Dobšináite, Ca2Ca(AsO4)2·2H2O, a new member of the roselite group from Dobšiná (Slovak Republic)

Abstract

Dobšináite, ideally Ca 2 Ca(AsO 4 ) 2 •2H 2 O, 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 or polycrystalline aggregates up to 1-4 mm in size consisting of densely intergrown, slightly rounded thin tabular to platy crystals up to 0.1 mm in size. Dobšináite has a white streak, vitreous luster, does not fluoresce under either short-or long-wave ultraviolet light. Cleavage on {010} is good, the Mohs hardness is ~3, and dobšináite is brittle with an uneven fracture. The calculated density is 3.395 g/cm 3 . Dobšináite is optically biaxial negative, the indices of refraction are α´ = 1.601(2) and γ´ = 1.629(2) and 2V meas. is 60° (+/-20°). Dobšináite is monoclinic, space group P2 1 /c, a = 5.990( 2), b = 13.013(4), c = 5.726(2) Å, β = 108.47(3)°, V = 423.3(3) Å 3 , 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, MgO 0.89, NiO 0.37, CoO 0.51, P 2 O 5 0.03, As 2 O 5 52.75, SO 3 0.18, H 2 O calc. 8.31, total 99.78. The resulting empirical formula on the basis of 10 O atoms per formula unit is Ca 2 (Ca 0.84 Mg 0.10 Co 0.03 Ni 0.02 ) Σ0.99 [(AsO 4 ) 1.99 (SO 4 ) 0.01 ] Σ2.00 •2H 2 O. Dobšináite is the Ca-dominant member of the roselite group as confirmed by stoichiometry, X-ray powder diffraction data and Raman spectroscopy. It is named after its type locality Dobšiná, Slovak Republic. Dobšináite was formed during weathering of primary Co and As-bearing ore minerals (safflorite, arsenopyrite) in a Ca enriched environment under relatively dry conditions.