On the Crystal-Chemistry of Rosasite and Parádsasvárite

We report the results of mineralogical and structural studies of paradsasvarite from Rudabanya, Hungary, and rosasite from Hayden, Arizona (USA). A preliminary investigation of the two minerals, which belong to the rosasite-malachite group, was conducted using Raman spectroscopy, X-ray diffraction, and EPMA. Paradsasvarite has the chemical formula (Zn1.91Cu0.06Mg0.02)(Sigma 1.99)(CO3)(OH)(2), ideally Zn-2(CO3)(OH)(2), and rosasite has the formula (Cu1.14Zn0.84Mg0.02)(Sigma 2)(CO)(3)(OH)(2), matching the general formula CuZn(CO3)(OH)(2). The first single-crystal full structural study of rosasite from Hayden, based on synchrotron X-ray data, is reported here and fully confirms the results obtained by Perchiazzi (2006) on the basis of X-ray powder data. The presence of structural disorder in Cu-Zn distribution in these minerals is indicated by the streaking of reflections along c*. Rosasite is monoclinic, P2(1)/a, a 12.2436(29) angstrom, b 9.3555(19) angstrom, c 3.1535(6) angstrom, beta = 98.69(3)degrees, and its crystal structure was refined to R-1 = 12.4%, wR(2) = 35.5%. The presence of Cu2+ causes a Jahn-Teller distortion of coordination polyhedra, a major feature of the rosasite crystal structure, particularly evident in the Me1 coordination polyhedron, which is fully occupied by Cu. A Rietveld study of paradsasvarite, based on synchrotron radiation data, shows it is isostructural with rosasite, monoclinic P2(1)/a, a 12.253(4) angstrom, b 9.348(3) angstrom, c 3.167(1) angstrom, beta = 97.700(4)degrees, and its crystal structure was refined to R-1 = 1.45%, wR(2) = 2.45%. The Me1 and Me2 coordination polyhedra in paradsasvarite, the former hosting Zn with a minor presence of Cu, the latter fully occupied by Zn, are as expected both more regular than the corresponding polyhedra in rosasite. Crystal-chemical considerations, based on literature chemical data for rosasite and Zn-rich malachite, indicate that a minimum Cu content of 0.4-0.5 apfu is necessary to stabilize the malachite-type structure.