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Rock-forming moissanite (natural -silicon carbide)

¹ Department of Geosciences, Mineralogy and Petrography, University of Fribourg, Switzerland
² Institute for Geology, Baltzerstrasse 1-3, University of Bern, Switzerland
³ Laboratory of Chemical and Mineralogical Crystallography, University of Bern, Switzerland.
⁴ Department of Earth Sciences, ETH Zürich, Switzerland

We report the first occurrence of moissanite (SiC) as a rock-forming mineral (8.4 vol%) in one unique specimen of a terrestrial rock. The sample has a homogeneous, porphyritic texture, and was found as a beach pebble thought to be derived from a Tertiary volcanic province of the Aegean Sea region. The matrix is bluish-colored and consists of very fine-grained brucite, calcite, and magnesite, in which macrocrysts of quartz (25.3 vol%) and moissanite are found. Other accessory phases are phlogopite-3T, magnesiochromite, an Fe-rich phase, Cl-bearing brucite, Al-rich orthopyroxene, and unidentified MgFe-silicates (4 vol%). The bulk-rock composition shows a "kimberlitic" chemistry (55.8 wt% SiO₂, 28.5 wt% MgO, 1.4 wt% CaO, 18.1 wt% LOI). Colorless gemmy, and blue or black moissanite crystals are subhedral and display characteristic hexagonal symmetry (6H polytype). Most moissanite grains contain metallic Si and Fe-silicide (Fe₃Si₇) inclusions, and more rarely, other Fe-silicides with varying amounts of Al (24.5 wt%), Ca (8.0 wt%), Mn (6.8 wt%), Ti (16.3 wt%), and Ni (2.6 wt%). The ¹³C value of the moissanite is –28.1. According to available data, the fs_O2 stability field of SiC is five to six log units below the iron-wüstite (IW) buffer curve. Therefore, the observed Fe-bearing silicates cannot have been equilibrated with SiC under ambient pressure. Instead, our finding indicates that the rock most likely formed at the ultrahigh-pressure conditions of the upper mantle or transition zone.

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