2008 Joint Annual Meeting (5-9 Oct. 2008): Albite Dissolution Kinetics Under near-Equilbrium Conditions

In weathering environments, the ability to predict how mineral dissolution rates respond to environmental change requires a fundamental understanding of the relationship between reaction kinetics and aqueous solution composition. The need for this understanding is particularly acute in silicate minerals such as plagioclase feldspars close to thermodynamic equilibrium, where dissolution rates are slow and the role of solution chemistry is potentially complex. Here we present experimental results under near equilibrium conditions and < 100°C that confirm our earlier hypothesis of critical path dependence in albite dissolution kinetics, in which the DG and temperature history of the surface determine the dissolution rate actually observed.

We derive several key conclusions from our experimental observations: first, rates measured as a function of DG do not compose a continuous rate function, and second, the concept of “steady state” cannot be applied indiscriminately to this system, as our discussion will illustrate. These results have significant implications for the application of laboratory rate data to natural and synthetic problems ranging from bedrock weathering to the stability of glasses used, for example, in nuclear waste disposal. These results cast doubt on the conventional treatment of laboratory rates, where data are fit as smooth, continuous functions of environmental variables. Reaction history is likely a critically overlooked variable relevant to the poor “fit” and large variation of rate data commonly observed in natural environments, and the large inconsistency between field and laboratory rates.