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

Graulichite-(Ce), ideally CeFe3+3(AsO42(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 μm spherical aggregates of rhombohedral crystals, 50 to 80 μm length, which show the forms 10-12 and 10-11. 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, Fe2O3 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 X-ray powder-diffraction pattern [d(in Å)(I)(hkil)] are: 3.052(100)(11-23), 3.636(40)(11-20), 2.239(35)(10-17), 1.817(35)(22-40), 2.792(30)(0006), 5.906(25)(10-11), 1.968(25)(30-33). 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