Graulichite-(Ce), a new arsenate mineral from the Stavelot Massif, Belgium

摘要

Graulichite-(Ce), ideally CeFe_3~(3+)(AsO_4)_2(OH)_6, is a new mineral species from Hourt, Stavelot Massif, Belgium. It occurs in the most altered parts of a Devillian quartzite (Lower to Middle Cambrian), in close association with arsenopyrite, scorodite, mimetite, pharmacosiderite, and geothite. Graulichite-(Ce) forms 80 to 150 μm spherical aggregates of rhombohedral crystals, 50 to 80 μm length, which show the forms {10(1-bar)2} and {10(11)-bar}. The mineral is transparent and exhibits a light-green to brownish colour, with a resinous lustre. It is non-fluorescent and shows an irregular fracture, without any perceptible cleavage. The calculated density is 4.42 g/cm~3. Graulichite-(Ce) is uniaxial negative, with a mean refractive index close to 1.97(1) (λ = 590 nm), pleochroic from light green to yellowish. Electron microprobe analyses gave As_2O_5 31.20, P_2O_5 0.03, SO_3 0.06, SiO_2 0.03, Al_2O_3 3.09, Fe_2O_3 30.65, SrO 0.24, PbO 0.07, BaO 3.95, CaO 0.03, La_2O_3 2.26, Ce_2O_3 15.73, Nd_2O_3 2.08, H_2O (calc.) 8.37, total 97.79 wt.. The resulting empirical formula, calculated on the basis of 6 cations, is (Ce_(0.67)Ba_(0.10)Nd_(0.09)Sr_(0.02))_(Σ1.06) (Fe_(2.68)~(3+)Al_(0.42))_(Σ3.10)(As_(1.89)S_(0.01)□_(0.10))_(Σ2.00)o_8(OH)_(5.57)(H_2O)_(0.43)_(Σ6.00). The single-crystal unit-cell parameters are a = 7.288(2) and c = 16.812(9) A, space group R3m. The seven strongest lines in the X-ray powder-diffraction pattern d(in A)(I)(hkil) are: 3.052(100)(11(2-bar)3), 3.636(40)(11(2-bar)0), 2.239(35)(10(1-bar)7), 1.817(35)(22(4-bar)0), 2.792(30)(0006), 5.906(25)(10(1-bar)1), 1.968(25)(30(3-bar)3). Graulichite(Ce) is the iron analogue of arsenoflorencite-(Ce), and belongs to the crandallite mineral group, with the alunite-type structure. The crystal structure of graulichite-(Ce) has been refined, based on single-crystal X-ray diffraction data, to R = 0.048.