『Abstract
The large discrepancy between field and laboratory measurements
of mineral reaction rates is a long-standing problem in earth
sciences, often attributed to factors extrinsic to the mineral
itself. Nevertheless, differences in reaction rate are also observed
within laboratory measurements, raising the possibility of intrinsic
variations as well. Critical insight is available from analysis
of the relationship between the reaction rate and its distribution
over the mineral surface. This analysis recognizes the fundamental
variance of the rate. The resulting anisotropic rate distributions
are completely obscured by the common practice of surface area
normalization. In a simple experiment using a single crystal and
its polycrystalline counterpart, we demonstrate the sensitivity
of dissolution rate to grain size, results that undermine the
use of “classical” rate constants. Comparison of selected published
crystal surface step retreat velocities (Jordan and Rammensee,
1998) as well as large single crystal dissolution data (Busenberg
and Plummer, 1986) provide further evidence of this fundamental
variability. Our key finding highlights the unsubstantiated use
of a single-valued “mean” rate or rate constant as a function
of environmental conditions. Reactivity predictions and long-term
reservoir stability calculations based on laboratory measurements
are thus not directly applicable to natural settings without a
probabilistic approach. Such a probabilistic approach must incorporate
both the variation of surface energy as a general range (intrinsic
variation) as well as constraints to this variation owing to the
heterogeneity of complex material (e.g., density of domain borders).
We suggest the introduction of surface energy spectra (or the
resulting rate spectra) containing information about the probability
of existing rate ranges and the critical modes of surface energy.』
1. Introduction
2. Materials and methods
3. Results
4. Discussion
4.1. VSI dissolution rates
4.2. Relevance to published data
4.3. An environmental example
5. Conclusions
Acknowledgements
References