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 R_p = 4.56%, wR_p = 6.51%. Kolwezite, P2₁/a, a = 12.201(1), b = 9.354(1), c = 3.1494(3) Å, β = 98.922(7)°, is isostructural with rosasite. Because of the Jahn-Teller effect, the Me1 polyhedron hosting Cu²⁺ only is strongly distorted towards a (4+2) coordination, while the Me2 octahedron, with major Co, is neatly a more regular one. The Me²⁺ 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 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, 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.