Oxygen Diffusion and Water-Related Defects in Feldspar Minerals: Information from Experimental and Modelling Studies

Abstract

The diffusion of oxygen in a variety of silicate and oxide minerals has been studied under both dry conditions (exchange with a dry gas, frequently oxygen or CO2) and hydrothermal conditions ('wet'; exchange with fluid enriched in t80 at high pressure). An early summary of the data (Freer and Dennis, 1982) showed that oxygen diffusion under dry conditions was associated with slow diffusivities and high activation energies (typically 300-400 kJ mol I ) whilst oxygen diffusion under 'wet' conditions was associated with faster diffusion rates and small activation energies (of the order 100-150 kJ mol-t). Direct comparison between the types of experiment is difficult since many of the early 'dry' experiments employed powdered specimens whilst most of the hydrothermal, 'wet', experiments were performed on single crystals, with the short (~ I pro) diffusion prof'fles analysed directly by SIMS or nuclear reaction analysis (Elphick and Graham, 1991). During the last decade good quality data from experiments performed under wet and dry conditions (using single crystals) has confirmed that the p ~ c e of water causes oxygen diffusion rates to be higher and activation energies lower in both anorthite (Elphick and Graham 1988) and quartz (Elphick et al., 1988). It has been postulated that protons somehow mediate in the jump process leading to a lowering of the activation barrier and hence faster diffusion. However, the exact mechanism responsible for this phenomena remains unknown. Oxygen could migrate in 'wet' conditions as either OH groups or as H20 molecules. In order to elucidate the effects of water in oxygen diffusion in feldspar at an atomistic level, we have used computer simulation techniques to probe the energetics and geometry of a range of defects and to ascertain migration energies for oxygen and OH groups in Albite. Complementary experimental studies of oxygen diffusion in sanidine have been performed under 'wet' and dry conditions. An assessment is made of experimental data for oxygen diffusion in various feldspar minerals under wet and dry conditions.