Abstract
Abstract. 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=3.49(7) Å, c=11.70(23) Å, V=124(4) Å3, and crystallizes in space group P63∕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) Å, b=4.85(10) Å, c=8.42(17) Å, V=334(12) Å3, 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 + 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.
Highlights
Chromitite orebodies in the Luobusa ophiolite have yielded a great variety of minerals for which the most prolific has been the Cr-31 chromitite orebody near Luobusa and the Cr-11 chromitite orebody near Kangjinla (Fig. 1, Table 1), which are located about 11 km apart in harzburgite near the contact with the transition zone dunite
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
Superreduced intermetallic phases have been found in the Cr-11 podiform chromitite orebody in the Luobusa ophiolite (Table 1, Fig. S1), which is located at 29◦11 N, 92◦18 E with an elevation of 5300 m in the Kangjinla district
Summary
Chromitite orebodies in the Luobusa ophiolite have yielded a great variety of minerals for which the most prolific has been the Cr-31 chromitite orebody near Luobusa and the Cr-11 chromitite orebody near Kangjinla (Fig. 1, Table 1), which are located about 11 km apart in harzburgite near the contact with the transition zone dunite. Superreduced intermetallic phases have been found in the Cr-11 podiform chromitite orebody in the Luobusa ophiolite (Table 1, Fig. S1), which is located at 29◦11 N, 92◦18 E with an elevation of 5300 m in the Kangjinla district. Xu et al (2009, 2013, 2018) reported compounds that appear to correspond to known minerals, such as native titanium (Fang et al, 2013), Ti–Fe–Si (probably zangboite, FeTiSi2, Li et al, 2009), Ti–N (osbornite), and Ti–C (khamrabaevite), as well as several Ti–Si, Ti–Si–P, and Ti–B intermetallic phases. Compositions of four of the new intermetallic compounds, including badengzhuite and zhiqinite, lie in the Ti–Si–P system and provide new information on this little-studied ternary system, which is information that can add a valuable perspective for understanding the conditions under which the Luobusa ophiolite was exhumed following burial to the upper mantle
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