Ti10(Si,P)6-7 and Ti11(Si,P)10, new phases from the Luobusa ophiolite, China: Implications for crystallization of Ti-Si-P melts

Corundum extracted from chromitite body Cr-11 in the Luobusa ophiolite near Kangjinla (Tibet, China) contains inclusions of super-reduced phases in the Ti-Si-P system, four of which are found in a spheroid 20 μm across, one half of which is composed of TiSi2 enclosing globules of TiP, the other half comprises two ternary phases for which chemical analysis (EDX) yielded compositions Ti10(Si5.4P1.4) and Ti11(Si7.5P2.5). 3-dimensional electron diffraction collected on the same crystals allowed structure solution and refinement (dynamical) in the hexagonal space group P63/mcm with a = 7.30(14) Å c = 5.09(10) Å (Z = 1) and tetragonal I4/mmm with a = 9.4(2) Å, c = 13.5(3) Å (Z = 4), respectively. The hexagonal phase belongs to the same structural type as mavlyanovite (Mn5Si3) and the synthetic end-members Ti5Si3 and Ti5P3.15, but is unique in being a ternary Ti-Si-P solid solution. Cell parameters, symmetry and structure of the tetragonal phase closely resemble binary and ternary synthetic G-phases, e.g., Grytsiv et al., 2006 [1], although no natural or synthetic G-phase has been reported in the Ti-Si-P system. The spheroid could have been a melt droplet that crystallized with decreasing temperature suggested by liquidi for the Ti-P [2] and Ti-Si [3] binary systems: Ti10(Si,P)6-7 → Ti11(Si,P)10 → TiSi2 + TiP. Such melt droplets were products of the interaction of mantle CH4 ± H2 fluids with basaltic magmas in the shallow lithosphere (∼30–100 km) [4, 5].