『Abstract
The micas are a unique class of minerals because of their layered
structure. A frequent question arising in mica dissolution studies
is whether this layered structure radically changes the dissolution
mechanism. We address this question here, using data from VSI
and AFM experiments involving muscovite to evaluate crystallographic
controls on mica dissolution. These data provide insight into
the dissolution process, and reveal important links to patterns
of dissolution observed in framework minerals. Under our experimental
conditions (pH 9.4, 155℃), the minimal global rate of normal surface
retreat observed in VSI data was 1.42×10-10 mol/m2/s
(σ= 27%) while the local rate observed at deep each pits reached
416×10-10 mol/m2/s (σ= 49%). Complementary
AFM data clearly show crystallographic control of mica dissolution,
both in terms of step advance and the geometric influence of interlayer
rotation (stacking periodicity). These observations indicate that
basal/edge surface area ratios are highly variable and change
continuously over the course of reaction, thus obviating their
utility as characteristic parameters defining mica reactivity.
Instead, these observations of overall dissolution rate and the
influence of screw dislocations illustrate the link between atomic
step movement and overall dissolution rate defined by surface
retreat normal to the mica surface. Considered in light of similar
observations available elsewhere in the literature, these relationships
provide support for application of the stepwave model to mica
dissolution kinetics. This approach provides a basic mechanistic
link between the dissolution kinetics of phyllosilicates, framework
silicates, and related minerals, and suggests a resolution to
the general problem of mica reactivity. 』
1. Introduction
2. Methods
3. Results
3.1. VSI data of etch pit populations
3.2. VSI data: dissolution rates
3.3. AFM data
4. Discussion
4.1. Etch pit morphology, structure, and distribution
4.2. Distribution of dissolution rates and their control by the
phyllosilicate structure
4.3. Application of the stepwave model
5. Summary
Acknowledgments
References