Fumarolic arsenates − a special type of arsenic mineralization

This paper is a review devoted to a special type of arsenic mineralization related to volcanic fumaroles of the oxidizing type. Fifty-four hydrogen-free arsenates (40 valid species and 14 insufficiently characterized phases) with Cu²⁺, Mg, Al, Fe³⁺, Ti⁴⁺, Zn, Ca, Na, and K are identified here and 46 of them are endemics of fumarolic formation. Fifty-three of them have been found in active fumaroles at the Tolbachik volcano, Kamchatka, Russia. Thirty different structure types have been established for fumarolic arsenates, including 19 previously unknown among minerals. The alkali-richest (>17 wt% Na₂O + K₂O) and Ti-richest (12–28 wt% TiO₂) natural arsenates occur in Tolbachik fumaroles. Nineteen fumarolic arsenates with Cu²⁺, Al, Fe³⁺ or Ti contain additional O²⁻ anions. The major fumarolic arsenates at Tolbachik are alluaudite-group members (johillerite, calciojohillerite, nickenichite, bradaczekite and badalovite), lammerite, svabite, tilasite, berzeliite and urusovite. All Tolbachik arsenates crystallized at temperatures ≥450 °C (the major assumed temperature interval is 500–750 °C), under atmospheric pressure and high oxygen fugacity. They were precipitated directly from the gas phase as volcanic sublimates, or were formed as a result of gas–rock interaction in fumarolic cameras, where the host basalt was the source of the less volatile elements Al, Ti, Mg and Ca. The empirical data on fumarolic As⁵⁺ minerals are summarized and discussed, including their chemistry, crystal chemistry (with special attention paid to isomorphic substitutions and solid solutions) and occurrence. Electron-microprobe analyses illustrating the chemical diversity and variability of 53 fumarolic arsenates are given as supplementary data, as well as a photographic documentation of their morphology and paragenetic relationships.