Zincobotryogen, ZnFe3+(SO4)2(OH)•7H2O: validation as a mineral species and new data

Zincobotryogen occurs in the oxidation zone of the Xitieshan lead-zinc deposit, Qinghai, China. The mineral is associated with jarosite, copiapite, zincocopiapite, and quartz. The mineral forms prismatic crystals, 0.05 to 2 mm in size. It is optically positive (2V(calc) = 54.1A degrees), with Z parallel to b and X boolean AND c = 10 degrees. The elongation is negative. The refractive indices are n (alpha) = 1.542(5), n (beta) = 1.551(5), n (gamma) = 1.587(5). The pleochroism scheme is X = colorless, Y = light yellow, Z = yellow. Microprobe analysis gave (in wt%): SO3 = 38.04, Al2O3 = 0.04, Fe2O3 = 18.46, ZnO = 13.75, MgO = 1.52, MnO = 1.23, H2O = 31.06 (by calculation), Total = 104.10. The simplified formula is (Zn,Mg)Fe3+(SO4)(2)(OH).7H(2)O. The mineral is monoclinic, P12(1)/n1, alpha = 10.504(2), b = 17.801(4), c = 7.1263(14) angstrom, and beta = 100.08(3)degrees, V = 1311.9(5) (3), Z = 4. The strongest lines in the powder X-ray diffraction pattern d(I)(hkl) are: 8.92 (100)(110), 6.32 (77)(-101), 5.56 (23)(021), 4.08 (22)(-221),3.21 (31)(231), 3.03 (34)(032), 2.77 (22)(042). The crystal structure was refined using 2816 unique reflections to R1(F) = 0.0355 and wR2(F (2)) = 0.0651. The refined formula is (Zn0.84Mg0.16)Fe3+(SO4)(2)(OH).7H(2)O. The atomic arrangement is characterized by chains with composition [Fe3+(SO4)(2)(OH)(H2O)](2-) and similar to 7 angstrom repeat distance running parallel to the c-axis. The chain links to a [MO(H2O)(5)] octahedron (M = Zn, Mg) and an unshared H2O molecule, and forms a larger chain building module with composition [M Fe-2+(3+)(SO4)(2)(OH)(H2O)(6)(H2O)]. The inter-chain module linkage involves only hydrogen bonding.