On the crystal-chemistry of rosasite and paradsasvarite

We report the results of mineralogical and structural studies of para´dsasva´rite from Rudaba´nya, 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. Para´dsasva´rite has the chemical formula (Zn1.91Cu0.06Mg0.02)R1.99(CO3)(OH)2, ideally Zn2(CO3)(OH)2, and rosasite has the formula (Cu1.14Zn0.84Mg0.02)R2(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, P21/a, a 12.2436(29) A? , b 9.3555(19) A? , c 3.1535(6) A? , b¼98.69(3)8, and its crystal structure was refined to R1¼12.4%, wR2¼35.5%. The presence of Cu2þ 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 para´dsasva´rite, based on synchrotron radiation data, shows it is isostructural with rosasite, monoclinic P21/a, a 12.253(4) A? , b 9.348(3) A? , c 3.167(1) A? , b ¼ 97.700(4)8, and its crystal structure was refined to R1 ¼ 1.45%, wR2 ¼ 2.45%. The Me1 and Me2 coordination polyhedra in para´dsasva´rite, 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.