| Abstrakt: |
Šlikite, Zn2Mg(CO3)2(OH)2·4H2O is a new supergene post-mining mineral from the Jáchymov (formerly St. Joachimsthal) ore district, Czech Republic, associated with brianyoungite, hexahydrite, hydromagnesite, nesquehonite, serpierite, smithsonite and a ktenasite-like mineral. It forms snow-white radial aggregates, up to 2 mm across, in vugs or on surfaces of fragments of skarn rocks. Individual crystals are very thin blades, elongated along [110] and flattened on {001}, up to 200 µm in length. Šlikite has a white streak, a vitreous lustre, and does not fluoresce under either short- or long-wave ultraviolet light. Cleavage is perfect on {001}, the Mohs hardness is ~2, and the fracture is uneven. The mineral is brittle, although thin blades (fibres) are slightly flexible. The calculated density is 2.613 g/cm3. Šlikite is optically biaxial negative, the indices of refraction are a = 1.50(1), ß = 1.55(1), ? = 1.59(1) and 2V meas. = 80(1)°. The mineral is triclinic, space group P 1 ¯ , a = 6.335(4), b = 6.340(1), c = 13.923(4) Å, a = 99.985(7)°, ß = 92.74(1)°, ? = 114.93(2)°, V = 494.8(4) Å3, Z = 2. The six strongest lines in the X-ray powder diffraction pattern are [d (Å)/I(hkl)]: 13.575/100(001), 4.525/31(003), 3.573/2(1 1 ¯ 3), 3.406/5(11 1 ¯ ), 2.996/3(11 3 ¯ ) and 2.773/3(112). The chemical analyses by electron microprobe yielded MgO 8.39, ZnO 46.28, CuO 0.05, MnO 0.68, CdO 0.04, CO2calc. 23.10, H2Ocalc. 23.65, total 102.19 wt%. The empirical formula on the basis of 12 O atoms per formula unit is Zn2.00(Mg0.79Zn0.17Mn0.04)S1.00(CO3)2(OH)2·4H2O. The crystal structure of šlikite was refined using synchrotron single-crystal data to R obs = 0.094 for 1594 reflections with I > 3s(I). The structure of šlikite is based on heteropolyhedral layers of composition [Zn4Mg(CO3)4(OH)4]2-, parallel to {001}, with interlayer H-bonded [Mg(H2O)6]2+ and H2O. Within the layers, [110] chains of corner-connected ZnO2(OH)2 tetrahedra share corners with Mg-centred octahedra. The two types of polyhedra are connected via CO3 groups. The Raman and infrared spectra of šlikite as well as tentative assignment of observed bands are given in this paper. The mineral is named in honour of the prominent Czech aristocrat Štepán Šlik (1487–1526), founder of town St. Joachimsthal (now Jáchymov). |
