Tuite, γ-Ca_3(PO_4)_2: a new mineral from the Suizhou L6 chondrite

摘要

Tuite, γ-Ca_3(PO_4)_2, the high-pressure polymorph of whitlockite, was found in a shock melt vein of the Suizhou chondrite. It occurs as polycrystalline aggregates in association with coarse-grained aggregates of ringwoodite, majorite and NaAlSi_3O_8-hollandite. These high-rpessure mineral assemblages are enclosed in the fine-grained matrix of the vein that consists of majorite-pyrope_(SS) garnet, metal and troilite. This new mineral is trigonal, R3-barm, a = 5.258 A, c = 18.727 A. The tuite is colourless and transparent with vitreous lustre and white streak; it is uniaxial (+) with high refractive indices (ε = 1.706, ω = 1.701) and low birefringence (0.005). Microprobe analyses of tuite yield an empirical formula (Ca_(2.51)Mg_(0.29)_(2.80)Na_(0.28)(P_(1.01)O_4)_2. The simplified formula, Ca_3(PO_4)_2, is the same as whitlockite outside the veins. Stronger reflections on synchrotron radiation X-ray powder diffraction pattern of tuite are d = 2.628 (100), 2.891 (80), 1.945 (47), 1.730 (25) and 1.567(22). The figures in brackets are relative intensities. Tuite in the shock vein of this meteorite was transformed from whitlockite via a shock-produced solid-state reaction, while the pressure and temperature in the shock vein attained 23 GPa and 2000 ℃. This new mineral and the mineral name (tuite) have been approved by the Commission on New Minerals and Mineral Names of the International Mineralogical Association.