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The malachite-rosasite group: crystal structures of glaukosphaerite and pokrovskite

Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126 Pisa, Italy

^* Corresponding author, e-mail: natale{at}dst.unipi.it

The 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), ß = 98.085(5)° for glaukosphaerite and a = 12.2396(4), b = 9.3506(4), c = 3.1578(1), ß = 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.

Key-words: glaukosphaerite, pokrovskite, powder data, structural solution, malachite-rosasite group.

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