Crystal chemistry of the low-symmetry (P4nc) vesuvianite from the KharmankuP Cordon (the South Urals) Golubev Ε. Α.,

Panikorovskii T. L., Krivovichev S. V., Zolotarev A. A. jr., Antonov A. A.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2016. V. 145. N 3. P. 94-104

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Language: Russian

Abstract

Crystal chemistry of low-symmetry vesuvianite from the Kharmankul’ Cordon (in specimen from the Mineralogical Museum of the SPbU) has been studied by electron microprobe analysis, single crystal X-ray diffraction and infrared spectroscopy. The crystal structure have been refined in the Pnc space group (P4nc, a = 15.6218(8), c = 11.8775(6) Å, V = 2898.6(3) ų) to R₁ = 0.036. Chemical composition of investigated sample (wt %): SiO₂ 38.22, Al₂O₃ 15.68, MgO 4.03, CaO 37.35, FeO 4.36, total 99.64. The crystal chemical formula can be written as Ca₁₉(Al_9.70Mg_1.48)₁₃Si₁₈O₆₉(OH)₉. IR spectrum bands are 438, 476, 563, 607, 632(w), 800, 881, 918, 964, 1016, 3200(shoulder), 3435, 3530 cm^–1. Different occupancies of the YIa and YIb sites (92 and 8 %, respectively) results in dominant orientation of chains of the intra-channels sites along the c axis, which causes reduction of symmetry from P4/nnc to P4nc. Detected average length of the <Fe‑O> bond for Y1A position is 1.96 Å, corresponding to Fe³⁺ in this position. The mineral-forming took place at temperatures lower that 300 °C and the high oxygen fugacity, which is in good agreement with supposed conditions of the contact metasomatic rocks formation in the Kharmankul’ Cordon area.

Key words: vesuvianite, space group P4nc, vesuvianite group, crystal structure, cation ordering, low-temperature metasomatic rocks, the South Urals.