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

Hematite from fumaroles of the Tolbachik volcano (Kamchatka, Russia) was studied in detail. There is characterized epitaxy of rutile-group minerals (cassiterite, rutile, and tripuhyite), oxide spinels (magnesioferrite, gahnite, and cuprospinel), and corundum on hematite. 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 antimony (up to 2.6 wt % Sb₂O₅) – which is a novel for this mineral. Other significant admixtures are represented by Ti, Mg, Mn, Al, and Cr. There were revealed previously unknown Sn-Cu- and Sn-Cu-Sb-bearing varieties of hematite and new for this mineral isomorphic schemes: Sn⁴⁺ + Cu²⁺ → 2Fe³⁺ and Sb⁵⁺ + 2(Cu,Fe)²⁺ → 3Fe³⁺. It is shown that in oxidizing-type fumaroles of Tolbachik hematite is the major concentrator of Sn and Sb. Hematite enriched in such admixtures was formed in the temperature range of 500–850 °C. Volcanic gas was the source of chalcophile elements (Sn, Sb, Cu, Zn), whereas a source of 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.