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

• Published in: Journal of geosciences (Prague) 2021-07, Vol.66 (2), p.127-235
• Main Authors: Sejkora, J., Števko, M., Škoda, R., Víšková, E., Toman, J., Hreus, S., Plášil, J., Dolníček, Z.
• Format: Article
• Language: English
• Subjects: Analytical methods; Aragonite; Arsenopyrite; Chemical analysis; Crystals; Diffraction; Diffraction patterns; Electron microprobe; Electron probes; Empirical analysis; Enrichment; Gypsum; Mineralization; Mineralogy; Minerals; Mining; Museums; Phosphorus pentoxide; Quartz; Raman spectroscopy; Scratch hardness; Stoichiometry; Sulfur trioxide; Sulphates; Ultraviolet radiation; Water hardness; X ray powder diffraction; X rays
• ISSN: 1802-6222; 1803-1943
• DOI: 10.3190/jgeosci.324

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 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/cm3. Dobšináite is optically biaxial negative, the indices of refraction are α' = 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, ß = 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, MgO 0.89, NiO 0.37, CoO 0.51, P2O5 0.03, As2O5 52.75, SO3 0.18, H2Ocalc. 8.31, total 99.78. The resulting empirical formula on 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 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.