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

摘要

Ferrotaaffeite-2N'2S, ideally BeFe_3Al_8O_(16) with hexagonal symmetry P6_3mc, 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-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 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-2N'2S is Be(Fe_(1.39)Mg_(0.97)Zn_(0.42)Mn_(0.17)Sn_(0.03))_(Σ2.98)Al_(7.99)O_(16),. The simplified formula is Be(Fe,Mg,Zn)_3Al_8O_(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-2N'2S has empirical formula Be(Mg_(1.54)Fe_(0.98)Zn_(0.38)Mn_(0.14) Sn_(0.02))_(Σ3.06)Al_(7.95)O_(16). Its structure was solved and refined in space group P6_3mc, 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, wR_2 = 0.137.