(K,Sr, )(Ca, )3Al6Si10O32, a dmisteinbergite-like phase from the Luobusa ophiolite, China: Evidence for quenching at mantle depths?

Corundum extracted from chromitite body Cr-11 in the Luobusa ophiolite near Kangjinla (Tibet, China) contains inclusions of super-reduced Ti phases, largely TiN-TiC. The dmisteinbergite-like mineral consitutes part of a halo around a spheroid 20 μm across composed of 50% of a Ti10(Si,P)7 – Ti11(Si,P)10 mixture and 50% TiSi2 with minor TiP. Chemical analysis (EDX) of a crystal ca. 4 x 1 μm across yielded a composition (K0.50Sr0.25…0.25)(Ca0.83…0.17)3Al6Si10O32. 3- dimensional electron diffraction collected on the same crystal allowed structure solution and refinement (dynamical) in the hexagonal space group P6/mcc, a = 10.2(2) Å, c = 14.9(3) Å, Z = 2. The structure is topologically identical to that of dmisteinbergite, but site occupancies differ, e.g., one quarter of the Ca sites in dmisteinbergite are occupied by K and Sr in the halo phase, resulting in considerable rotation of the tetrahedra and doubling of a and b cell parameters. A phase of this composition and structure has not been reported either as a mineral or as a synthetic compound. Compositionally, the dmisteinbergite-like phase corresponds to 63% anorthite, 11% orthoclase, 5% SrAl2Si2O8 and 21% quartz, i.e., a Carich silica melt. The spheroid can be interpreted as a droplet of Ti-Si-P melt that crystallized to a mixture of ternary and binary phases. According to Griffin et al. [1] and Xiong et al. [2], super-reduced phases associated with corundum appear to reflect the local interaction of mantle-derived CH4 ± H2 fluids with basaltic magmas in the shallow lithosphere (∼30– 100 km). Quenching of the silica melt gave a metastable dmisteinbergite-like phase instead of feldspar [3].