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

Abstract

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): (Pb 3.54 Cd 1.06 ) ∑4.60 (Bi 5.45 In 0.29 ) ∑5.74 (S 13.30 Se 0.70 ) ∑14 . A single-crystal X-ray-diffraction study indicates a monoclinic symmetry, space group P 2 1 / m , with unit-cell parameters a 15.4172(9), b 4.0582(2), c 20.8071(12) A, β 98.031(2)°, V 1289.05(12) A 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 Pb 5 Bi 6 Se 14 . 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.