Crystal chemistry of the low-symmetry (P4nc) vesuvianite from the Kharmankul' Cordon (the South Urals)

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) Å ³ ) to R ₁ = 0.036. Chemical composition of investigated sample (wt %): SiO ₂ 38.22, Al ₂ O ₃ 15.68, MgO 4.03, CaO 37.35, FeO 4.36, total 99.64. The crystal chemical formula can be written as Ca ₁₉ (Al _9.70 Mg _1.48 Fe _1.30 ³⁺ Fe _0.52 ³⁺ ) ₁₃ Si ₁₈ O ₆₉ (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 YIA position is 1.96 Å, corresponding to Fe ³⁺ 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.