Prachařite, CaSb5+2(As3+2O5)2O2·10H2O, a new mineral from Lavrion, Greece

Prachařite, ideally CaSb⁵⁺₂(As³⁺₂O₅)₂O₂·10H₂O, is a new mineral found in underground workings of the Plaka Mine No. 80, Plaka, Lavrion Mining District, Attica, Greece. It occurs as colourless to white, thin tabular hexagonal, in general sharp crystals up to 2.5 mm in diameter, and is associated with pharmacolite, sulphur and very rare smamite {Ca₂Sb(OH)₄[H(AsO₄)₂]·6H₂O} on a matrix composed of sphalerite, galena and carbonate gangue. Prachařite is translucent to transparent, with a glassy lustre, white streak, a good cleavage parallel to {0001} and a distinct cleavage parallel to {10\(\overline{1 }\)0}. It is non-luminescent, brittle, and has an uneven fracture, a Mohs hardness of 2–2.5 and X-ray density D_x = 2.848 g/cm³, D_calc. = 2.836–2.853 g/cm³ (for two measured compositions). Optically, it is uniaxial negative, with ω = 1.619(1) and ε = 1.553(1). Prachařite is trigonal, space group P\(\overline{3 }\)c1 (no. 165), with a = 13.951(2), c = 19.899(2) Å, V = 3354.1(10) ų and Z = 6. Strongest lines in the X-ray powder diffraction pattern are [d in Å (I) hkl]: 9.894 (100) 002; 6.045 (8) 200; 5.156 (10) 202; 4.946 (11) 004; 3.297 (19) 311, 006, 222; 2.988 (22) 400, 313, 116. Two sets of independent electron probe micro-analyses yielded (wt%): CaO 6.28/7.12, MgO 0.09/-, Zn -/0.01, Sb₂O₅ 39.22/40.19, As₂O₃ 47.59/47.39, SO₃ -/0.02, H₂O 21.65/22.04 (calculated on the basis of ideal composition derived from crystal-structure determination), total 114.83/116.77; the total is reproducibly high due to a loss of a third of all water molecules under the electron beam. The empirical formulae, based on O = 22 atoms per formula unit, for the two datasets are very similar, (Ca_0.93Mg_0.02)_Σ0.95Sb_2.02(AsO₃)_4.00·10H₂O and Ca_1.04Sb_2.03(AsO₃)_3.92·10H₂O. The ideal formula is CaSb⁵⁺₂(As³⁺₂O₅)₂O₂·10H₂O, determined with the help of a crystal-structure determination based on single-crystal X-ray diffraction datasets collected at room temperature (R1 = 2.3%). The atomic arrangement of prachařite is unusual; it is based on two different layers containing a six-membered ring of corner-sharing SbO₆ octahedra, an eight-coordinated Ca1 atom in the centre of the ring, two non-equivalent AsO₃ groups corner-linked to form a (As₂O₅)⁴⁻ diarsenite group, and, on interlayer sites, a seven-coordination Ca2 atom and three water molecules (all only weakly hydrogen-bonded), one of which is only partially occupied (split position). The mineral is named in honour of Dr Ivan Prachař, a long-term researcher of the mineralogy and underground workings of Lavrion.