Hematite from Fumaroles of the Tolbachik Volcano (Kamchatka, Russia): Chemistry, Relationships with Associated Minerals, Morphological and Genetic Features

Hematite from the fumaroles of the Tolbachik volcano (Kamchatka, Russia) was studied in detail. This fumarolic hematite contains significant admixtures of chalcophile elements: tin (up to 9.2 wt % SnO₂, the highest content known for natural hematite), copper (up to 4.7 wt % CuO), and for the first time antimony (up to 2.6 wt % Sb₂O₅). Other significant admixtures are represented by Ti, Mg, Mn, Al, and Cr. The previously unknown Sn–Cu- and Sn–Cu–Sb-bearing varieties of hematite were revealed, as well as new isomorphic schemes for this mineral: Sn⁴⁺ + Cu²⁺ → 2Fe³⁺ and Sb⁵⁺ + 2(Cu,Fe)²⁺ → 3Fe³⁺. It is shown that, in oxidizing-type Tolbachik fumaroles, hematite is the major concentrator of Fe, as well as Sn and Sb. The epitaxy of rutile-group minerals (cassiterite, rutile, and tripuhyite), oxide spinels (magnesioferrite, gahnite, and cuprospinel), and corundum is described. Hematite enriched in such admixtures was formed in the temperature range of 500–850°C. Volcanic gas was the source for chalcophile elements (Sn, Sb, Cu, Zn), whereas a source for hardly volatile components (Mg, Al, Cr, Ti) was basalt surrounding fumarole chambers; a mixed source is suggested for Fe, Mn, and V. The chalcophile specificity of admixed components is an indicative feature of fumarolic hematite. The finding of martite in Tolbachik fumaroles implies that the oxidizing potential increases in time during the fumarolic mineral formation.