FERROTAAFFEITE-2N'2S, A NEW MINERAL SPECIES, AND THE CRYSTAL STRUCTURE OF Fe2+-RICH MAGNESIOTAAFFEITE-2N'2S FROM THE XIANGHUALING TIN-POLYMETALLIC ORE FIELD, HUNAN PROVINCE, CHINA

Abstract

Ferrotaaffeite-2 N ’2 S , ideally BeFe 3 Al 8 O 16 with hexagonal symmetry P 6 3 mc , 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 Fe 2+ -rich magnesiotaaffeite-2 N ’2 S , ferronigerite-2 N 1 S , cassiterite, liberite, pyrite, sphalerite, pyrrhotite, galena, spinel, and phlogopite. The mineral forms tabular crystals about 100 μ m 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 VHN 50 1801–2404 kg/mm 2 (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/cm 3 . The Gladstone-Dale relationship gives N calc = 1.82. The empirical formula for the investigated ferrotaaffeite-2 N ’2 S is Be(Fe 1.39 Mg 0.97 Zn 0.42 Mn 0.17 Sn 0.03 ) ∑2.98 Al 7.99 O 16 ,. The simplified formula is Be(Fe,Mg,Zn) 3 Al 8 O 16 . The strongest five reflections in the X-ray powder diffraction pattern [ d in A (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) A, and V 517.46(8) A 3 , Z = 2. The associated Fe 2+ -rich magnesiotaaffeite-2 N ’2 S has empirical formula Be(Mg 1.54 Fe 0.98 Zn 0.38 Mn 0.14 Sn 0.02 ) ∑3.06 Al 7.95 O 16 . Its structure was solved and refined in space group P 6 3 mc , with a 5.6978(8), b 5.6978(8), c 18.373(4) A, V 516.57(15) A 3 , to R 1 = 0.043, w R 2 = 0.137.