『Abstract
The more rapid dissolution of Ca-rich feldspars relative to Na,K-rich
feldspars has been attributed to the preferential leaching of
Al deep within the feldspar structure. Evidence from surface microanalysis
(e.g., Hellmann et al., 2003), however, shows that preferential
dissolution of Al is confined to the top layers of the feldspar
lattice and that the amorphous surface layer most likely results
from precipitation versus dissolution. It is thus critical to
examine the extent of preferential Al removal. Here we present
a theoretical study of plagioclase dissolution behavior using
parameterized Monte Carlo simulations. Two different dissolution
mechanisms, a mechanism involving preferential leaching of Al
and an interfacial dissolution-reprecipitation mechanism, are
tested using compositions representing the entire plagioclase
solid solution series. Our modeling results indicate that under
the control of the preferential Al leaching mechanism, the influence
of (Al,Si) disorder on the dissolution rate is significant. At
a fixed composition, an increase in the degree of (Al,Si) disorder
yields an increased dissolution rate, with an 8-fold increase
in dissolution rate observed for highly disordered albite (An0) compared to low albite. Increasing anorthite
content tends to decrease the variation in the dissolution rate
due to disorder. The difference in the dissolution rate of 293
tested oligoclase configurations with a composition of An20 is 3-fold, and the difference is reduced to
2-fold among 107 andesine configurations of An30.
Furthermore, feldspar configurations with completely disordered
(Al,Si) distributions yield a consistent log-linear dependence
of dissolution rate on the anorthite content (An), while other
feldspar configurations with modest degrees of (Al,Si) disorder
exhibit rates less than this trend. In contrast, when Al removal
is confined to the top surface layers, a variety of feldspar configurations
with different (Al,Si) disorder but a single fixed composition
have similar dissolution rates; and the dissolution rate of Ca-rich
feldspars departs positively from its log-linear relationship
with anorthite content. This departure occurs around An80
and is in good agreement with previous experimental studies. Subsequent
modeling results of aluminum inhibition, ΔG dependence, and formation
of altered surface layers in the framework of the interfacial
dissolution-reprecipitation mechanism are all comparable with
experimental investigations, and these results suggest that an
interfacial dissolution-reprecipitation mechanism governs the
dissolution of plagioclase feldspars.』
1. Introduction
2. Plagioclase mineralogy
3. Theoretical dissolution model
3.1. Variation of Si-O-Al and Si-O-Si in the entire plagioclase
series
3.2. Quantification of the solution saturation state
3.3. Formulation of the kinetic model
4. Results
4.1. Dependence of dissolution rate on anorthite content
4.1.1. Preferential Al leaching mechanism
4.1.2. Confining the Al preferential dissolution process to
the top layers
4.2. Effect of (Al,Si) disorder
4.3. Saturation state dependence
4.4. Surface relaxation
5. Discussions
5.1. Comparison with previous studies
5.2. Interstitial space of feldspar structures
5.3. The altered layer and the independent dissolution beneath
6. Summary and conclusion
Acknowledgments
References