Cristobalite and tridymite from deposits of the Arsenatnaya fumarole (Tolbachik volcano, Kamchatka, Russia)

Abstract

This article displays data on cristobalite and tridymite from the Arsenatnaya active fumarole, the Tolbachik volcano, Kamchatka, Russia. The minerals occur in associations with fumarolic sylvite, sanidine, cassiterite, hematite, pseudobrookite, johillerite, tilasite, badalovite. Fumarolic cristobalite is tetragonal (-modification); the unit-cell parameters for one of samples are: а = 4,975 (7) Å, с = 6,944 (13) Å, V = 171,89 Å3. There are two types of tridymite — monoclinic (MC) and orthorhombic (PO-10) — in the Arsenatnaya fumarole. The unit-cell parameters of these tridymite modifications are: a = 18,553 (5), b = 5,006 (1), с = 25,952 (10) Å,   = 117,68 (2)o, V = 2134,3 (11) Å3 (MC); a = 9,941 (2), b = 17,165 (4), с = 82,362 (18) Å, V = 14053,4 (29) Å3 (PO-10). Mineral assemblages of cristobalite and tridymite indicate high-temperature formation conditions of these minerals — not lower 450–500 °С — with a high participation degree of HCl and HF in process of basalt alteration by fumarolic gas. The surrounding basalt was a source of silicon. This element was, probably, transported in the form of SiX4, where X = F, Cl.