Svenekite, Ca[AsO2(OH)2]2, a new mineral from Jachymov, Czech Republic

Svenekite (IMA 99-007), Ca[AsO2(OH)(2)](2), is a rare supergene arsenate mineral occurring in the Geschieber vein, Jachymov ore district, Western Bohemia, Czech Republic. It grows directly on the granite rocks and occurs isolated from other arsenate minerals otherwise common in Jachymov. Svenekite 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 mu m long. Svenekite 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 similar to 2. Svenekite is biaxial, non-pleochroic. The refractive indices are alpha' = 1.602(2), gamma' = 1.658(2). The empirical formula of svenekite (based on As + P + S = 2 a.p.f.u., an average of 10 spot analyses) is (Ca1.00Mg0.01)(Sigma 1.01)[AsO2(OH)(2)](1.96)[PO2(OH)(2)](0.03)(SO4)(0.01). The simplified formula is Ca[AsO2(OH)(2)](2) and requires CaO 17.42, As2O5 71.39, H2O 11.19, total 100.00 wt.%. Raman and infrared spectroscopy exhibit dominance of O-H vibrations and vibration modes of distorted tetrahedral AsO2(OH)(2) units. Svenekite is triclinic, space group P (1) over bar, with a = 8.5606(5), b = 7.6926(6), c = 5.7206(4) angstrom, alpha = 92.605(6), beta = 109.9002(6), gamma = 109.9017(6)degrees, and V = 327.48(4) angstrom(3), Z = 2, D-calc = 3.26 g.cm(-3). 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 (angstrom)/I(%)/(hkl)]: 3.968(33)(2 (1) over bar0); 3.766(35)(2 (1) over bar(1) over bar); 3.697(49)(101); 3.554(100)(020); 3.259(33)(2 (2) over bar0); 3.097(49)(1 (2) over bar1). The crystal structure of svenekite was refined from single-crystal X-ray diffraction data to R-1 = 0.0250 based on 1309 unique observed, and to wR(2) = 0.0588, for all 1588 unique reflections (with GOF(all) = 1.20). The structure of svenekite consists of sheets of corner-sharing CaO8 polyhedra and AsO2OH2 groups, stacked parallel to (001). Adjacent sheets are linked by hydrogen bonds. The svenekite structure possesses very short symmetrical hydrogen bonds (with the D-H lengths similar to 1.22 angstrom). The mineral is named to honour Jaroslav Svenek, the former curator of the mineralogical collection of the National Museum in Prague, Czech Republic.