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, P 21 / 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 Me 1 polyhedron hosting Cu2+ only is strongly distorted towards a (4 + 2) coordination, while the Me 2 octahedron, with major
Co, is neatly a more regular one. The Me
2+ 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 Me 2 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.
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SYNCHROTRON POWDER DATA;
SYNTHETIC COBALTOAN MALACHITES
Document Type: Research Article
Publication date: June 1, 2018
This article was made available online on April 3, 2018 as a Fast Track article with title: "Incorporation of Co in the rosasite–malachite carbonate group of minerals: crystal structure studies of kolwezite and synthetic cobaltoan malachites".
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The European Journal of Mineralogy publishes original papers, review articles and letters dealing with the mineralogical sciences s.l. These include primarily mineralogy, petrology, geochemistry, crystallography and ore deposits, as well as environmental, applied and technical mineralogy. Nevertheless, papers in any related field, including cultural heritage, will be considered.
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