Svenekite, Ca[AsO_2(OH)_2]_2, a new mineral from Jachymov, Czech Republic

摘要

Svenekite (IMA 99-007), Ca[AsO_2(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 urn 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 ~2. Svenekite is biaxial, non-pleochroic. The refractive indices are a' = 1.602(2), y' = 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 (Ca_(1.oo)Mg_(o.o1))[AsO_2(OH)_2]_(1.96)[PO_2(OH)_2]_(o.o3)(SO_4)_(o.o1) The simplified formula is Ca[AsO_2(OH)_2]_2 and requires CaO 17.42, As_2O_5 71.39, H_2O 11.19, total 100.00 wt.%. Raman and infrared spectroscopy exhibit dominance of O—H vibrations and vibration modes of distorted tetrahedral AsO_2(OH)_2 units. Svenekite is triclinic, space group PI, with a = 8.5606(5), b = 7.6926(6), c = 5.7206(4) A, a = 92.605(6), p = 109.9002(6), y = 109.9017(6)°, and V= 327.48(4) A3, Z= 2, D_(calo) = 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 (A)/I(%)/(hkl)]: 3.968(33)(2l0); 3.766(35)(2ll); 3.697(49)(101); 3.554(100)(020); 3.259(33)(220); 3.097(49)(121). 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_(a11) = 1.20). The structure of svenekite consists of sheets of corner-sharing CaO_8 polyhedra and AsO_2OH_2 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 -1.22 A). The mineral is named to honour Jaroslav Svenek, the former curator of the mineralogical collection of the National Museum in Prague, Czech Republic.