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

Graulichite-(Ce), ideally CeFe³⁺₃(AsO₄)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 goethite. Graulichite-(Ce) forms 80 to 150 μ spherical aggregates of rhombohedral crystals, 50 to 80 μ length, which show the forms {1012} and {1011}. 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/cm³. 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 As₂O₅ 31.20, P₂O₅ 0.03, SO₃ 0.06, SiO₂ 0.03, Al₂O₃ 3.09, Fe₂O_3.30.65, SrO 0.24, PbO 0.07, BaO 3.95, CaO 0.03, La₂O₃ 2.26, Ce₂O₃ 15.73, Nd₂O₃ 2.08, H₂O (calc.) 8.37, total 97.79 wt. %. The resulting empirical formula, calculated on the basis of 6 cations, is (Ce_0.67Ba_0.18La_0.10Nd_0.09Sr_0.02)E]_σ1.06 (Fe²⁺_2.68Al_0.42)_Σ3.10[(As_1.89S_0.01□_0.00)Σ_2.00O₈][(OH)_5.57(H₂O)_0.43]Σ_6.00. The single-crystal unit-cell parameters are a = 7.288(2) and c = 16.812(9) Å, space group R3m. The seven strongest lines in the X-ray powder-diffraction pattern [d(in Å)(l)(hkil)] are: 3.052(100)(1123), 3.636(40)(1120), 2.239(35)(1017), 1.817(35)(2240), 2.792(30)(0006), 5.906(25)(1011), 1.968(25)(3033). 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.