OBSERVATIONS ON MAGNESITE INCLUSIONS IN NATURAL SILICON-VACANCY GEM DIAMONDS

Magnesite (MgCO₃), as an inclusion in diamond, has important implications for the chemistry of the Earth’s mantle, possible carbon reservoirs, and potential diamond formation mechanisms. While models predict magnesite can occur throughout much of the mantle, only a few magnesite inclusions have been positively identified in diamond. We observed abundant magnesite inclusions in three near colorless, natural gem diamonds in association with peridotitic olivine and graphite inclusions. Unlike previously reported occurrences, all three of these diamonds also contained Si-V defects. Although silicon impurities have been observed in natural diamonds before, it remains a very rare occurrence and the origin of the Si and the growth environment are unclear. Raman and photoluminescence spectroscopy were used characterize these three diamonds and their inclusions in an effort to provide some insight into their history. To the author’s knowledge, this is the first reported simultaneous occurrence of these impurities in diamond. Coexistence of magnesite and olivine in diamond suggests a very narrow range of ƒO₂ conditions near the highest ƒO₂ under which diamond is stable (EMOD – Enstatite-Magnesite-Oxygen-Diamond buffer) in peridotitic mantle. Given the rarity of both Si impurities and magnesite in natural diamond, the possibility that the unusual ƒO₂ conditions under which these diamonds likely formed may have also played a role in the incorporation of Si-V defects into the diamond lattice is explored.