Moissanite (Sic) from Kimberlites: Polytypes, Trace Elements, Inclusions and Speculations on Origin

Lithos 122 (2011) 152–164

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Moissanite (SiC) from kimberlites: Polytypes, trace elements, inclusions and speculations 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, Russia b GEMOC ARC Key Centre, Macquarie University, NSW 2109, Australia c Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California at Davis, Davis, CA 95616, USA article info abstract

Article history: An extensive collection of moissanite (SiC) grains from the Mir, Aikhal and Udachnaya kimberlite pipes of Received 26 July 2010 Yakutia has been characterized in terms of structural perfection, defects and the major- and trace-element Accepted 24 December 2010 chemistry of SiC and its included phases. The natural grains are clearly distinct from synthetic SiC produced by Available online 1 January 2011 various methods. Most of the natural SiC grains are 6H and 15R polytypes. Some of the grains (b10%) show extremely complex Raman spectra indicating strongly disordered structures. Some grains also show zoning in Keywords: impurities, C-isotope composition and cathodoluminescence brightness. Moissanite (SiC) Iron silicide Inclusions are heterogeneously distributed within the natural SiC; their size varies from a few nanometers to – – Oxycarbide hundreds of microns. The most abundant inclusions in SiC are Si metal and iron silicide (FeSi2); a Si C O Raman spectroscopy phase with stoichiometry close to Si4(C,O)7 probably is related to the silicon oxycarbides. FeSi2 commonly

LAM-ICP-MS appears to have exsolved from Si metal; in some cases Ti metal then has exsolved from FeSi2 to form Electrochemical deposition symplectites. 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 some

consistent minor differences between localities. The trace-element patterns of FeSi2 and Si4(C,O)7 are strongly enriched in LREE/HREE and are broadly similar to the patterns of kimberlites, carbonatites and some diamond-forming ﬂuids. However, extreme negative anomalies in Eu (and Sm) suggest highly reducing

conditions. Yb also shows strong negative anomalies in FeSi2 from all three localities, and in Si4(C,O)7 from Aikhal and Mir, but not in those from Udachnaya. Trace-element chemistry and the nature of the inclusions provide a reliable basis for distinguishing natural and synthetic SiC. Textural and chemical features and the

presence of oxidation products (Si4(C,O)7 and SiO2) suggest that moissanite grew at high temperatures and elevated pressures and was subsequently partly oxidised, also at high T. Several important features of moissanite grains from kimberlites are consistent with the formation of natural SiC by electrochemical processes in carbonate-silicate melts. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Diamonds have played an important role in defining the types of fluids that circulate in some parts of the SCLM, because some types of Recent research has brought an emerging recognition that the diamonds commonly contain visible fluid inclusions whose major- subcontinental lithospheric mantle (SCLM), especially in the ancient element, trace-element and isotopic compositions can be determined cratonic roots, has undergone major compositional modification by in-situ microanalytical techniques (e.g. Rege et al., 2010; Weiss through time. These studies suggest that the primary rocks of the et al., 2009) or by solution techniques (e.g. McNeill et al., 2009). The Archean SCLM were magnesian dunites and harzburgites, highly precipitation of diamonds also underlines the importance of redox depleted by the removal of melts, and that these rocks have been reactions between fluids and their mantle wall-rocks, and the need to repeatedly affected by metasomatic processes (Griffinetal.,2009and develop better tools for studying redox processes. In this paper we references therein). These fluid-mediated processes have refertilised present new data on natural silicon carbide (SiC), another highly the barren SCLM, adding some of the components originally extracted reduced phase from the deep lithosphere, and explore its implications during large-scale partial melting. These studies illustrate the impor- for redox processes in the SCLM. tance of understanding fluid-related processes in the deep lithosphere. SiC occurs in nature as the mineral moissanite, and there are several reviews of the extensive literature on its occurrence (e.g. Derkachenko et al., 1972; Kaminsky et al., 1968; Lyakhovich, 1979; ⁎ Corresponding author. Marshintsev, 1990). Moissanite has been found as inclusions in E-mail address: [email protected] (A.A. Shiryaev). diamonds (Jaques et al., 1989; Klein-BenDavid et al., 2007; Leung

0024-4937/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2010.12.011 A.A. Shiryaev et al. / Lithos 122 (2011) 152–164 153 et al., 1990; Moore and Gurney, 1989), and in mantle-derived addressed by avoiding the use of SiC-containing machinery and by magmatic rocks such as kimberlites (Bobrievich et al., 1957) and working in clean rooms. The kimberlitic moissanite grains are up to volcanic breccias (Bauer et al., 1963; Di Pierro et al., 2003; Gorshkov et 1 mm across. Most of the grains are transparent and they show al., 1995). These finds suggest that SiC may be rare but ubiquitous in at various colors, of which bluish-green is most common. As earlier least the deeper parts of the subcontinental lithospheric mantle. More observed (Derkachenko et al., 1972; Marshintsev, 1990) most grains enigmatic occurrences include high- and low-grade metamorphic are fractured, but some preserve well-formed crystallographic faces. rocks, limestones, pegmatites and chromitite pods within ophiolites Raman microspectroscopy was employed to determine the (Gnoevaja and Grozdanov, 1965; Lyakhovich, 1967, 1979; Marshint- polytypes, to assess the degree of crystalline perfection, and to sev, 1990 and references therein; Shiryaev et al., 2008a; Trumbull et identify some inclusions. Most analyses were performed using a al., 2009). These occurrences will not be dealt with here, but will be 514.4 nm laser in nearly back-scattering geometry either on polished the subject of future studies aimed at a general understanding of grains placed in epoxy for chemical analysis or on loose grains. The redox processes in the lithosphere. laser spot was 5–20 μm in diameter and the explored wavelength Grains of natural SiC often contain inclusions of highly reduced range was between 90 and 3750 cm− 1. This geometry allows rapid phases such as native Si (Marshintsev, 1990) and Fe-, Mg-, Ti-, identification of polytypes, but the relative intensities of various peaks Cr-silicides (Di Pierro et al., 2003; Marshintsev, 1990; Marshintsev cannot be compared for the polished samples since the exact et al., 1967; Mathez et al., 1995), which imply formation under orientation of the polished grains is sample-dependent. A silicon extremely reducing conditions, well below the iron-wustite buffer chip was employed for wavelength calibration. The influence of the that is commonly regarded as a lower limit on the fO2 of the mantle. If laser power on line positions is not pronounced for SiC; nevertheless, this implication is accepted, it requires a mechanism for drastically the power was kept low to reduce possible artefacts. Photolumines- lowering the fO2 of the subcontinental mantle, at least locally. cence was generally weak and posed no problems in data analysis. Alternatively, other mechanisms must be proposed and tested. In The SiC grains were cast in epoxy and polished prior to analysis. either case, moissanite can potentially provide significant insights into Major elements were analysed at GEMOC, Macquarie University, redox processes in the deep continental lithosphere. using a CAMECA SX100 electron microprobe, fitted with 5 wave- In the past, most finds of moissanite in nature have been ascribed length dispersive spectrometers, using an accelerating voltage of to industrial contamination during sample preparation. Discrimina- 15 kV and a sample current of 20 nA. The diameter of the electron tion of natural vs synthetic SiC is indeed a problem; Bauer et al. (1963) beam was b5 μm. Standards included natural minerals and synthetic noted that “the morphological, physical and chemical properties of oxides; matrix corrections were done as described by Pouchou and natural moissanite and synthetic silicon carbide agree very closely”. Pichoir (1984). Counting times were 10 s for peaks and 5 s for Several criteria have been proposed for distinguishing natural background on either side of the peak. moissanites from their synthetic analogues and the most commonly Trace-element contents of minerals were analysed at GEMOC mentioned are the polytype abundance and inferred narrow compo- using an Agilent 7500cs ICPMS attached to a 266 nm Nd-YAG laser sitional range of “synthetic” inclusions. However, many of these operating at 10 Hz; the beam diameter was varied between 40 and criteria are unsuitable for such discrimination. The composition of 150 μm depending on the type of material being analysed. The NIST inclusions in synthetic SiC varies considerably between the samples 610 glass was used as the external standard and EMP data for Si were grown by different methods. It is frequently mentioned that the used as the internal standard. The NIST 610 glass was run twice at the number of polytypes present in synthetic SiC is very high (more than beginning and the end of each analytical session and the USGS BCR-2 100), whereas moissanite is represented by very few polytypes. glass was run as an unknown to verify the quality of the data. The data However, the vast majority of the polytypes in synthetic SiC exist only were processed using the software programme GLITTER 4.4, which as unique samples and are not representative. Moreover, most of the allows the recognition and exclusion of inclusions encountered during so-called long-period polytypes are not, strictly speaking, indepen- the time-resolved analysis (www.es.mq.edu.au/GEMOC; Griffin et al., dent species, since they commonly consist of stacks of “ordinary” 2008). Data were normalised to chondrites using values recom- polytypes such as 4H and 6H. This issue is discussed, for example, by mended by GERM (http://www-ep.es.llnl.gov/germ/). For compara- van Landuyt et al. (1983). tive purposes synthetic SiC samples produced by two different However, a growing body of data (see references above) confirms methods, the Acheson and the modified Lely (sublimation) methods, the widespread existence of small SiC grains in a variety of geological were also studied. settings. Petrologists are thus faced with the problem of defining the Transmission Electron Microscopy (TEM) was used at Bayerisches mechanism(s) by which SiC might crystallise in such a wide range of Geoinstitute (BGI) for direct investigation of submicroscopic inclu- environments, most of which would not provide the extremely sions. Loose grains were mechanically crushed in ethanol and the reducing conditions implied by experimental studies of SiC stability. suspension brought to the Cu grid. A Philips FEG CM20 instrument SiC is commonly reported from kimberlites (and diamonds), and the operating at 200 keV was used. industrial-scale separation of heavy minerals from kimberlites makes it feasible to recover trace mineral phases such as SiC. We therefore have begun our investigations of natural SiC with a study of 3. Results moissanite from kimberlites. In this paper we report the results of a detailed investigation, using a 3.1. Polytypes and SiC lattice perfection range of techniques, of natural SiC grains recovered from the heavy fractions of the Mir, Aikhal and Udachnaya kimberlite pipes (Yakutia). From the mineralogical point of view one of the most interesting Data are presented on the structural perfection and the major- and features of SiC is its ability to form numerous polytypes, i.e. structural trace-element chemistry of the natural SiC grains, as well as descriptions modifications, each of which may be regarded as built by stacking and analyses of syngenetic inclusions of other phases. Finally we layers of (nearly) identical structure and composition, where different propose a tentative explanation for the formation of SiC in nature. crystallographic types differ only in their stacking sequence (Verma and Krishna, 1966). In the Ramsdell notation used in this paper to 2. Samples and methods indicate the polytype, the figure indicates the number of nonequiv- alent layers and the letter the type of lattice packing: cubic, hexagonal The moissanite grains were hand-picked from the heavy fraction or rhombohedral. The most common polytypes in synthetic SiC are of crushed kimberlite samples. Issues of contamination were carefully hexagonal 4H and 6H, rhombohedral 15R and cubic 3C. 154 A.A. Shiryaev et al. / Lithos 122 (2011) 152–164

Although many physical properties of the different polytypes are similar, some important differences do exist, e.g., the band gap varies from 2.4 eV in 3C to 3.23 eV in 2H. Such variations in properties are partly due to the different coordination of Si and C atoms in the polytypes, which induces differences in the electronic structure. It is generally believed that in case of equilibrium growth the SiC polytype is largely determined by pressure and temperature, although impurities and kinetics may also play a role. The extensive modern literature on polytypic transformations generally confirms the well- known SiC phase diagram of Knippenberg (1963), which shows that the hexagonal and rhombohedral SiC polytypes form at temperatures N1300 °C at ambient pressure. Growth of synthetic SiC by various methods (vapour, sublimation, and melt) at low temperatures always produces the cubic (3C) variety. One of the important observations for understanding the formation of natural SiC is the remarkable predominance of the hexagonal over the cubic polytype in natural occurrences: only 3 natural occurrences 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) suggest that these finds of the 3C-SiC could, in fact, reflect transformation of the 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 to reconcile with reported occurrences of 6H SiC in limestones and similar rocks. Due to differences in band gap the color of SiC is polytype- dependent. Therefore, already from optical examination the majority of natural SiC grains were identified as the green- and blue-colored 6H variety. However, doping strongly influences SiC coloration and a more reliable method should be employed to identify the polytypes. Fig. 1. Raman spectra of moissanite grains. (A) TA and TO modes of SiC grains with Raman spectroscopy is a powerful tool for this purpose and for various degrees of perfection (disorder increases from bottom to top); (B) LO mode assessing the degree of lattice disorder (Nakashima and Harita, 1997). illustrating various concentration of uncompensated impurities and zonation. Concen- Typical spectra are shown in Fig. 1A. Grains of pure 6H SiC make up tration of uncompensated dopants increases from top to bottom. The middle curve shows superposition of two domains. 55% of the Mir sample set. The second most abundant pure polytype is 15R: 8% of the Mir population. Some grains show the presence of 6H +15R mixtures in different proportions. In total the grains made of growth zonation. A SIMS study (Shiryaev et al., 2008b) has shown that pure or intermixed 6H and 15R polytypes make up 83% of the sample. some grains from Mir are zoned in terms of C and Si isotopic Similar proportions were observed for the Aikhal and Udachnaya composition; both elements are isotopically heavier in the 6H sample suites. Several grains (~5%) contain mixtures of 6H and 4H polytype than in the 15C. polytypes. The rest of the studied populations consist of various Two grains from the Mir pipe show features in the OH vibration mixtures of 6H, 8H, 15R and 21R polytypes; such crystals may be region (Fig. 3). However, the Raman spectra of these grains contain thought of as containing long-period polytypes. Disorder in SiC is not lines not related to SiC, and the H-related bands may be due to completely random; disordered regions are mixtures of randomly submicroscopic H-rich inclusions rather than structural defects (C–H disordered simple polytype domains and those containing stacking complexes) in the SiC matrix. faults distributed periodically or near-periodically. Some of the grains Second-order Raman scattering is also clearly observed between (b10%) show extremely complex spectra, indicating strongly disor- 1450 and 1950 cm− 1. The spectra are sample- and polytype- dered structures (Fig. 1A). dependent, but no attempt to extract quantitative information was The shape and position of the SiC Raman peak related to the made due to the uncertainty in the orientation of the polished grains Longitudinal Optic (or LO) phonon mode provide information about relative to the laser beam. stresses (Falkovskii, 2004) and the concentrations of charge carriers Some rare features observed in our earlier TEM study (Shiryaev et (e.g., Nakashima and Harita, 1997). Spectra of some grains show al., 2008a) resemble extended defects called micropipes, which are a stress-related features. However, the spectra of most grains show the very special type of growth dislocation with a huge Burgers vector. If presence of various levels of uncompensated charge carriers (pre- the existence of micropipes in moissanite grains can be proved, this sumably Al, B, perhaps N; see below), whose concentrations reach would indicate spiral growth, presumably on a substrate. Significant- several hundred ppm in some grains. On average, the 15R grains ly, micropipes are not observed in SiC grown for abrasive purposes by contain higher concentrations of uncompensated impurities than the the Acheson process. Indirect support for the existence of micropipes 6H ones. Many spectra show weak peaks around 640 cm− 1, which are in moissanite comes from the similarity of some defect-related tentatively ascribed to the local vibrational mode of nitrogen conoscopic figures in natural SiC (Bauer et al., 1963) with calculations substituted on a Si site (Colwell and Klein, 1972). by Presser et al. (2008). However, despite an extensive search, we The LO peak of some grains clearly reflects the superposition of at failed to observe such birefringence patterns in the grains from our least two components (Fig. 1B). This suggests the existence of collection. The birefringence of SiC grains from our collection domains with markedly different types and/or concentrations of generally indicates the presence of various degrees of stress. uncompensated dopants. Judging from the size of the laser spot this Another remarkable feature of the moissanite grains studied by heterogeneity is present at a scale of b10 μm. Cathodoluminescence TEM (this work, Shiryaev et al., 2009) is the surprisingly low density images (Fig. 2) confirm that at least some of the grains show clear of dislocations. Prolonged annealing at high temperatures may have A.A. Shiryaev et al. / Lithos 122 (2011) 152–164 155

Fig. 3. H-related bands in the Raman spectrum of a Yakutian SiC grain.

(Di Pierro et al., 2003; Gnoevaja and Grozdanov, 1965; Lyakhovich, 1979; Marshintsev, 1990; Mathez et al., 1995). However, the chemical composition and crystal chemistry of previously reported compounds differ from those reported here. Typical BSE images of inclusion-bearing kimberlitic SiC grains are shown 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 was trapped as a liquid within the growing SiC crystal. Si0 also forms networks between grains of SiC (Fig. 4C). Many of the Si0 inclusions

contain globules or rims of FeSi2 (Fig. 4D; see below); the rounded 0 interfaces between Si and FeSi2 in these complex inclusions suggest 0 that the FeSi2 has exsolved from the molten Si as an immiscible melt. A Si(C,O) phase (20–30 μm in size) is commonly found on contacts between Si lamellae and on the edges of SiC grains. This phase shows intense green (occasionally blue) CL and some grains show lamellar variations in CL brightness. In some cases the SiC partly encloses aggregates of Si(C,O) grains that show pronounced CL zoning (Fig. 2C); single homogeneous grains of the Si(C,O) phase also are found enclosed in SiC (Fig. 2A). A more complex inclusion assemblage is illustrated in Figs. 4B and 5. A semi-euhedral crystal of SiC (150×350 μm; Fig. 4B) contains three inclusions with negative-crystal forms; each consists of Si0 with blebs of

FeSi2 nucleated along the contacts. At the top of the crystal, a large irregular grain of FeSi2 is surrounded by a halo of cauliﬂower-shaped SiC grains set in a matrix of SiO2. The SiO2 also forms botryoidal outgrowths along the edge of the SiC crystal. The large FeSi2 grain has a complex 0 internal structure, with anastomosing “worms” of Ti set in the FeSi2 matrix (Fig. 5A). The microstructure strongly suggests the subsolidus 0 exsolution of Ti from the FeSi2, and the presence of subgrains within the Fig. 2. Cathodoluminescence images. (A) SiC grain showing zoning and inclusions of 0 0 0 FeSi2, deﬁned by different orientation of the Ti “worms”.TheTi metallic Si. Inclusions of FeSi2 are scattered through the Si . The upper edge of the SiC crystal is embayed by Si0, which runs between the main crystal and another mass of SiC. strongly concentrates Ni and Mn.

The bright spot on one margin of the main SiC grain is an irregular grain of the Si(C,O) The complex intergrowth of SiC and SiO2 around the FeSi2 grain phase, characterised by intense greenish CL. It is surrounded by a zone of apparent suggests a secondary replacement process, which may have oxidised alteration, with variable contents of Si, O and Mg. (B) Euhedral crystal of SiC (Aikhal-2- Si0 and to some extent SiC. The relationships between the phases in 35) showing apparent growth zoning. (C) Bright CL shows an aggregate of zoned grains of the Si–C–O phase bordering SiC (grain Aikhal-2-86). these complex grains suggest that SiC coexisted with a melt phase dominated by Si0 and Fe0; this melt was trapped as inclusions in the

SiC and also formed a grain-boundary phase. On cooling, FeSi2 appears to have crystallised (exsolved?) from the Si0, typically along contacts decreased the density of extended defects and led to exsolution of with SiC. impurities as submicron inclusions. The TEM study shows that submicroscopic inclusions of FeSi2 and the Si(C,O) phases are also dispersed in the SiC matrix (Fig. 6).

3.2. Inclusion–host relationships Electron diffraction conﬁrms the identiﬁcation of the silicide as FeSi2. Failure to detect the Raman signal of β-FeSi2 may be explained by Several SiC grains from each locality contain inclusions of other deviation of its composition from an ideal formula. Electron phases. The most common is silicon metal, followed by an iron silicide diffraction indicates that the Si(C,O) phase is not amorphous; it is

(FeSi2) and an oxygen-bearing Si(C,O) phase. The presence of Si metal poorly crystalline, but cannot be readily identiﬁed. Interestingly, the and various silicides in natural SiC has been reported previously distribution of the submicroscopic inclusions in the volume of SiC 156 A.A. Shiryaev et al. / Lithos 122 (2011) 152–164

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 exsolution 0 of FeSi2 in a negative-crystal inclusion of Si .

grains is heterogeneous (Fig. 6A, B): inclusion-rich domains coexist of Ba (N0.05 ppm) were found in only a few point analyses in the with rather perfect regions. The reason for such heterogeneity is not present study. The most plausible explanation of chemical differences yet clear, since inclusions apparently are not associated with cracks between natural and synthetic materials lies in the abundance and 0 and other defects. This relationship between SiC, Si and FeSi2 makes composition of submicroscopic accessory phases. The SiC lattice is trace-element analysis of both Si0 and SiC problematic due to the similar to diamond in its extremely low capacity for hosting probability of contamination by submicroscopic silicide inclusions. impurities, except for some substitutional elements (Al, N, and B). Therefore, most trace elements analysed in natural SiC probably 3.3. Compositions of phases reside in submicroscopic syngenetic inclusions and not in the SiC crystalline lattice. Averaged major-element compositions of SiC and included phases The contents of most trace elements in the natural SiC grains are are given in Table 1; trace-element data are summarized in Table 2. low (1–200 ppm), but several observations can be made. SiC grains from Mir are generally lowest in trace elements, and those from 3.3.1. Moissanite Udachnaya are highest. Contents of Al, Mg, Ti and Zr show wide Moissanite is typically quite a pure stoichiometric SiC, with mean ranges, while B contents are more homogeneous. The low contents of Si contents of 73–75 wt.%. Oxygen contents vary between 0.0–0.43 wt. chalcophile elements (Cu, Zn, Mo, Pb, and Ni) could indicate that a

%. The irregular grains around the FeSi2 in Fig. 4B are an exception, sulfide phase coexisted with the SiC but has not been trapped as containing 0.25 wt.% Fe and nearly 3% O (Table 1). In the synthetic inclusions. The REE patterns of natural SiC are poorly defined because samples only Si and C were present at levels detectable by EMP. The most analyses are below the minimum detection limits (MDL; cf synthetic SiC also contains only a few trace elements (B, Al, Sc and Ba) Acheson SiC, Table 2), but the available data suggest a flat or weakly at levels above the LAM-ICPMS detection limits (Table 2). These negative slope in a chondrite-normalised plot (not shown). Overall impurities (as well as N) are typical of synthetic SiC. The levels of Al in REE contents are higher in the samples from Aikhal and Udachnaya, the synthetic SiC are higher than observed in natural samples from whereas the MREE–HREE is below the detection limits in SiC from Mir. Mir and Aikhal, but lower than the mean values in SiC from There is no obvious correlation between the trace element Udachnaya. Mg contents in the synthetic SiC are below detection chemistry of the moissanite matrix and its polytype composition (b0.4 ppm) but the natural samples typically contain 1–8 ppm Mg. and/or degree of lattice perfection as determined by Raman The major- and trace-element data therefore define some clear spectroscopy. However, on the basis of limited statistics, a tentative differences between the synthetic SiC and the populations extracted conclusion is that the 15R grains are somewhat richer in trace from the kimberlites. elements than the more abundant 6H. The 15R SiC also shows larger The presence of incompatible elements such as Ba in natural SiC scatter in C and Si isotopic ratios than the 6H (Shiryaev et al., 2008b). was noted by Gnoevaja and Grozdanov (1965), but detectable levels These observations might suggest that impurities stabilize certain SiC A.A. Shiryaev et al. / Lithos 122 (2011) 152–164 157

0 Fig. 5. Exsolution phenomena. (A) Map of TiKα distribution showing exsolution of Ti in FeSi2 grain attached to SiC (grain 1–10; see Fig. 4A). B–D: X-ray distribution maps of O, Si and 0 Ni show intergrowths of SiC and SiO2 around the FeSi2 grain, and the concentration of Ni in the exsolved Ti ;E–F — maps of Mn and Fe distributions. polytypes (Tairov and Tsvetkov, 1983), but more complete data are available analyses suggest that the intrinsic trace-element contents of required to address this question. the Si0 are extremely low.

3.3.3. Iron silicide 3.3.2. Metallic Si The iron silicide phase shows considerable compositional vari- Raman spectroscopy and EMP analysis provide unambiguous ability. In all but two analyses, Fe ranges from 43.1–46.4 wt.% (mean conﬁrmation of the presence of metallic Si. A slight shift of the 45.2–45.4 in Mir and Aikhal; 43.1–44.1 in Udachnaya); Si ranges from Raman peak of Si indicates a residual pressure of several kbar 46.8–56.1 wt.% (mean from 50.8–62.0). Ti varies widely; many grains (Shiryaev and Kagi, unpublished). Metallic Si contains an average contain 2.5–3.8 wt.% Ti, while others contain b0.5 ppm. X-ray maps 98.4–99.2 wt.% Si and 0.34 wt.% O. Most LAM-ICPMS analyses of the suggest that the pre-exsolution Ti content of the Fe–Si phase shown in Si0 show measurable contents of a range of elements, but these Fig. 6 was signiﬁcantly higher. Ni (LAM-ICPMS) varies from b1 ppm to analyses may be contaminated by minute inclusions of FeSi2. The N800 ppm, and Mn from 1.5 ppm to 1.3 wt.%. The calculated structural 158 A.A. Shiryaev et al. / Lithos 122 (2011) 152–164

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) — iron silicide inclusions in SiC matrix; (D) SiC matrix with a silicide inclusion. At least two stacking faults (other SiC polytypes) are observed (marked byanarrow).

formula for nearly all analyses is (Fe, Ti, Mn, Cr, and Ni) Si2. This is localities. All analyses of FeSi2 from Mir show a striking negative Eu clearly different from the Fe3Si7 composition reported as inclusions in anomaly; Eu is below detection limits (0.02 ppm) in all analyses, even moissanite by Di Pierro et al. (2003), which contains N5% Mn and little those with several hundred ppm of Nd and Gd. Negative anomalies

Ti. A second Fe–Si phase was found as an inclusion in one SiC grain also are apparent in Sm, Y and Yb. The patterns of the Aikhal FeSi2 from Udachnaya (Table 1); it has a stoichiometry close to (Fe,Ti)Si3. grains are similar, but the negative anomalies in Eu are less deep, and The FeSi2 phase has high contents of many trace elements; it Y shows minor negative or positive anomalies, or none. The FeSi2 appears to concentrate most of these elements in the system, and thus grains from Udachnaya show a range of negative Eu anomalies; only may give an indication of the nature of the environment in which the one grain has a negative Yb anomaly, and the same grain also has a

SiC and its associated phases formed. Most of the analysed FeSi2 grains weakly negative Y anomaly. Two grains (those with the least negative are smaller than the laser beam, and the analyses are contaminated to Eu anomalies) have small positive Ce anomalies. Within each locality, varying degrees by Si0 or SiC; this is clearly seen as a deﬁcit in Fe the depth of the Sm, Y and Yb anomalies varies from grain to grain, but relative to the EMP analyses. Since the SiC and Si0 have very low trace- the average values for the anomalies in Sm and Yb are similar between element contents, the overall patterns of the different analyses are localities (Fig. 7A). parallel to one another, although they may differ in absolute abundance by up to 3 orders of magnitude. The average patterns 3.3.4. Si–C–O phase shown in Fig. 7A therefore were constructed by averaging the 3–5 Most grains of the Si–C–O phase contain 55.2–57.7% Si (mean 56.3) analyses with highest Fe contents in each locality, and normalising to and 8.3–20.8 wt.% O; we assume that the balance (12.0–32.4 wt.%) is

45% Fe. These averages show enrichment of the LREE over the HREE, made up of C. The stoichiometry of this phase appears to be close to Si4 with unusual negative anomalies in Sm, Eu and Yb. Zr levels are (C,O)7, which would require a mixed valence (between 3 and 4) for Si. strikingly high; Zr/Hf ranges from subchondritic in some grains to EMP analysis shows that parts characterised by darker CL (see above) suprachondritic in others; on average both Zr/Hf and Nb/Ta are close contain minor amounts of Na (0.1–0.2 wt.%) and Al (0.2–0.3 wt.%). to the chondritic values. Qualitative EDX analysis in the TEM of microscopic inclusions of

In detail, the chondrite-normalised REE patterns of the FeSi2 compositionally similar phases conﬁrms the occasional presence of Al (Fig. 7A) show signiﬁcant variations within each locality, and between and K. Most likely, this phase belongs to the rich family of silicon A.A. Shiryaev et al. / Lithos 122 (2011) 152–164 159

Table 1 Means and ranges for moissanite matrix (SiC) and principal included phases — EMP data.

Source

SiC Mir Aikhal Udachnaya

Mean, n=11 Range Mean, n=21 Range Mean, n=22 Range

Na 0.003 0–0.005 0.002 0–0.007 0.002 0–0.01 Cr 0.003 0–0.012 0.004 0–0.01 0.003 0–0.01 Mn 0.007 0–0.02 0.008 0–0.02 0.004 0–0.01 Fe 0.050 0–0.26 0.018 0–0.05 0.007 0–0.04 Al 0.002 0–0.01 0.035 0–0.15 0.009 0–0.02 Mg 0.003 0–0.01 0.003 0–0.01 0.005 0–0.01 Si 72.65 69.3–74.1 74.64 73.3–76.5 73.75 72.6–74.7 K 0.003 0–0.01 0.003 0–0.01 0.003 0–0.02 Ca 0.002 0–0.01 0.002 0–0.01 0.009 0–0.02 Ti 0.003 0–0.01 0.001 0–0.01 0.002 0–0.01 Ni 0.006 0–0.02 0.006 0–0.03 0.004 0–0.03 O 0.379 0–2.9 0.195 0–0.5 0.358 0–0.87 Total 73.11 69.3–75.5 74.92 73.5–76.8 74.15 73.1–76

Si metal Mir Aikhal Udachnaya

Mean, n=9 Range Mean, n=15 Range Mean, n=6 Range

Na 0.001 0–0.005 0.001 0–0.01 0.000 0–0.001 Cr 0.004 0–0.10 0.003 0–0.013 0.003 0–0.007 Mn 0.007 0–0.25 0.005 0–0.027 0.003 0–0.012 Fe 0.059 0.009–0.28 0.122 0–0.49 0.379 0–1.56 Al 0.002 0–0.007 0.002 0–0.004 0.021 0–0.122 Mg 0.003 0–0.007 0.007 0–0.027 0.005 0–0.012 Si 98.44 98–99 99.13 97.7–99.7 98.98 97.6–99.5 K 0.002 0–0.006 0.002 0–0.014 0.003 0–0.007 Ca 0.002 0–0.01 0.003 0–0.019 0.023 0–0.14 Ti 0.009 0–0.06 0.004 0–0.013 0.007 0–0.023 Ni 0.005 0–0.02 0.008 0–0.05 0.009 0–0.028 O 0.338 0.17–0.47 0.490 0.26–1.55 0.401 0.33–0.57 Total 98.87 98.4–99.5 99.78 99.2–100.6 99.83 99.3–100.3

FeSi2 Mir Aikhal Udachnaya Mean, n=11 Range Mean, n=20 Range Mean, n=3 Range

Na 0.002 0–0.012 0.009 0–0.009 0.004 0–0.01 Cr 0.039 0.001–0.06 0.029 0.01–0.047 0.032 0.03–0.034 Mn 0.252 0.005–1.31 0.096 0.011–0.15 0.539 0.11–1.32 Fe 45.42 43.6–46.4 45.15 44.0–45.9 43.73 43.1–44.1 Al 0.005 0–0.019 0.039 0–0.12 0.002 0–0.007 Mg 0.003 0–0.026 0.009 0–0.1 0.007 0–0.018 Si 50.78 46.8–55.4 54.43 52.6–56.1 54.29 51.5–55.8 K 0.002 0–0.014 0.007 0–0.012 0.007 0–0.016 Ca 0.003 0–0.009 0.007 0–0.011 0.005 0–0.008 Ti 2.01 0–4.95 0.652 0–3.2 1.207 0–3.6 Ni 0.134 0.02–0.33 0.052 0.002–0.09 0.045 0.032–0.061 O 0.499 0.41–0.66 0.520 0.3–1.69 0.517 0.35–0.65 Total 99.17 96.8–100.9 101.01 98.4–102.4 100.39 100–101.1

SiCO phase(s) Mir Aikhal Udachnaya

Mean, n=2 Range Mean, n=15 Range Mean, n=3 Range

Na 0.12 0.07 0.02–0.17 0.10 0.01–0.21 Cr 0.00 0.00 0–0.01 0.00 0–0.01 Mn 0.00 0.01 0–0.02 0.02 0–0.02 Fe 0.02 0.41 0–5.69 0.02 0.01–0.03 Al 0.15 0.17 0.05–0.39 0.32 0.02–0.7 Mg 0.01 0.00 0–0.01 0.01 0–0.016 Si 56.07 56.43 56–57.5 56.75 56.8–57.7 K 0.00 0.00 0–0.01 0.01 0.01–0.02 Ca 0.02 0.02 0–0.03 0.07 0.02–0.13 Ti 0.00 0.01 0–0.18 0.00 0.00 Ni 0.01 0.01 0–0.02 0.01 0–0.01 O 11.19 13.36 7.4–17.4 16.29 13.8–20.8 Total 67.64 70.49 64.4–74.3 73.59 70.6–78

oxycarbides (Si4C4 − xO2-like compounds) commonly formed on the chemical conditions of oxidation and it is not surprising that natural surfaces of oxidised SiC (see recent review by Presser and Nickel, and synthetic compounds should differ from each other, nor that the 2008). In experiments on SiC oxidation such phases have been analyses should show a wide range of C/O. observed not only on surfaces, but also in the interiors of SiC grains The few Si4(C,O)7 grains that could be analysed by LAM-ICPMS (Lavrenko et al., 1981). The exact composition of the oxycarbide show high levels of trace elements and smooth REE-HFSE patterns formed during such oxidation processes depends on the physio- (Fig. 7B) with pronounced negative Sr and Eu anomalies; one analysis 160 A.A. Shiryaev et al. / Lithos 122 (2011) 152–164

Table 2 LAM-ICPMS Analyses of Trace Elements (ppm).

Acheson SiC Mir SiC Aikhal SiC Udachnaya SiC Mir FeSi2 Aikhal FeSi2 Udachnaya FeSi2 Mir Aikhal Si–C–O Udachnaya Si–C–O Si–C–O Mean, Mean, Mean, Mean, Mean, Mean, Mean, n=4 n=44 n=4 n=4 n=11 n=5 n=5

B 2.8 2.0 2.2 1.3 8.1 7.2 4.0 1.08 b0.64 4.1 8.6 Mg b0.4 1.9 0.96 1.2 148 133 39 b0.95 2.6 124 5773 Al 174 46 85 511 71 289 74 22 25 274 11,992 Sc 21 5.9 14 6.4 14 13 7.5 6.4 11 8.6 17 Ti b0.5 7.0 5.6 5.7 2178 469 1751 1.3 46 284 1050 V b0.1 1.1 98 18 53 b0.23 1.6 8.5 25 Cr b1.1 2.1 1.8 2.0 79 61 64 3.8 8.5 27 88 Mn b 0.2 1.2 1.6 211 334 819 b0.55 73 91 87 Fe b7 276 223 45,000 45,000 45,000 b18.3 2899 13,383 57,400 Co b0.7 0.43 0.70 37 20 19 0.19 0.41 3.6 13 Ni b0.15 0.79 137 77 87 b0.42 3.4 13 81 Cu b0.1 1.1 0.76 44 24 58 b0.33 11 50 131 Zn b0.25 1.7 0.35 24 32 14 b0.66 0.67 15 572 Ga b0.15 0.80 0.40 11.8 7.2 5.7 b0.45 3.9 21 6.7 Ge b0.27 1.5 0.62 10.2 5.2 3.6 b0.71 2.6 1.9 3.1 As b0.3 5.2 2.2 1.4 b0.66 0.83 1.9 6.0 Ba 0.10 0.53 0.87 0.48 b0.28 b0.24 1.54 53 La b0.01 0.28 13 59 17 b0.015 371 225 172 Ce b0.01 0.54 0.59 861 173 381 b0.020 340 2069 420 Pr b0.01 0.08 0.14 1.2 0.9 1.8 b0.014 49 32 45 Nd b0.04 0.08 0.36 6.3 2.5 4.6 b0.103 170 122 227 Sr b0.01 35 5.7 375 b0.033 0.06 1.1 48 Sm b0.04 2.4 10 1.0 b0.088 13 28 38 Eu b0.01 122 710 126 b0.028 0.04 0.13 0.33 Gd b0.03 0.11 431 693 1088 b0.077 21 18 17 Tb b0.01 0.02 57 83 45 b0.026 1.9 1.5 2.0 Dy b0.02 0.10 199 354 179 0.068 11 5.8 11 Ho b0.01 3.4 5.5 7.7 b0.014 1.5 0.84 1.6 Y b0.01 b0.01 0.03 0.17 b0.047 14 31 61 Er b0.015 27 22 22 b0.034 3.0 1.6 4.5 Yb b0.02 3.2 2.4 2.4 b0.053 0.09 b0.09 3.1 Lu b0.01 13 15 10 b0.014 0.15 0.21 0.7 Zr b0.03 0.24 0.51 2.2 1.8 1.3 b0.081 31 202 159 Hf b0.04 4.3 4.7 3.7 b0.084 0.81 6.6 3.1 Nb b0.03 0.43 0.31 0.18 0.14 b0.080 0.29 1.7 4.1 Ta b0.01 0.02 0.28 0.46 0.22 b0.012 0.03 0.20 0.28 Pb b0.02 0.07 0.86 17 5.7 12 b0.049 1.7 7.7 586 Mo b0.3 0.31 0.28 0.10 0.15 b0.69 0.69 3.4 4.8

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. shows a negative Yb anomaly. Zr/Hf and Nb/Ta are chondritic to especially the Fe-silicides, which concentrate trace elements. Several slightly super-chondritic. The chalcophile elements are present in factors indicate that the moissanite grains from the kimberlites were high but variable amounts. Given the aggregate nature of the formed at high temperatures and elevated pressures: a) exsolution of individual inclusions of the Si–C–O phase(s) (Fig. 2C) it is highly Ti from FeSi2; b) precipitation of FeSi2 from metallic Si; c) clearly probable that the LAM-ICPMS analyses represent mixtures of two or observed oxidation of SiC. In addition, the strongly negative Eu more phases. anomalies require that, at least locally, the oxygen fugacity was low.

The FeSi2 phase clearly accepts highly charged small ions such as 4. Discussion the REE and HFSE. The REE patterns with deep negative anomalies in Eu and Sm suggest a strongly reducing environment, in which It is not surprising that the authors of most reports on natural SiC essentially all Eu, and much of the Sm, has been reduced to the 2+ have faced serious problems explaining their findings. The formation of state and entered another phase. However, the great variability in the natural SiC is usually ascribed to hydrothermal activity (Lyakhovich, degree of Eu-, Sm- and Yb depletion suggests that the redox state also 1979; Marshintsev, 1990), serpentinisation (Mathez et al., 1995), or varied even during the crystallisation of the SiC in the different unspecified HPHT processes (e.g., Trumbull et al., 2009). Some natural localities. The apparent lack of Eu and Sm anomalies in the moissanite moissanite grains have been ascribed to disintegrated meteorites, since further suggests that the SiC does not discriminate against Eu2+ or minute SiC grains are present in some chondrites (Bernatowicz et al., Sm2+. Without samples of other coexisting phases, it is not clear 1987). However, meteoritic provenance cannot be very common even where the Eu and Sm are concentrated in this environment. for non-kimberlitic SiC grains, because of contrasting C isotope If the SiC and FeSi2 precipitated from a fluid phase, the residual compositions, the large size of the terrestrial grains and the predom- fluid might be left with a strong positive Eu anomaly, which might inance of hexagonal polytypes in such grains. impose on other metasomatised domains, depending on the mass The results of this study allow distinction of natural from synthetic balance (fluid/rock ratios). Griffin and O'Reilly (2007) have noted that moissanites. The most obvious discrimination criterion is the many Cr-garnets from cratonic mantle peridotites show positive (as markedly different trace element chemistry, which reflects differ- well as negative) Eu anomalies that cannot be reconciled with the ences in the composition of the growth media. As noted above, the former presence of plagioclase, the usual explanation for Eu anomalies trace-element composition of the bulk SiC in natural samples reflects in mantle-derived rocks. The inferred positive Eu anomaly of the the composition and abundance of microinclusions of other phases, fluids residual from the crystallisation of SiC (+ FeSi2) may offer an A.A. Shiryaev et al. / Lithos 122 (2011) 152–164 161

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. explanation for Eu anomalies in deep mantle rocks, independent of operate in nature are in space and during impact-related events. subduction processes. Another synthetic process, liquid epitaxy, proceeds at geologically

The overall trace-element patterns of the FeSi2 and the Si4(C,O)7 reasonable temperatures (N800 °C). Besides epitaxial growth on a SiC- phase are broadly similar to those of many fibrous diamonds (Fig. 7A; substrate, spontaneous SiC crystallization has also been achieved. Araujo et al., 2008; Rege et al., 2005, 2010; Weiss et al., 2008). The However, this process requires the presence of a Si-saturated metal patterns of such diamonds reflect the presence of inclusions melt such as Fe–Al–Si or Al–Sn (e.g., Chaussende et al., 2001; representing a range of saline to hydrous to carbonatitic high-density Derkachenko et al., 1972; Yakimova et al., 1996). Usually such fluids, which may be derived by low-degree melting of carbonated processes lead to the formation of 3C-SiC, although early experiments peridotites and eclogites (Weiss et al., 2008). The trace-element data by Ellis (1960) allowed speculation that certain metals might stabilize suggest that the formation of the kimberlitic moissanite and its the hexagonal polytypes. The reaction of diamond with Si-rich melt associated inclusions could be related to similar high-density, low- can lead to crystallisation of oriented SiC on the diamond (Varshavskii volume fluids. However, thermodynamic calculations (Mathez et al., and Shulpyakov, 1967). Similar processes might be operative in 1995) and experiments (Ulmer et al., 1998) show that in the Mg– certain exotic mantle domains and could be responsible for 3C-SiC Si–C–O system (a proxy for metasomatism in a mantle setting), SiC inclusions in diamonds, but are unlikely to be common. In addition, occurs only under highly reducing conditions: 4–7 log units below the several papers report SiC formation from low-temperature solutions iron-wustite (IW) buffer. in the presence of strong reducing agents, but such experiments are Before discussing possible mechanisms of moissanite growth in mostly laboratory curiosities. the lithosphere, we must briefly review the main methods of Occurrences of moissanite in very different geological settings synthetic production of SiC. The most common (Acheson and probably indicate that several mechanisms could be responsible for sublimation) processes require very high temperatures (N2000 °C) moissanite formation. We believe that two possibilities are the most and gas flow (Tairov and Tsvetkov, 1978). Chemical-vapour deposi- important: 1) growth in deep mantle during interaction of Si (or C)-rich tion (CVD) is used to grow SiC films, most commonly by the metal melts with carbonaceous (or siliceous) materials; and 2) redox decomposition of SiCl4, but the only plausible niches where it could reactions. These mechanisms are certainly viable from thermodynamic 162 A.A. Shiryaev et al. / Lithos 122 (2011) 152–164 point of view and experiments have shown that SiC could indeed be produced in these ways. Redox reactions such as serpentinisation can produce 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 to many reduced phases, may survive changes in conditions. Numerous reports of SiC in association with carbonates (Gnoevaja and 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 of moissanite in relatively oxidised carbonate-silicate melts. Carbonate- rich fluids are as an important metasomatic agent in the deep lithosphere, 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 and Walker, 2006; Kavner et al., 2007) suggests that electrochemical Fig. 8. Generalised scheme of electrodeposition of SiC (adapted from Moller et al., reactions under high pressures might be responsible for a range of 1997). A — in the absence of an external electric field; B — in the presence of an external redox reactions under mantle conditions. Several studies have argued field. that electrochemical processes are important in the genesis of sulfide ores (e.g., Mironov et al., 1981; Moller and Kersten, 1994; Moller et al., 1997; Nyussik and Komov, 1981; Yakhontova and Grudev, 1978). “broken” grains are rather common among SiC crystals grown Other experimental studies have demonstrated the possibility of from experimental melts. Once formed, many of the moissanite forming silicon carbide in the course of electrochemical processes in grains must have been subjected to oxidation or etching, leading systems containing carbonates and silicates (Devyatkin, 2003; Devyat- to the formation of silicon oxycarbides and oxides. For silicon kin et al., 2002; Elwell et al., 1982). In these experiments the SiC layers carbide, the oxidation kinetics are generally slow and the were deposited on several types of electrodes at temperatures as low as oxidation rate depends strongly on polytype, crystallographic

720 °C, from carbonate melts containing a few percent of SiO2.The orientation and type of surface (C- or Si-face; Presser and Nickel, deposited layers consist of hexagonal SiC, as confirmed by XRD, 2008). Therefore, a substantial fraction of moissanite grains may sometimes with excess Si or C. The reaction leading to SiC formation survive post-growth alteration, though with some modifications of by electrodeposition is described as M2CO3 +SiO2 =M2O+SiC+2O2, shape. where M=Li, Na, and K. The minimum temperature of the process is This work has some important implications for the geochemistry determined by the melting point of the carbonate mixture. The and petrology of the deep lithosphere. First of all, we can confirm that electromotive force at which these reactions proceed depends on moissanite is indeed a natural mineral and can be present in different electrode composition and is between 0.5 and 1.6 V. SiC electrodepo- lithologies. Second, this work shows that redox processes in the sition also has been achieved in the presence of pressurised CO2 over the lithospheric mantle may lead to the generation of locally highly melt, following reactions such as CO2 +M2SiO3 =M2O+SiC+O2 reduced conditions with oxygen fugacity reaching 5–7 log units below (Devyatkin, 2003; Devyatkin et al., 2002). Electrolysis of silumin the IW buffer. Third, the consistency of the moissanite properties with (Al alloyed with a few percent of Si) in carbonate melts at 500 and an electrochemical mechanism for its synthesis may indicate that 600 °C has also produced SiC layers (Sannikov et al., 1993). Moreover, electrochemical redox processes are common in natural settings. They the experiments at 600 °C produced hexagonal plate-like crystallites, could be responsible for the formation of other reduced phases such as 0 similar to those observed among moissanites. Fe3C and Fe (Bulanova and Zayakina, 1990; Shiryaev et al., 2010)by To the best of our knowledge, electrodeposition is the only process creation of locally reduced environments within the relatively documented as leading to the formation of hexagonal SiC at low oxidised deep lithospheric mantle. temperatures; this may shed light on the still unsolved problem of SiC polytype genesis (e.g., Tairov and Tsvetkov, 1983). The electrodepo- 5. Conclusions sition mechanism for the formation of natural SiC can overcome the requirement of highly reducing conditions and temperatures in This investigation of the chemistry, defects and inclusions of excess of 1300 °C. Electrochemical processes may operate in many moissanite (SiC) grains separated from kimberlites shows general geochemical environments. External electric fields could be produced, similarities between three sample sets and marked differences with for example, by deformation processes or by percolation of fluids synthetic silicon carbide produced by various processes. The moissa- through porous media (Gershenzon et al., 1993; Gokhberg et al., nite found in kimberlites is a naturally occurring mineral that 2007; Guglielmi, 2007; Hunt et al., 2007; Sgrigna et al., 2004). crystallised at high temperatures and elevated pressures, probably However, the presence of the external fields is not always within the stability fields of diamond and molten Si. The trace- necessary. Virtually all conducting minerals that contain oxidising element chemistry of the growth medium, as reflected in trapped components could serve as electrodes with fluids acting as electro- inclusions of Fe-silicides, is similar to those of the different high- lytes, and such conditions might be locally realized in many different density fluids involved in the growth of diamond. Many features of the settings. The interaction of carbonatitic melts with other mantle studied moissanite grains are consistent with its formation by phases seems to be a very promising mechanism, since their electrical localised electrochemical processes within a relatively oxidised conductivities are very different (Gaillard et al., 2008). Other environment, from carbonate-silicate melts similar to those that possibilities include contact between melts with different oxygen may have precipitated many diamonds. fugacities and/or water contents. Solid minerals in the immediate vicinity of such contacts may serve as nucleation sites for SiC. Very Acknowledgements generalised schemes of the processes leading to SiC elecrodeposition are shown in Fig. 8. AAS is grateful to the Humboldt Foundation and to Russian The extreme rarity of completely euhedral SiC grains may be President grant MK-147.2007.5 for partial financial support. This partially due to detachment from the original substrate. Similar study used instrumentation funded by ARC LIEF and DEST A.A. Shiryaev et al. / Lithos 122 (2011) 152–164 163

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