Tashelgite, CaMgFe^{2 +} Al₉O₁₆(OH), a new mineral species from calc-skarnoid in Gorny Shoria

Abstract

A new mineral species has been discovered at the calc-skarnoid occurrence near the mouth of the Tashelga River, Kuznetsky Alatau, Gorny Shoria, Russia, and named after the locality of its discovery. Associated minerals are calcite, hibonite, grossular, vesuvianite, hercynite, magnetite, corundum, perovskite, scapolite, diopside, and apatite. The new mineral occurs as prismatic or finely fibrous crystals up to 1.5–2.0 mm in length, their parallel intergrowths, and felty aggregates as large as 10 mm across. Tashelgite is bluish green, translucent to transparent, with vitreous luster; D _calc = 3.67 g/cm³. The IR spectrum does not contain bands of OH groups. Tashelgite is biaxial (−), with α = 1.736(2), β = 1.746(2), γ = 1.750(2); 2V _meas = −20(2)°. Dispersion is strong, r < ν. Pleochroism is distinct: X (blue-green) > Y (yellowish green) > Z (almost colorless). Chemical composition (electron microprobe, average of five-point analyses, Fe₂O₃ is estimated from the ratio of intensities $I(Fe_{K\beta _5 } )/I(Fe_{K\beta _1 } )$ in the X-ray spectrum, H₂O was determined as a weight loss on heating in vacuum up to 1000°C), wt %: 7.98 CaO, 6.75 MgO, 0.45 MnO, 11.32 FeO, 1.40 Fe₂O₃, 70.70 Al₂O₃, 1.8(2) H₂O, 100.40 in total. The empirical formula calculated on the basis of 17 oxygen atoms is H_1.27Ca_0.90Mg_1.06Mn_0.04 Fe ²⁺_1.00 Fe ³⁺_0.11 Al_8.80O_17.00. The idealized formula is CaMgFe²⁺Al₉O₁₆(OH). According to single-crystal X-ray structural data, tashelgite is monoclinic, pseudoorthorhombic, space group Pc; unit cell parameters are: a = 5.6973(1), b = 17.1823(4), c = 23.5718(5)Å; β = 90.046(3)°; V = 2307.5(1)ų, Z = 8. The crystal structure of tashelgite is unique and characterized by ordering of all cations; Al occupies sites with octahedral and tetrahedral coordination. The cation ordering has also been confirmed by IR spectroscopy. The strongest lines of the X-ray powder diffraction pattern (d, Å]-I[hkl] are: 11.79–48 [002], 2.845–43 [061], 2.616–100 [108], 2.584–81 [146], 2.437–44 [163], 2.406–61 [057], 2.202–72 [244]. The type specimen of tashlegite has been deposited at the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia.