Tuite, gamma-Ca3(PO4)2: a new mineral from the Suizhou L6 chondrite

Abstract

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 3 O 8 -hollandite. These high-pressure 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, R 3 m,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°C. 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.