Dobšináite, Ca 2 Ca(AsO 4 ) 2 ·2H 2 O, a new member of the roselite group from Dobšiná (Slovak Republic)

[摘要] Dobšináite, ideally Ca2Ca(AsO4)2·2H2O, is a new supergene mineral from the Dobšiná deposit, Slovak Republic, associated with phaunouxite, picropharmacolite, erythrite-hörnesite, gypsum and aragonite. It forms white to pink clusters orpolycrystalline aggregates up to 1–4 mm in size consisting of densely intergrown, slightly rounded thin tabular to platycrystals up to 0.1 mm in size. Dobšináite has a white streak, vitreous luster, does not fluoresce under either short- orlong-wave ultraviolet light. Cleavage on {010} is good, the Mohs hardness is ~3, and dobšináite is brittle with an unevenfracture. The calculated density is 3.395 g/cm3. Dobšináite is optically biaxial negative, the indices of refraction areα´ = 1.601(2) and γ´ = 1.629(2) and 2Vmeas. is 60° (+/– 20°). Dobšináite is monoclinic, space group P21/c, a = 5.990(2),b = 13.013(4), c = 5.726(2) Å, β = 108.47(3)°, V = 423.3(3) Å3, Z = 2. The seven strongest lines in the X-ray powder diffraction pattern are as follows: d (Å)/I(hkl): 5.197/37(110); 3.443/38(130); 3.385/66(031); 3.249/77(040); 3.201/42(13–1);3.026/100(121) and 2.822/60(10–2). The chemical analyses by electron microprobe yielded (in wt. %) CaO 36.74, MgO0.89, NiO 0.37, CoO 0.51, P2O5 0.03, As2O5 52.75, SO3 0.18, H2Ocalc. 8.31, total 99.78. The resulting empirical formulaon the basis of 10 O atoms per formula unit is Ca2(Ca0.84Mg0.10Co0.03Ni0.02)Σ0.99[(AsO4)1.99(SO4)0.01]Σ2.00·2H2O. Dobšináite isthe Ca-dominant member of the roselite group as confirmed by stoichiometry, X-ray powder diffraction data and Ramanspectroscopy. It is named after its type locality Dobšiná, Slovak Republic. Dobšináite was formed during weatheringof primary Co and As-bearing ore minerals (safflorite, arsenopyrite) in a Ca enriched environment under relatively dryconditions.

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[关键词] dobšináite;calcium arsenate;new mineral;roselite group;Dobšiná;Slovak Republic