Enigmatic super-reduced phases in corundum from natural rocks: Possible contamination from artificial abrasive materials or metallurgical slags

Abstract

Recently a number of reports claimed enigmatic appearance of high-pressure and super-reduced minerals in ophiolitic chromitite and peridotite. Diamond, moissanite, various metal alloys, and native metals, carbides and nitrides were found in mineral separates from bulk rock probes of chromitite and peridotite from Tibet, Polar Ural and other localities. Similar findings of super-reduced phases were reported for pyroclastic rocks and alluvial deposits of Mt. Carmel in Israel. We performed the study of microinclusions in corundum grains from abrasive materials produced industrially in an electric arc furnace and found that they are very similar to microinclusions in corundum grains from natural samples. The key similar phases are Ti3+4Al2(Zr,Ti4+)4O11 carmeltazite, Ti2O3 tistarite, TiN1-x, TiC1-x, FeSi and Fe-Si-Ti alloys, hibonite, grossite, anorthite, and residual feldspatic glass. Although some differences between abrasive corundum and corundum from Tibet and Mt. Carmel are obvious, the morphology of melt pockets and amount of mineralogical similarities is more than critical to suggest that they can be of the same origin. The additional possible source of corundum grains with super-reduced inclusions is various Al-bearing slags after aluminothermic reactions during steelmaking. Moreover, taking into account the spectacular similarity of diamonds from ophiolite with synthetic diamonds, we claim for a thorough reconsideration of ultrahigh-pressure and super-reduced phases in natural rocks and argue that we need to find criteria for discrimination between natural and artificial samples. One of the strongest criteria would be textural and structural features that clearly demonstrate the indigenous character of the host minerals.