THE CRYSTAL STRUCTURE OF (Cd,In)-RICH CANNIZZARITE FROM KUDRIAVY VOLCANO, ITURUP ISLAND, KURILES, RUSSIA

A (Cd,In)-rich variety of cannizzarite has been found in sublimates from high-temperature fumaroles of the Kudriavy volcano, on Iturup Island, Kurile Archipelago, Russia. An electron-microprobe analysis gave the unit formula (calculated on the basis of 14 anions): (Pb3.54Cd1.06)(Sigma 4.60)(Bi5.45In0.29)(Sigma 5.74)(S13.30Se0.70)(Sigma 14). A single-crystal X-ray-diffraction study indicates a monoclinic symmetry, space group P2(1)/m, with unit-cell parameters a 15.4172(9), b 4.0582(2), c 20.8071(12) angstrom, beta 98.031(2)degrees, V 1289.05(12) angstrom(3), and Z = 2. Its crystal structure has been solved and refined to R-1 = 0.0504 on the basis of 3158 unique reflections. There are 25 atom positions, corresponding to 11 cations and 14 anions. Among cations, bond-valence calculations allow us to distinguish three Pb sites, two Bi sites, and six mixed sites. Selenium seems to concentrate at two specific S positions of the H layer. In the cannizzarite variable-fit series of the composite layer type, the two H and Q sublattices are here in the ratio 3H : 5Q, like in the synthetic isotypic selenide Pb5Bi6Se14. The structural features of H and Q layers and the common pseudotranslational sublattices of cations and anions are examined. The distribution of Pb, Bi, Cd and In among the mixed sites is discussed; cadmium and In could not be differentiated. In the final proposal, the Q layer contains the three Pb sites, with two mixed (Pb, Bi) sites; the H layer contains the two Bi sites, one [Pb, Bi,(Cd,In)] site, and three [Bi,(Cd,In)] sites.