Two new minerals, badengzhuite, TiP, and zhiqinite, TiSi2, from the Cr-11 chromitite orebody, Luobusa ophiolite, Tibet, China: Is this evidence for super-reduced mantle-derived fluids?

Titanium minerals enclosed in corundum separated from the Cr-11 orebody include native Ti, zamboite (FeTiSi2), osbornite (TiN)-khamrabaevite (TiC) solid solutions, and jingsuiite (TiB2), as well as the new minerals badengzhuite (TiP) and zhiqinite (TiSi2) and two potentially new minerals, Ti11(Si,P/10 and Ti10(Si,P,/7, where indicates a vacancy. These minerals together constitute a spheroid 20 μm across inferred to have crystallized from a droplet of Ti-Si-P intermetallic melt. Energy-dispersive spectroscopy and three-dimensional electron diffraction were applied to characterize the two new minerals. Badengzhuite has a primitive hexagonal cell with a D 3.49.7/A, c = 11.70.23/A, V = 124.4/A3, and crystallizes in space group P 63=mmc (Z = 4). It is isostructural with synthetic TiP. Two EDX (energy dispersive X-ray spectroscopy) analyses of badengzhuite gave 60.56 wt %Ti and 39.44 wt % P and 62.74 wt % Ti and 37.26 wt % P from which an empirical formula of Ti1.020P0.980 was calculated on the basis of two atoms (ideally TiP). Zhiqinite has a primitive orthorhombic cell with a = 8.18.16/A, b = 4.85.10/A, c = 8.42.17/A, V = 334.12/A3, and crystallizes in space group Fddd (Z = 8). It is isostructural with synthetic TiSi2 (C54 type). Four EDX analyses of zhiqinite gave 39.58-44.79 wt % Ti and 55.21-60.42 wt % Si, from which an empirical formula of Ti0.905Si2.095 was calculated on the basis of three atoms (ideally TiSi2). We suggest that interaction of mantle-derived CH4 C H2 fluids with basaltic magmas in the shallow lithosphere (depths of ∼ 30-100 km) under conditions more reducing than 6 log units below the oxygen fugacities corresponding to the iron-wüstite buffer resulted in precipitation of corundum that entrapped intermetallic melts, some of which crystallized to ultra-reduced Ti-P-Si phases. Experimental work on the Ti-Si and Ti-P systems indicates that the minerals enclosed in corundum could have crystallized from the alloy melt at the lowest temperature accessible on the liquidus. It has been alleged that these ultra-reduced phases are anthropogenic contaminants inadvertently introduced with fused alumina abrasive during preparation of mineral separates. Nonetheless, we conclude that the differences between the ultra-reduced minerals in the separates and the ultra-reduced phases in fused alumina are more convincing evidence for these minerals having a natural origin than the similarities between them are evidence for an anthropogenic origin.