Pseudobrookite from Active Fumaroles of the Tolbachik Volcanic Complex (Kamchatka), and Composition and Chemistry of Minerals of the Pseudobrookite Group

Pseudobrookite from the Arsenatnaya and Yadovitaya active volcanic fumaroles (Tolbachik volcanic complex, Kamchatka) is characterized for the first time. The chemical composition and peculiarities of isomorphism of minerals of the pseudobrookite group of various genetic groups are studied on the basis of 203 analyses of 40 samples and published literature data on 44 geological objects. It is shown that the subsurface conditions are most favorable for the crystallization of pseudobrookite including that with the composition close to end-member $${\text{Fe}}_{2}^{{3 + }}{\text{Ti}}{{{\text{O}}}_{5}}$$ ; lunar rocks contain minerals of the ferropseudobrookite Fe2+Ti2O5–armalcolite MgTi2O5 series characterized by an intermediate Fe2+/Mg ratio, a maximum Ti content, and the absence of Fe3+; and the impact minerals and their technogenic analogs are close in composition to Fe2+Ti2O5. The Tolbachik fumarolic pseudobrookite has a high Al2O3 content (up to 7.1 wt %) and a permanent content of CuO and SnO2 (up to 1.8 wt % both), as well as a low Sc2O3 content (up to 0.3 wt %). The presence of Cu and Sn is typical of pseudobrookite of this genetic type. These elements often occur together in pseudobrookite following the scheme Sn4+ + Cu2+ → 2Fe3+, which was previously unknown for minerals of this group. Pseudobrookite in Tolbachik fumaroles formed during gaseous–metasomatic alteration of basalt at 500–750°С. It is suggested that Cu and Sn were transported by volcanic gas; Ti, Al, and Sc were taken from basalt hosting the fumarolic chambers; and a Fe source could be mixed.