The novel solid solutions with titanite structure: arsenates, phosphates, and vanadates of the tilasite group

The solid-solution system between natural arsenates, vanadate, and phosphate of the tilasite group are characterized. The material originates from sublimates of the active Asenatnaya fumarole, Tolbachik volcano, Russia and paleofumaroles of Thomas Range, Utah, USA. The tilasite-group minerals were formed under the oxidizing conditions at temperatures > 300°C. The minerals belong to the titanite structure type and have the general formula AM(TO4)X in which A are Ca and Na in sevenfold coordination, M are octahedrally coordinated Mg, Al, Fe3+, and Ti, T are tetrahedrally coordinated pentavalent As, P, and V, and X are O or F. The studied minerals show wide-range isomorphous substitutions at all these sites except of O atoms coordinating T. Maxwellite NaFe3+(AsO4)F and durangite NaAl(AsO4)F form a solid-solution series with only homovalent substitution Fe3+ ↔ Al. Two systems with heterovalent substitutions are observed: (1) with the major substitution Mg ↔ Fe3+ between tilasite CaMg(AsO4)F, maxwellite and a hypothetic end-member CaFe3+(AsO4)O and (2) with the major substitution M3+ ↔ Ti4+ between arsenatrotitanite NaTi(AsO4)O, durangite, maxwellite and CaFe3+(AsO4)O. In both cases the charge balance is achieved by coupled substitutions ANa+ ↔ ACa2+ and XF− ↔ XO2−. Two novel mineral varieties belong to these solid solutions, namely a MFe3+-dominant variety of tilasite with the formula close to (Ca2/3Na1/3)(Fe3 + 2/3Mg1/3)(AsO4)(F2/3O1/3) and an XO-dominant variety of maxwellite with the formula (Na2/3Ca1/3)(Fe3 + 2/3Ti1/3)(AsO4)(O2/3F1/3). Another solid solution forms between reznitskyite CaMg(VO4)F, tilasite and isokite CaMg(PO4)F with the major substitutions TAs5+ ↔ TV5+ ↔ TP5+. All these solid solutions are novel and highlight the specific crystal chemistry of fumarolic oxysalts with the titanite-type structure.