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The malachite-rosasite group: crystal structures of glaukosphaerite and pokrovskite
No full textThe crystal structures of glaukosphaerite (Cu,Ni)2(CO3)(OH)2 and pokrovskite Mg2(CO3)(OH)2, two carbonates belonging to the malachite-rosasite group, have been determined from powder diffraction data, and refined up to wRp = 3.94% and 1.63% respectively. Both minerals are isostructural with rosasite (Cu,Zn)2 (CO3)(OH)2, with P21/a space group and cell parameters a = 12.0613(4) Å, b = 9.3653(4), c = 3.1361(1), β=98.085(5)° for glaukosphaerite and a = 12.2396(4), b = 9.3506(4), c = 3.1578(1), β = 96.445(5)° for pokrovskite. Their structures are built up by ribbons of edge-sharing octahedra running along the c-axis; the ribbons are linked together through corner-sharing giving rise to corrugated layers parallel to (100) that are interconnected through carbonate groups. The same layers and carbonate groups, arranged in a different orientation with respect to the symmetry operators, build up the structure of malachite: the relationships between the rosasite-like and the malachite-like structural models are discussed. The octahedral distortion of the two independent Me sites in the malachite-rosasite known structures is evaluated and attributed to the Jahn-Teller effect of Cu2+. According to the available chemical data for the examined material, a partial occupancy in Mg sites of pokrovskite is maintained, with a coupled partial substitution of hydroxyls by water molecules. On the basis of the structural results, the difficulties and ambiguities in the powder pattern indexing by preceeding authors are discussed and explained, and reliable guesses for the arrangements of kolwezite and nullaginite are drawn
THE CRYSTAL-STRUCTURE OF HIORTDAHLITE-I
No full textThe crystal structure of hiortdahlite I (triclinic, {Mathematical expression}, a=11.0149(9), b=10.9409(9), c=7.3534(3), α=109.350(3), β=109.879(4), γ=83.434(4)) was determined and refined using 2910 reflections to R1=0.058 and R2=0.055. The results of the structure determination confirmed that hiortdahlite presents the features common to the whole series of minerals in the cuspidine group, namely "octahedral walls", four columns large and running parallel to c, interconnected by corner sharing and through Si2O7 groups. The cation distribution in the polyhedral walls and the location of the fluorine anions were determined and discussed. The crystal-chemical formula obtained from the structural study is (Na3.2Ca0.8)Ca8Zr2M2(Si2O7)4O2.8F5.2 where M represents a mixing site with average charge three
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