Abstract

Ferrotaaffeite-2N’2S, ideally BeFe3Al8O16 with hexagonal symmetry P63mc, is a new mineral species of the taaffeite group (IMA No. 2011-025) occurring as a skarn mineral in the Xianghualing Sn-polymetallic ore field, Linwu County, Hunan Province, People’s Republic of China. It was found near the outer contact of the Laizhiling A-type granite pluton and in the Middle-Upper Devonian carbonate rocks of the Qiziqiao Formation. The associated minerals are Fe2+-rich magnesiotaaffeite-2N’2S, ferronigerite-2N1S, cassiterite, liberite, pyrite, sphalerite, pyrrhotite, galena, spinel, and phlogopite. The mineral forms tabular crystals about 100 μm in size. It is dark green to dark grey with a white streak, transparent, and has a vitreous lustre. Cleavage is fair on {001}. It is brittle, with a conchoidal fracture. The hardness is VHN50 1801–2404 kg/mm2 (8.5–9 on the Mohs scale). No fluorescence in long- or short-wave ultraviolet radiation was observed. The calculated density from the empirical chemical formula is 3.99 g/cm3. The Gladstone-Dale relationship gives Ncalc = 1.82. The empirical formula for the investigated ferrotaaffeite-2N’2S is Be(Fe1.39Mg0.97Zn0.42Mn0.17Sn0.03)∑2.98Al7.99O16,. The simplified formula is Be(Fe,Mg,Zn)3Al8O16. The strongest five reflections in the X-ray powder diffraction pattern [d in Å (I) hkl] are: 2.43(100) 114, 2.60(90) 016, 1.425(90)220, 2.86(80) 110, and 1.473(80) 0210. The unit-cell parameters refined from the powder data are a 5.706(8), c 18.352(3) Å, and V 517.46(8) Å3, Z = 2. The associated Fe2+-rich magnesiotaaffeite-2N’2S has empirical formula Be(Mg1.54Fe0.98Zn0.38Mn0.14 Sn0.02)∑3.06Al7.95O16. Its structure was solved and refined in space group P63mc, with a 5.6978(8), b 5.6978(8), c 18.373(4) Å, V 516.57(15) Å3, to R1 = 0.043, wR2 = 0.137.

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