Effects of Al/Si ordering on feldspar dissolution: Part I. Crystallographic control on the stoichiometry of dissolution reaction

Abstract

The incongruent dissolution of feldspar is associated with the formation and evolution of Si-rich amorphous interfacial structures during its chemical weathering. The stoichiometry of dissolution is compositionally dependent, and this dependence reflects the nature of dissolution reaction as a combination of surface renewal and heterogeneous chemical reaction. We hypothesize that during continuous surface renewal, reactive sites on a feldspar’s surface will inherit characteristics from the bulk structure of the primary mineral. Hence, the dissolution rate of feldspar depends not only on water chemistry but also on its crystallographic properties. We propose a new formalism to quantify the dependence of the dissolution rates of feldspars on their crystallographic properties. A correlation between the degree of Al/Si ordering and the stoichiometry of feldspar dissolution is predicted based on this formalism. This correlation is verified by a combination of water chemistry analysis, synchrotron X-ray diffraction with structure refinement (HR-XRD), and Fourier transform infrared spectroscopy (FTIR). We find that the rates of dissolution and the degree to which dissolution is incongruent depend on the frequency of SiOSi and SiOAl linkages (chemistry) and the crystallographic characteristics of these linkages (bond lengths, bond angles, and ordering) in the dissolving mineral. The knowledge of Al/Si ordering provides a means to quantify the relative abundances of Si atoms with different reactivities. Using a C1¯-based structure model, we show that the distribution of Al within the Al-rich (T1) type of tetrahedral sites has only a minor effect on dissolution stoichiometry, whereas the distribution of Al between the T1-type and the Si-rich (T2)-type of sites affects dissolution incongruence predominantly. A greater extent of disorder results in higher dissolution incongruence. As an implication, our results suggest that the formation of interfacial structures during silicate dissolution should at least be partially affected by reactivity differences among the framework atoms of a silicate.