Pseudobrookite from Active Fumaroles of the Tolbachik Volcanic Massif (Kamchatka). Chemistry of Pseudobrookite-Group Minerals and Its Indicator Role

For the first time there is characterized pseudobrookite from deposits of active volcanic fumaroles Arsenatnaya and Yadovitaya of the Tolbachik volcanic massif (Kamchatka). Chemical composition of pseudobrookite-group minerals and isomorphism in these oxides of different origin were studied on the base of 203 analyses (literature data and original analyses of 40 samples) from 44 localities. It is shown that at the near-surface levels pseudobrookite crystallized mainly with composition chemically close to the end-member Fe\(_{2}^{{3 + }}\)TiO₅. It is known that lunar rocks contain only minerals of the “ferropseudobrookite” Fe²⁺Ti₂O₅ ‒ armalcolite MgTi₂O₅ series characterized by an intermediate value of the Fe²⁺:Mg-ratio, the highest Ti content and the absence of Fe³⁺. Impact minerals and there technogenic analogues are chemically close to Fe²⁺Ti₂O₅. Pseudobrookite from Tolbachik fumaroles is characterized by the high Al content (up to 7.1 wt % Al₂O₃), a stable level of Cu (up to 1.8 wt % CuO) and Sn (up to 1.8 wt % SnO₂) admixtures, and, sporadically by the small admixture of Sc (up to 0.3 wt % Sc₂O₃). Cu and Sn admixtures are indicative features of the pseudobrookite of this genetic type. These elements usually are incorporated into pseudobrookite together, according to the isomorphic scheme Sn⁴⁺ + Cu²⁺ → 2Fe³⁺, which is revealed for the first time for minerals of this group. Pseudobrookite of Tolbachik fumaroles was formed participating the process of gas–metasomatic alteration of basalt at temperature 500–750 °С. Presumably, Cu and Sn were brought by volcanic gas, while basalt was a source of Ti, Al and Sc; for Fe a source could be mixed.