Distribution and chemical composition of halotrichite in geothermal fields of kamchatka

Halotrichite has been identified in geothermal fields of the Kambalny-Pauzhetka-Koshelev region, the Bolshoi Semiachik complex, and the Mutnovsky volcano (Kamchatka, Russia). Halotrichite forms efflorescence on soils heated up to 70°C, around steam-gas vents and boiling springs. Tschermigite, rozenite, szomolnokite, gypsum, alunogen, barite, melanterite, hexahydrite, and minerals of the copiapite, alunite and voltaite groups were found in close association with halotrichite. The chemical composition of halotrichite samples from different geothermal fields is similar and characterized by Mg substitution in the Fe2+ position, with Fe2+:Mg ratio ranging from 90:10 to 50:50, and also by Fe3+ substitution in the Al position in some samples, with Al:Fe3+ reaching up to 85:15. Halotrichite is a typical mineral of low-temperature volcanic setting, formed as a result of alteration of parent minerals by hydrothermal fluid and represents an intermediate form of crystallization of leached elements. At the same time, the local conditions of mineral formation — such as variations in Eh, pH, surface temperature, and elemental composition — do not reflect in the chemical composition or other thin typomorphic features of halotrichite. The mechanism of halotrichite formation is probably identical in both geothermal fields and zones of oxidation of sulfide deposits.