Langbeinite-Group Minerals and Vanthoffite from Fumarole Exhalations of the Tolbachik Volcano (Kamchatka)

In this paper, we provide characteristics of sulfates of exhalation origin-langbeinite K2Mg2(SO4)(3), two modifications of calciolangbeinite K2Ca2(SO4)(3) (new data), and vanthoffite Na6Mg(SO4)(4) (first mineralogical data for this genetic type)-from active fumaroles of the Tolbachik volcano in Kamchatka. These minerals are associated with anhydrous copper sulfates and arsenates, minerals of the aphthitalite and alluaudite groups, krasheninnikovite, anhydrite, sanidine, cristobalite, tridymite, tenorite, hematite, etc. Langbeinite and calciolangbeinite form a series of solid solutions, in which most of the compositions correspond to the ranges of (Mg2.0-1.6Ca0.0-0.4) and (Ca1.2-2.0Mg0.8-0.0). It is shown that, in calciolangbeinite, with a content of more than 20 mol % of K2Mg2(SO4)(3), decomposition into cubic calciolangbeinite with a lower Mg content and langbeinite can occur upon slow cooling. For the first time, in minerals of the langbeinite group, impurities of copper and zinc, the maximal concentrations of which are noted in langbeinite with a low Ca content and reach 0.53 atoms per formula unit (below, apfu) for Zn (10.0 wt % of ZnO) and 0.18 apfu for Cu (3.3 wt % of CuO), were revealed. These elements replace Mg and Ca. Varieties of langbeinite and calciolangbeinite enriched in Na (up to 0.31 apfu = 2.3 wt % of Na2O) were found. Other significant impurities in these minerals are represented by Rb, Cs, Mn, Cd, Al, and Fe. The fumarole vanthoffite contains impurities of K, Ca, Mn, Zn, Cu, and Fe (up to 0.47 apfu in total). This significant manifestation of cationic isomorphism in langbeinite-group minerals and vanthoffite is observed only at Tolbachik volcano and is caused primarily with the peculiar conditions of their crystallization in high-temperature volcanic fumaroles.