Erssonite, CaMg7Fe3+2(OH)18(SO4)2⋅12H2O, a new hydrotalcite-supergroup mineral from Långban, Sweden

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Abstract
The new wermlandite-group mineral erssonite, ideally CaMg7Fe23+(OH)(18)(SO4)(2)center dot 12H(2)O (or [Mg7Fe23+(OH)(18)][Ca(SO4)(2)].12H(2)O), was discovered in a late-stage, low-temperature assemblage in cavities of a magnetite-bearing dolomitic rock from the Langban deposit, Varmland county, Bergslagen ore province, Sweden. The associated minerals are dolomite, calcite, members of the magnetite-magnesioferrite solid-solution series, phlogopite, chrysotile, pyroaurite and norbergite. Erssonite has a vitreous lustre and forms colourless, platy hexagonal crystals flattened on [0001], up to 0.5 mm across and up to 10 mu m thick, occurring mainly as aggregates in cavities of dolomitic rock. Erssonite is malleable; separate crystals are flexible and non-elastic, with a perfect, mica-like cleavage on {0001}. The calculated density is equal to 2.02 g.cm(-3). Raman spectroscopy shows the presence of typical bands for S-O bonds attributed to intercalated SO42- anions and structural OH- anions together with the absence of C-O bonds, attributed to carbonate anions. The chemical composition is (wt.%, electron microprobe, H2O content is calculated from structure data): MgO 28.67, CaO 2.76, Al2O3 0.23, Cr2O3 0.23, Fe2O3 16.00, SiO2 0.48, SO3 14.80, H2O 35.58, total 98.75. The empirical formula based on 38 O atoms is H41.48Ca0.52Mg7.47Fe2.113+Al0.05Cr0.03S1.94Si0.08O38. The ideal formula is CaMg7Fe23+(OH)(18)(SO4)(2).12H(2)O or {Mg7Fe23+(OH)(18)}{[Ca(H2O)(6)] (SO4)(2)(H2O)(6)}. The crystal structure was determined using single-crystal X-ray diffraction data and refined to R = 0.093. Erssonite is trigonal, P (3) over bar c1, with a = 9.3550(6), c = 22.5462(14) angstrom, V = 1708.8(2) angstrom(3) and Z = 2. The strongest lines of the powder X-ray diffraction pattern [d, angstrom (I, %)(hkl)] are: 11.22 (90)(002), 5.63 (64)(004), 4.670 (100)(110, 104, 014), 2.626 (64)(032, 302), 2.435 (66)(034, 304) and 1.951 (45)(038, 308). The mineral is named in honour of the Swedish amateur mineralogist Dr. Anders Ersson (b. 1971).