Volcanic emissions of molecular chlorine

Up to 60ppmv (180mg/m3) of Cl2 together with 40–80ppmv HCl were measured in gas emissions from the Tolbachik scoria cones, Kamchatka, which are still hot after the 1975–1976 eruption. Other gas components were atmospheric air (94–99vol %), water vapour (1–6vol %) and other acid species (HF, CO2 and H2SO4, total less than 0.1vol %). Two different processes can account for the existence of Cl2 in the Tolbachik emissions. The catalytic oxidation of volcanic HCl by oxygen is probably the main source of Cl2. Fine crystals of Fe2O3, and oxides and chlorides of other transition metals on the surface of altered basalt can serve as catalysts. The oxidative decomposition of Na, K and Mg chloroferrates formed as a result of basalt acid leaching, can also create high concentrations of molecular chlorine in volcanic gases. The processes described represent a previously unknown case of abiogenic heterogeneous catalysis in nature and examples of gas–rock interactions that affect the composition of volcanic gases.