Abstract

The rate-limiting step for the dissolution of many primary silicate minerals is thought to be a surface-controlled reaction. This should result in dissolution rates that are linear functions of time. However, the observed dissolution rates for aluminosilicate minerals have commonly been characterized as “parabolic”—that is, the apparent rates of release of silicon and of the alkali ions are linear functions of the square root of time (t½). Here, data are presented which suggest that this discrepancy may result in part from the precipitation of secondary aluminosilicates during the dissolution experiments. In simulated dissolution studies, the kinetics of formation and the stoichiometries of precipitates can be studied unambiguously as functions of time, component concentrations, and pH. Results of these studies indicate that significant amounts of silicon are incorporated into amorphous aluminum-rich precipitates at dissolved silicon concentrations greater than 170 μm. At higher concentrations, silicon precipitation rates increase. These precipitation reactions result in apparent rates of addition of dissolved silicon to solution which are linear functions of t½. As a result of the type of precipitation reactions observed here, estimates for the rates of dissolution obtained for aluminum-rich silicate minerals may be low by as much as a factor of two.

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