FERROTAAFFEITE-2N2S,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】：正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 100μm 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) ~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, 7516.57(15) ~3,to R_1 = 0.043,wR_2 = 0.137.