Incorporation of Co in the rosasite-malachite carbonate group of minerals: crystal structure studies of kolwezite and of synthetic cobaltoan malachites

The crystal structure of kolwezite, (Cu,Co)2(CO3)(OH)2, has been refined by Rietveld method from synchrotron X-ray powder data. Kolwezite, P21/a, a = 12.201(1), b = 9.354(1), c = 3.1494(3) Å, ß= 98.922(7)°, is isostructural with rosasite, and its crystal structure was refined up to Rp = 4.56%, wRp = 6.51%. Because of the Jahn-Teller effect, Me1 polyhedron hosting Cu2+only is strongly distorted towards a (4+2) coordination, while Me2 octahedron, with major Co, is neatly a more regular one. Me2+ polyhedra are connected through edge-sharing, forming "ribbons" running along [001] direction. These ribbons are interconnected by corner sharing to form "corrugated" layers, interlinked by the carbonate groups, giving rise to an infinite framework structure, with notation M−M=M−T. The Rietveld study of synchrotron X-ray powder data of synthetic cobaltoan malachites shows a decrease in unit-cell volume, with an anisotropic contraction of unit-cell parameters, directly related to the increasing Co content. The major progressive shortening of apical bonds in Me2 octahedra, with consequent regularization of the Jahn-Teller distorted Cu octahedra, drives a progressive compacting of the malachite structure, the main factor accounting for volume contraction in cobaltoan malachites.