Honzaite, (Ni,Co)2(AsO3OH)2(H2O)5, is a new supergene mineral from the Jáchymov (formerly St. Joachimsthal) ore district, Czech Republic, associated with arsenolite, zeunerite, Ni-rich burgessite and a red amorphous Co–Ni–Cu arsenate phase. It forms pale pink (some with purplish or violet tints) irregular to hemispherical microcrystalline aggregates up to 5 mm in diameter on strongly weathered ore gangue. Tiny prismatic crystals, up to 30 μm long, are rarely observed on the surface of these aggregates. Honzaite has a pale pink streak, a vitreous luster, does not fluoresce under either short- or long-wave ultraviolet light. Cleavage on {0 1 0} is good, the Mohs hardness is ∼3, and honzaite is brittle with an irregular fracture. The calculated density is 2.993 g/cm3. Honzaite is optically biaxial positive, the indices of refraction are α 1.601(2), β 1.608(2), γ 1.629(2), and 2Vmeas. is 60(1)°. Honzaite is monoclinic, space group P21/n, a = 4.6736(6), b = 9.296(1), c = 12.592(1) Å, β = 99.115(8)°, V = 540.2(1) Å3, Z = 2. The seven strongest lines in the X-ray powder diffraction pattern are as follows: d (Å)/I(hkl): 7.431/100(0 1 1); 6.215/18(002); 3.717/9(0 2 2); 3.254/7(121); 3.078/7(121); 3.005/5(0 3 1) and 2.568/7(1 3 0). The chemical analyses by electron microprobe yielded (in wt%) MgO 0.24, CaO 0.10, FeO 0.21, NiO 16.51, CoO 12.71, CuO 0.55, ZnO 0.84, P2O5 0.26, As2O5 45.82, SO3 0.52, H2Ocalc. 22.15, total 99.91. The resulting empirical formula on the basis of 13 O atoms per formula unit is (Ni1.08Co0.83Zn0.05Cu0.03Mg0.03Fe0.01Ca0.01)Σ 2.04(AsO3OH)1.94(SO4)0.03(PO3OH)0.02 ∙ 5H2O. The crystal structure of honzaite was refined from powder X-ray data using Rietveld refinement (Rp = 0.0116, Rwp = 0.0174) of the structure model of isostructural burgessite. The structure of honzaite is based upon infinite chains of Ni octahedra and protonated As tetrahedra linked by a network of hydrogen bonds. Honzaite is the Ni-dominant analogue of burgessite. It is named in honor of the prominent Czech mineralogist and famous collector of minerals from the Jáchymov ore district, Dr. Jan “Honza” Hloušek (1950–2014). It forms during weathering of primary nickelskutterudite and tennantite under strongly acidic conditions.

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