Švenekite (IMA 99-007), Ca[AsO<sub>2sub>(OH)<sub>2sub>]<sub>2sub>, is a rare supergene arsenate mineral occurring in the Geschieber vein, Jáchymov ore district, Western Bohemia, Czech Republic. It grows directly on the granite rocks and occurs isolated from other arsenate minerals otherwise common in Jáchymov. Švenekite usually forms clear transparent coatings composed of indistinct radiating to rosette-shaped aggregates up to 3 mm across. They are composed of thin lens- or bladed-shaped crystals, usually 100–150 μm long.Švenekite is transparent to translucent and has a white streak and a vitreous lustre; it does not fluoresce under ultraviolet light. Cleavage is very good on {010}. The Mohs hardness is ~2. Švenekite is biaxial, non-pleochroic. The refractive indices are α′ = 1.602(2), γ′ = 1.658(2). The empirical formula ofšvenekite (based on As + P + S = 2 a.p.f.u., an average of 10 spot analyses) is (Ca<sub>1.00sub>Mg<sub>0.01sub>)<sub>Σ1.01sub>[AsO<sub>2sub>(OH)<sub>2sub>]<sub>1.96sub>[PO<sub>2sub>(OH)<sub>2sub>]<sub>0.03sub>(SO<sub>4sub>)<sub>0.01sub>. The simplified formula is Ca[AsO<sub>2sub>(OH)<sub>2sub>]<sub>2sub> and requires CaO 17.42, As<sub>2sub>O<sub>5sub> 71.39, H<sub>2sub>O 11.19, total 100.00 wt.%. Raman and infrared spectroscopy exhibit dominance of O–H vibrations and vibration modes of distorted tetrahedral AsO<sub>2sub>(OH)<sub>2sub> units.Švenekite is triclinic, space group P1İ, with a = 8.5606(5), b = 7.6926(6), c = 5.7206(4) Å<sub>,sub> α = 92.605(6), β = 109.9002(6), γ = 109.9017(6)°, and V = 327.48(4) Å<sup>3sup>, Z = 2, D<sub>calcsub> = 3.26 g·cm<sup>−3sup>. The a:b:c ratio is 0.7436:1:1.1082 (for single-crystal data). The six strongest diffraction peaks in the X-ray powder diffraction pattern are [d ( Å)/<sub>Isub>(%)/(hkl)]: 3.968(33)(21İ0); 3.766(35)(21İ1İ); 3.697(49)(101); 3.554(100)(020); 3.259(33)(22İ0); 3.097(49)(12İ1). The crystal structure of švenekite was refined from single-crystal X-ray diffraction data to R<sub>1sub> = 0.0250 based on 1309 unique observed, and to wR<sub>2sub> = 0.0588, for all 1588 unique reflections (with GOF<sub>allsub> = 1.20). The structure of švenekite consists of sheets of corner-sharing CaO<sub>8sub> polyhedra and AsO<sub>2sub>OH<sub>2sub> groups, stacked parallel to (001). Adjacent sheets are linked by hydrogen bonds. Thešvenekite structure possesses very short symmetrical hydrogen bonds (with the D–H lengths ~1.22 Å). The mineral is named to honour JaroslavŠvenek, the former curator of the mineralogical collection of the National Museum in Prague, Czech Republic.
