MINERALS OF THE SYSTEM ZnS CdS FROM FUMAROLES OF THE KUDRIAVY VOLCANO, ITURUP ISLAND, KURILES, RUSSIA

Abstract

Minerals of the system ZnS–CdS formed between 400 and 725°C in active fumaroles at the Kudriavy volcano, Iturup Island, Russia, were studied using scanning electron microscopy with energy dispersion, X-ray diffraction and with an electron microprobe. The composition of these minerals varies widely: Zn 2.46–57.7, Cd 2.38–69.5, Fe 0.72–13.6 wt.%, and thus covers almost the entire ZnS–CdS series. Two types of solid solutions occur: cubic, from 0 to ∼8 mol.% CdS, and hexagonal, from α17 to 100 mol.% CdS, separated by a two-phase transition zone. Three different mechanisms of substitution account for the range of composition: Zn-for-Fe in sphalerite, Zn-for-Cd in wurtzite–greenockite, and Cd-for-Fe in greenockite. The incorporation of Cu and In, up to 2.92 and 4.75 wt.%, respectively, is noted in sphalerite, and up to 1.27 wt.% Cu and 2.75 wt.% In, respectively, in minerals of the wurtzite–greenockite series. Both elements evidently occur as the roquesite component in solid solution. Zinc-rich species are metal-deficient, whereas Cd-rich species are sulfur-deficient. The range of nonstoichiometry in minerals of the system ZnS–CdS studied is in the order of 2.6 at.%, or 0.05 apfu S. We conclude that there is not a complete solid-solution between wurtzite and greenockite in the temperature range studied owing to a phase transition in ZnS and a change in the structure type. Phase relations for the temperature range studied proved to be identical with those established for synthetic compounds. In a fumarolic environment, crystals of Cd-rich sulfides of the system ZnS–CdS precipitate mainly at lower temperatures (400°C) than those of Zn (up to 725°C).