Abstract

Examination of published feldspar dissolution data indicates that the relative rates of release of Na, K, Ca, and Si do not conform to the bulk stoichiometry of the parent feldspar. This lack of conformity is of two types: (1) nonstoichiometric release of Na, K, and Ca relative to each other and (2) nonstoichiometric release of Na, K, and Ca relative to Si. The former is attributed to inhomogeneity of the feldspar due to exsolution intergrowths and chemical zonation in crystals. The latter, which persists after accounting for secondary mineral precipitation, is due to the two-step reaction mechanism that results in the release of Si. This involves the exchange of H+ for Na+, K+, and Ca++ on the surface of the feldspar, followed by release of Al and Si to solution. Mass-balance calculations indicate that the first step accounts for a substantial proportion of the overall reaction progress and that a steady-state quantity of the H-feldspar complex is not produced. Thus, the general rate law of Aagaard and Helgeson cannot be applied to the early stages of dissolution. However, if each step is considered elementary and substrate limited, rate laws can be written for each that simulate the delayed release of Si relative to Na, K, and Ca. These rate laws can also be shown to result in linear kinetics under certain conditions (e.g., substrate saturation and steady-state H-feldspar).

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