Abstract
Moissanite, SiC, is an uncommon accessory mineral that forms under low oxygen fugacity. Here, we analyze natural SiC from a Miocene tuff-sandstone using synchrotron Laue microdiffraction and Raman spectroscopy, in order to better understand the SiC phases and formation physics. The studied crystals of SiC consist of 4H- and 6H-SiC domains, formed from either, continuous growth or, in one case, intergrown, together with native Si. The native Si is polycrystalline, with a large crystal size relative to the analytical beam dimensions (>1–2 μm). We find that the intergrown region shows low distortion or dislocation density in SiC, but these features are comparatively high in Si. The distortion/deformation observed in Si may have been caused by a mismatch in the coefficients of thermal expansion of the two materials. Raman spectroscopic measurements are discussed in combination with our Laue microdiffraction results. Our results suggest that these SiC grains likely grew from an igneous melt.
Highlights
IntroductionSiC may be the archetypal polytypic material (e.g., polymorphs only differ in the stacking sequence of identical sheets or structural units) with greater than 250 known synthetic polytypes, ~11 of which are reported as naturally occurring [1]
SiC may be the archetypal polytypic material with greater than 250 known synthetic polytypes, ~11 of which are reported as naturally occurring [1]
Initial indexing attempts focused on the 4H-SiC polytype, which we anticipated based on previous TEM and Raman spectroscopic work [41]
Summary
SiC may be the archetypal polytypic material (e.g., polymorphs only differ in the stacking sequence of identical sheets or structural units) with greater than 250 known synthetic polytypes, ~11 of which are reported as naturally occurring [1]. The polytypes are typically described with a number that refers to the number of layers that repeat along the stacking direction, and a letter that refers to the crystal system [2]. They generally lie on a continuum between the wurtzite (2H) and zinc-blende (3C). Transformation from one polytype to another is thought to occur by periodic slip around dislocations [4], diffusional rearrangement with the nucleation and expansion of stacking faults [5], and/or changes in Si/C ratios during crystallization [6]
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