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

The crystal structure of kolwezite, (Cu, Co)2(CO3)(OH)2, was refined by Rietveld method from synchrotron X-ray powder data to Rp =4.56%, wRp =6.51%. Kolwezite, P21/a, a= 12.201(1), b=9.354(1), c=3.1494(3)A , b = 98.922(7)°, is isostructural with rosasite. Because of the Jahn-Teller effect, the Me1 polyhedron hosting Cu2þ only is strongly distorted towards a (4þ2) coordination, while the Me2 octahedron, with major Co, is neatly a more regular one. The Me2þ polyhedra are connected through edge-sharing, forming "ribbons" running along [0 0 1] 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 notationMM=MT. 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, with 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 compaction of the malachite structure; it is the main factor accounting for volume contraction in cobaltoan malachites.