presence of strong reducing agents, but such experiments aremostly laboratory curiosities.
Occurrences of moissanite in very different geological settingsprobably indicate that several mechanisms could be responsible formoissanite formation. We believe that two possibilities are the mostimportant: 1) growth in deepmantle during interaction of Si (or C)-richmetal melts with carbonaceous (or siliceous) materials; and 2) redoxreactions. These mechanisms are certainly viable from thermodynamic
Fig. 8. Generalised scheme of electrodeposition of SiC (adapted from Moller et al.,1997). A— in the absence of an external electric field; B— in the presence of an externalfield.
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point of view and experiments have shown that SiC could indeed beproduced in these ways. Redox reactions such as serpentinisation canproduce extremely reduced phases such as native iron; most of them,however, disappear during later stages of the serpentinisation process.Silicon carbide is rather stable against oxidation and, in contrast tomanyreduced phases, may survive changes in conditions.
Numerous reports of SiC in association with carbonates (Gnoevajaand Grozdanov, 1965; Klein-BenDavid et al., 2007; Miyano et al.,1982; Shiryaev et al., 2008a) suggest another type of redox process,electrochemical deposition, which might lead to the formation ofmoissanite in relatively oxidised carbonate-silicate melts. Carbonate-rich fluids are as an important metasomatic agent in the deeplithosphere, where they are linked to the formation of diamonds(e.g. Weiss et al., 2008; references therein).
Recent experimental work (Havens and Kavner, 2009; Kavner andWalker, 2006; Kavner et al., 2007) suggests that electrochemicalreactions under high pressures might be responsible for a range ofredox reactions under mantle conditions. Several studies have arguedthat electrochemical processes are important in the genesis of sulfideores (e.g., Mironov et al., 1981; Moller and Kersten, 1994; Moller et al.,1997; Nyussik and Komov, 1981; Yakhontova and Grudev, 1978).
Other experimental studies have demonstrated the possibility offorming silicon carbide in the course of electrochemical processes insystems containing carbonates and silicates (Devyatkin, 2003; Devyat-kin et al., 2002; Elwell et al., 1982). In these experiments the SiC layerswere deposited on several types of electrodes at temperatures as low as720 °C, from carbonate melts containing a few percent of SiO2. Thedeposited layers consist of hexagonal SiC, as confirmed by XRD,sometimes with excess Si or C. The reaction leading to SiC formationby electrodeposition is described as M2CO3+SiO2=M2O+SiC+2O2,where M=Li, Na, and K. The minimum temperature of the process isdetermined by the melting point of the carbonate mixture. Theelectromotive force at which these reactions proceed depends onelectrode composition and is between 0.5 and 1.6 V. SiC electrodepo-sition alsohas been achieved in thepresence of pressurised CO2 over themelt, following reactions such as CO2+M2SiO3=M2O+SiC+O2
(Devyatkin, 2003; Devyatkin et al., 2002). Electrolysis of silumin(Al alloyed with a few percent of Si) in carbonate melts at 500 and600 °C has also produced SiC layers (Sannikov et al., 1993). Moreover,the experiments at 600 °C produced hexagonal plate-like crystallites,similar to those observed among moissanites.
To the best of our knowledge, electrodeposition is the only processdocumented as leading to the formation of hexagonal SiC at lowtemperatures; this may shed light on the still unsolved problem of SiCpolytype genesis (e.g., Tairov and Tsvetkov, 1983). The electrodepo-sition mechanism for the formation of natural SiC can overcome therequirement of highly reducing conditions and temperatures inexcess of 1300 °C. Electrochemical processes may operate in manygeochemical environments. External electric fields could be produced,for example, by deformation processes or by percolation of fluidsthrough porous media (Gershenzon et al., 1993; Gokhberg et al.,2007; Guglielmi, 2007; Hunt et al., 2007; Sgrigna et al., 2004).
However, the presence of the external fields is not alwaysnecessary. Virtually all conducting minerals that contain oxidisingcomponents could serve as electrodes with fluids acting as electro-lytes, and such conditions might be locally realized in many differentsettings. The interaction of carbonatitic melts with other mantlephases seems to be a very promising mechanism, since their electricalconductivities are very different (Gaillard et al., 2008). Otherpossibilities include contact between melts with different oxygenfugacities and/or water contents. Solid minerals in the immediatevicinity of such contacts may serve as nucleation sites for SiC. Verygeneralised schemes of the processes leading to SiC elecrodepositionare shown in Fig. 8.
The extreme rarity of completely euhedral SiC grains may bepartially due to detachment from the original substrate. Similar
“broken” grains are rather common among SiC crystals grownfrom experimental melts. Once formed, many of the moissanitegrains must have been subjected to oxidation or etching, leadingto the formation of silicon oxycarbides and oxides. For siliconcarbide, the oxidation kinetics are generally slow and theoxidation rate depends strongly on polytype, crystallographicorientation and type of surface (C- or Si-face; Presser and Nickel,2008). Therefore, a substantial fraction of moissanite grains maysurvive post-growth alteration, though with some modifications ofshape.
This work has some important implications for the geochemistryand petrology of the deep lithosphere. First of all, we can confirm thatmoissanite is indeed a natural mineral and can be present in differentlithologies. Second, this work shows that redox processes in thelithospheric mantle may lead to the generation of locally highlyreduced conditions with oxygen fugacity reaching 5–7 log units belowthe IW buffer. Third, the consistency of themoissanite properties withan electrochemical mechanism for its synthesis may indicate thatelectrochemical redox processes are common in natural settings. Theycould be responsible for the formation of other reduced phases such asFe3C and Fe0 (Bulanova and Zayakina, 1990; Shiryaev et al., 2010) bycreation of locally reduced environments within the relativelyoxidised deep lithospheric mantle.
5. Conclusions
This investigation of the chemistry, defects and inclusions ofmoissanite (SiC) grains separated from kimberlites shows generalsimilarities between three sample sets and marked differences withsynthetic silicon carbide produced by various processes. The moissa-nite found in kimberlites is a naturally occurring mineral thatcrystallised at high temperatures and elevated pressures, probablywithin the stability fields of diamond and molten Si. The trace-element chemistry of the growth medium, as reflected in trappedinclusions of Fe-silicides, is similar to those of the different high-density fluids involved in the growth of diamond. Many features of thestudied moissanite grains are consistent with its formation bylocalised electrochemical processes within a relatively oxidisedenvironment, from carbonate-silicate melts similar to those thatmay have precipitated many diamonds.
Acknowledgements
AAS is grateful to the Humboldt Foundation and to RussianPresident grant MK-147.2007.5 for partial financial support. Thisstudy used instrumentation funded by ARC LIEF and DEST
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Systematic Infrastructure Grants, Macquarie University and Indus-try. We thank E.S. Efimova and M. Tomshin for providing thekimberlitic SiC grains; T.E. Scherbakova for help with the heavy-fraction separation; Norman Pearson and Debora Araujo for theirassistance with the EMP and LAM-ICPMS analyses. We are gratefulto Dr. S.V. Devyatkin for discussions and the provision of electro-deposited SiC. Comments of Drs, Gaillard, D.Harlov, A.C. Kerr and ananonymous reviewer are highly appreciated. This is contribution697 from the Australian Research Council National Key Centrefor the Geochemical Evolution and Metallogeny of Continents(http://www.gemoc.mq.edu.au).
References
Araujo, D.P., Griffin, W.L., O'Reilly, S.Y., 2008. Micro-inclusions in monocrystallineoctahedral diamonds from Diavik, Slave Craton: clues to diamond genesis: 9thInternational Kimberlite Conference, Frankfurt, Extended abstract 9IKC-A-00138http://www.cosis.net/abstracts/9IKC/00138/9IKC-A-00138-1.pdf.
Bauer, J., Fiala, J., Hrichova, R., 1963. Natural α-silicon carbide. American Mineralogist48, 620–634.
Bernatowicz, T., Fraundorf, G., Tang, M., Anders, E.,Wopenka, B., Zinner, E., Fraundorf, P.,1987. Evidence for interstellar SiC in the Murray carbonaceous meteorite. Nature330, 728–730.
Bobrievich, A.P., Kalyuzhnii, V.A., Smirnov, G.I., 1957. Moissanite in the kimberlites ofthe East Siberian platform. Akademii Nauk SSSR Doklady 115 (6), 1189–1192.
Bulanova, G.P., Zayakina, N.V., 1990. A graphite–cohenite–iron mineral association inthe core of a diamond form the XXIII CPSU congress pipe. Doklady Akademii NaukSSSR, Geosciences section 317, 706–709.
Colwell, P.J., Klein, M.V., 1972. Raman scattering from electronic excitations in n-typesilicon carbide. Physical Review B 6, 498–515.
Derkachenko, L.I., Zaretskaya, G.M., Obuhov, A.P., Sokolova, T.V., Filonenko, N.E., 1972.Mineralogy of Silicon Carbide (Silicon Carbide in Industrial and Natural Rocks).Nauka, Leningrad.
Devyatkin, S.V., 2003. Electrochemistry of silicon in chloro-fluoride and carbonatemelts. Journal of Mining and Metallurgy 39B, 303–307.
Devyatkin, S.V., Pisanenko, A.D., Shapoval, V.I., 2002. Chemical and electrochemicalbehavior of carbonate melts containing silicon oxide. Russian Journal of AppliedChemistry 75, 562–564.
Di Pierro, S., Gnos, E., Grobety, B.H., Armbruster, T., Bernasconi, S.M., Ulmer, P., 2003.Rock-forming moissanite (natural α-silicon carbide). American Mineralogist 88,1817–1821.
Ellis Jr., R.C., 1960. Growth of silicon carbide from solution. In: O'Connor, J.R., Smiltens,J. (Eds.), Silicon Carbide: A High Temperature Semiconductor. Pergamon press,pp. 124–129.
Elwell, D., Feigelson, R.S., Simkins, M.M., 1982. Electrodeposition of silicon carbide.Materials Research Bulletin 17, 697–706.
Falkovskii, L.A., 2004. Raman investigations of semiconductors with defects. Physics–Uspekhi 174, 249–272.
Gaillard, F., Malki, M., Iacono-Marziano, G., Pichavant, M., Scaillet, B., 2008. Carbonatitemelts and electrical conductivity in the asthenosphere. Science 322, 1363–1365.
Gershenzon, N.I., Gokhberg, M.B., Yunga, S.L., 1993. The electromagnetic field of anearthquake focus. Physics of the Earth and Planetary Interiors 77, 13–19.
Gnoevaja, N., Grozdanov, L., 1965. Moissanite from Triassic rocks, NW Bulgaria.Proceedings of Bulgarian Geological Society 26, 89–95.
Gokhberg, M.B., Kolosnitsyn, N.I., Nikolaev, A.I., 2007. Tidal deformations and theelectrokinetic effect in a two-layer fluid-saturated porous medium. Izvestiya-Physics of the Solid Earth 43, 702–706.
Griffin, W.L., O'Reilly, S.Y., Afonso, J.C., Begg, G.C., 2009. The composition and evolutionof lithospheric mantle: a re-evaluation and its tectonic implications. Journal ofPetrology 50, 1185–1204.
Griffin, W.L., Powell, W.J., Pearson, N.J., O'Reilly, S.Y., 2008. GLITTER: data reductionsoftware for laser ablation ICP-MS. In: Sylvester, P. (Ed.), Laser Ablation–ICP–MS inthe Earth Sciences: Mineralogical Association of Canada Short Course Series,Volume 40, pp. 204–207. Appendix 2.
Guglielmi, A.V., 2007. Ultra-low-frequency electromagnetic waves in the Earth's crustand magnetosphere. Physics-Uspekhi 50, 1197–1216.
Havens, K., Kavner, A., 2009. High pressure solid electrolyte redox experiments:Application to silver iodide. Physics of the Earth and Planetary Interiors 174,315–322.
Hunt, A., Gershenzon, N., Bambakidis, G., 2007. Pre-seismic electromagnetic phenomena inthe framework of percolation and fractal theories. Tectonophysics 431, 23–32.
C-isotope composition of Argyle and Ellendale diamonds. : Kimberlites and relatedrocks, 2. Blackwell Scientific, Cambridge, England, pp. 966–989.
Kaminsky, F.V., Bukin, V.I., Potapov, S.V., Arkus, N.G., Ivanova, V.G., 1968. Discoveries ofsilicon carbide under natural conditions and their genetic importance: IzvestiyaAkademii Nauk SSSR, ser. Geologicheskaya no. 6, pp. 57–66 (English translation:International Geology Review 1968, 11, 561–569).
Kavner, A., Walker, D., 2006. Core/mantle-like interactions in an electric field. Earth andPlanetary Science Letters 248 (1–2), 316–329.
Kavner, A., Walker, D., Newville, M., Sutton, S., 2007. Externally-driven charge transferin silicates at high pressure and temperature: a XANES study. Earth and PlanetaryScience Letters 256, 314–327.
Klein-BenDavid, O., Wirth, R., Navon, O., 2007. Micrometer-scale cavities in fibrous andcloudy diamonds — a glance into diamond dissolution events. Earth and PlanetaryScience Letters 264, 89–103.
Lavrenko, V.A., Jonas, S., Pampuch, R., 1981. Petrographic and x-ray identification ofphases formed by oxidation of silicon carbide. Ceramics International 7, 75–76.
Leung, I., Guo,W., Freidman, I., Gleason, J., 1990. Natural occurrence of silicon carbide ina diamondiferous kimberlite from Fuxian. Nature 346, 352–354.
Lyakhovich, V.V., 1967. Accessory Minerals in Granitoids of Soviet Union. Nedra Press,Moscow.
Lyakhovich, V.V., 1979. Origin of accessory moissanite: Izvestiya Akademii Nauk SSSR,ser. Geologicheskaya no. 4, pp. 63–74 (English translation: International GeologyReview 1980, 22, 961–970).
Marshintsev, V.K., Shchelchkova, S.G., Zol'nikov, G.V., Voskresenskaya, V.B., 1967. Newdata on moissanite from the Yakutian kimberlites. Geologiya i Geofizika 12, 22–31.
Mathez, E.A., Fogel, R.A., Hutcheon, I.D., Marshintsev, V.K., 1995. Carbon isotopiccomposition and origin of SiC from kimberlites of Yakutia, Russia. Geochimica etCosmochimica Acta 59, 781–791.
McNeill, J., Pearson, D.G., Klein-BenDavid, O., Nowell, G.M., Ottley, C.J., Chinn, I., 2009.Quantitative analysis of trace element concentrations in some gem-qualitydiamonds. Journal of Physics: Condensed Matter 21, 364207 (13 pp).
Mironov, A.G., Zhmodik, S.M., Maksimova, E.A., 1981. An experimental investigation ofsorption of gold by pyrites with different thermoelectric properties. GeochemistryInternational 18, 153–160.
Miyano, S., Sueno, S., Ohmasa, M., Fujii, T., 1982. A new SiC polytype, 45Rb. ActaCrystallographica A 38, 477–482.
Moller, P., Kersten, G., 1994. Electrochemical accumulation of visible gold on pyrite andarsenopyrite surfaces. Mineralium Deposita 29, 404–413.
Moller, P., Sastri, C.S., Kluckner, M., Rhede, D., Ortner, H., 1997. Evidence forelectrochemical deposition of gold on arsenopyrite. European Journal of Mineralogy9, 1217–1226.
Moore, R.O., Gurney, J.J., 1989. In: Ross, J., Jaques, A.L., Ferguson, J., Green, D.H., O'Reilly,S.Y., Danchin, R.V., Janse, A.J.A. (Eds.), Mineral inclusions in diamond from theMonastery kimberlite, South Africa. : Kimberlites and Related Rocks, 2. BlackwellScientific, Cambridge, England, pp. 1029–1041.
Nakashima, S., Harita, H., 1997. Raman investigation of SiC polytypes. Physica StatusSolidi A162, 39–64.
Nyussik, Ya.M., Komov, I.L., 1981. Electrochemistry in Geology. Nauka, Leningrad.Pouchou, J.L., Pichoir, F., 1984. A newmodel for quantitative X-ray microanalysis. Part 1.
Applications to the analysis of homogeneous samples. Recherche Aerospatiale 3,11–38.
Presser, V., Nickel, K.G., 2008. Silica on silicon carbide. Critical Reviews in Solid State andMaterials Sciences 33, 1–99.
Presser, V., Loges, A., Nickel, K.G., 2008. Scanning electron and polarization microscopystudy of the variability and character of hollow macro-defects in silicon carbidewafers. Philosophical Magazine 88, 1639–1657.
Rege, S., Jackson, S.J., Griffin, W.L., Davies, R.M., Pearson, N.J., O'Reilly, S.Y., 2005.Quantitative trace element analysis of diamond by laser ablation inductively coupledplasma mass spectrometry. Journal of Analytical Atomic Spectroscopy 20, 601–610.
Rege, S., Griffin, W.L., Pearson, N.J., Araujo, D., Zedgenizov, D., O'Reilly, S.Y., 2010. Trace-element patterns of fibrous and monocrystalline diamonds: insights into mantlefluids. Lithos 118, 313–337.
Sannikov, V.I., Manukhina, T.I., Kudyakov, V.Ya., 1993. Investigation of the films at thesilumin surface oxidized in molten alkali metals carbonates. Melts 5, 86–91.
Sgrigna, V., Buzzi, A., Conti, L., Guglielmi, A.V., Pokhotelov, O.A., 2004. Electromagneticsignals produced by elastic waves in the Earth's crust. Nuovo Cimento C.Geophysics and Space Physics 27, 115–132.
Shiryaev, A.A., Wiedenbeck, M., Reutsky, V., Polyakov, V.B., Mel'nik, N.N., Lebedev, A.A.,Yakimova, R., 2008a. Isotopic heterogeneity in synthetic and natural silicon carbide.Journal of Physics and Chemistry of Solids 69, 2492–2498.
Shiryaev, A.A., Wang, S., Stoyanov, E., Pirouz, P., 2009. TEM study of microstructure ofnatural SiC (moissanite) from contrasting geological settings: inferences formoissanite formation: Extended abstracts of International Conference on SiliconCarbide and Related Materials 2009, Nurnberg, Germany.
Shiryaev, A.A., Zubavichus, Ya.V., Veligzhanin, A.A., McCammon, C., 2010. Localenvironment and valence state of iron in microinclusions in fibrous diamonds:X-ray Absorption and Mössbauer data. Russian Geology and Geophysics 51,1262–1266.
164 A.A. Shiryaev et al. / Lithos 122 (2011) 152–164
Sugiyama, S., Togaya, M., 2001. Phase relationship between 3C- and 6H-silicon carbideat high pressure and high temperature. Journal of American Ceramics Society 84,3013–3016.
Tairov, Yu.M., Tsvetkov, V.F., 1978. Investigation of growth processes of ingots of siliconcarbide single crystals. Journal of Crystal Growth 43, 209.
Tairov, Yu.M., Tsvetkov, V.F., 1983. Progress in controlling the growth of polytypiccrystals. Progress in Crystal Growth and Characterisation 7, 111–162.
Trumbull, R.B., Yang, J.-S., Robinson, P.T., Di Pierro, S., Vennemann, T., Wiedenbeck, M.,2009. The carbon isotope composition of natural SiC (moissanite) from the Earth'smantle: new discoveries from ophiolites. Lithos 113, 612–620.
Ulmer, G.C., Grandstaff, D.E., Woermann, E., Gobbels, M., Schonitz, M., Woodland, A.B.,1998. The redox stability of moissanite (SiC) compared with metal-metal oxidebuffers at 1773 K and at pressures up to 90 kbar. Neues Jahrbuch Mineralogie,Abhandlungen 172, 279–307.
van Landuyt, J., van Tendeloo, G., Amelinckx, S., 1983. High resolution electronmicroscopy of polytypes. Progress in Crystal Growth and Characterisation 7,343–378.
Varshavskii, A.V., Shulpyakov, Yu.F., 1967. Oriented crystallisation ofα-SiC on diamond.Doklady Akademii Nauk SSSR 173 (3), 573–574.
Verma, A.R., Krishna, P., 1966. Polymorphism and Polytypism in Crystals. John Wiley &Sons, New York.
Weiss, Y., Klein-BenDavid, O., Kessel, R., Griffin, W.L., Kiflawi, I., Bell, D.R., Harris, J.W.,Navon, O., 2008. Fluid microinclusions in diamonds from Kankan, Guinea and a newmodel for the evolution of a diamond forming fluids: 9th International KimberliteConference, Frankfurt, Extended abstract 9IKC-A-00115. http://www.cosis.net/abstracts/9IKC/00115/9IKC-A-00115-1.pdf.
Weiss, Y., Kessel, R., Griffin, W.L., Kiflawi, I., Klein-BenDavid, O., Bell, D.R., Harris, J.W.,Navon, O., 2009. A newmodel for the evolution of diamond-forming fluids: evidencefrom microinclusion-bearing diamonds from Kankan, Guinea. Lithos 112S, 660–674.
Yakhontova, L.K., Grudev, A.P., 1978. Hypergenesis Zone of Ore Deposits. MSU press,Moscow.
Yakimova, R., Tuominen, M., Bakin, A.S., Fornell, J.-O., Vehanen, A., Janzen, E., 1996.Silicon carbide liquid phase epitaxy in the Si–Sc–C system. Institute of PhysicsConference Series 142, 101–104.
