Volcanic emissions of molecular chlorine

Up to 60 ppmv (180 mg/m(3)) of Cl-2 together with 40-80 ppmv 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 (94-99 vol %), water vapour (1-6 vol %) and other acid species (HF, CO2 and H2SO4, total less than 0.1 vol %). Two different processes can account for the existence of Cl-2 in the Tolbachik emissions. The catalytic oxidation of volcanic HCl by oxygen is probably the main source of Cl-2. 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. (C) 2012 Elsevier Ltd. All rights reserved.