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The malachite-rosasite group, with general formula Me22+(CO3)(OH)2 , includes both common minerals, such as malachite (Me2+=Cu) and rosasite (Me2+=Cu, Zn), together with rare species such as chukanovite (Fe), kolwezite (Cu,Co), glaukosphaerite (Cu,Ni), mcguinnessite (Mg,Cu), nullaginite (Ni), pokrovskite (Mg). Besides the chemical composition, their structural relationships are suggested by the common features of their powder patterns.
All the above minerals are always customarily found as microcrystalline fibrous spherical aggregates, and just for this reason single crystal studies were performed for malachite [1] and rosasite [2] only. Rosasite crystal structure [3] was solved through powder diffraction data, revealing a structural arrangement related to that of malachite. Malachite is monoclinic, P21/a, with a=9.502, b=11.974, c=3.24 Å, β =98.75°, whereas rosasite, P21/a, has cell constants a=12.897, b=9.370, c=3.162 Å, β=110.26°. Both structures are built up by the same "modules", namely octahedral walls of edge-sharing Me2+ polyhedra, which are interconnected by corner sharing to form infinite "layers", and "triangular" CO3 groups, which ensure further connections between the layers. The two structures show close resemblance when seen in [001] projection, but given the different ways in which the above "modules" interconnect, they are in fact different. Subsequent structural studies in this group were carried out on powder data by the Rietveld method, leading to the structural solution for mcguinnessite [3], glaukosphaerite and pokrovskite [4], and lastly for chukanovite [5]. All the above mentioned phases are isostructural with rosasite. Kolwezite was defined as a new species [6] with chemical formula (Cu1.33Co0.67)(CO3)0.98(OH)2.07, and assigned to the triclinic system with a=9.50 Å, b=12.15 Å, c=3.189 Å, α= 93.32°, β=90.74°, γ=91.47°. According to (4) the presence in the powder pattern of kolwezite (PDF 29-1416) of an intense reflection (-121) at 2.591 Å and, contrarly to malachite, of only a weak (-201) reflection at 2.872 Å suggest the isostructurality with rosasite. Starting from the structural model of rosasite, the structure of kolwezite was successfully refined using synchrotron X-ray data, up to Rp= 1.08%, wRp= 1.68 %, RBragg= 0.68%.
Cu occupies one of the two independent Me sites, showing a Jahn-Teller distorted 4+2 coordination, whereas Co concentrates in the second independent site, with a slightly distorted octahedral coordination. References [1] Zigan, F., Joswig, W., Schuster, H.D., Mason, S.A., (1977), Zeit. Krist., 145, 412 [2] Roberts, A. C., Jambor, J. L., Grice, J. D., (1986), Powd. Diff., 1, 56. [3] Perchiazzi, N. (2006), Z. Kristallogr. Suppl. 23, 505-510 [4] Perchiazzi, N., Merlino, S. (2006) Eur. J. Mineral. 2006, 18, 787?792 [5] Pekov I., Perchiazzi N., Merlino S., Kalachev V., Merlini M., Zadov A. (2007), Eur. J. Mineral. 19 891-898 [6] Deliens M., Piret P. (1980), Bull. Mineral. 103 179-184.
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