Theoretical approach to evaluating plagioclase dissolution mechanisms

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 ( An 0) 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 An 20 is 3-fold, and the difference is reduced to 2-fold among 107 andesine configurations of An 30. 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 An 80 and is in good agreement with previous experimental studies. Subsequent modeling results of aluminum inhibition, Δ G 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.