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Tuite, -Ca₃(PO₄)₂

¹ Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou, 510640 China
² Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA

^* Author to whom correspondence should be addressed (xdxie{at}gis.sti.gd.cn).

Tuite, -Ca₃(PO₄)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 NaAlSi₃O₈-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, Rm,a = 5.258 Å, c = 18.727 Å. The tuite is colourless and transparent with vitreous lustre and white streak; it is uniaxial (+) with high refractive indices ( = 1.706, = 1.701) and low birefringence (0.005). Microprobe analyses of tuite yield an empirical formula (Ca_2.51Mg_0.29)_2.80Na_0.28(P_1.01 O₄)₂. The simplified formula, Ca₃(PO₄)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.

Key-words: Suizhou meteorite, high-pressure polymorph, whitlockite, new mineral, tuite.

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