Reactions of Feldspar and Mica with Water at Low Temperature and Pressure

Abstract Muscovite and K-feldspar (adularia) were dry-ground to — 200/inch particles, which were suspended in water. These suspensions were “titrated” with KCl, and the pH was recorded as a function of KCl concentration and temperature. The results indicate that muscovite and adularia react with water to produce a surface film in which H + has displaced K +. The “titration” curves show some characteristics attributable to exchange reactions, others apparently related to equilibria among solids of fixed composition. Maximum release of K + from adularia by reaction with water is much greater than that from mica. The interpretation is made that the first result of reaction of mica and adularia with water is a surface layer that grades from an outer portion that is structurally disrupted to an inner portion that retains the original silicate structure but with H + substituted for K+. Addition of K + as KCl to the suspending solution displaces H + from the disrupted zone, but all H + originally taken up by the solids was not returned to the solution by concentrations of KCl up to 1. 0 M. Experiments were of a few hours duration; work by others has shown that the disrupted zone releases appreciable concentrations of silica and alumina to solution over longer time intervals. These hydrolysis experiments indicate that at 25°C an H-feldspar or H-mica structure is favored over a K-feldspar or K-mica structure except in solutions in which the ratio of a K+ a H+ exceeds 10 9–10 or 10 7–8 respectively. These ratios decrease in the temperature range 25-65°C by a factor of about 10 0. 7. These results, where considered in relation to the observed behavior of feldspar and mica under weathering conditions, indicate that the major energy change for the reactions, 2\, mica + 5H₂O = \, 3\, kaolin + \, 2K^ + \, + \, 2OH^ - 3K - feldspar + 2H₂O = \, K - mica + 6quartz + \, 2OH^ - can be considered to result from H + -K + exchange, and that the energy contribution from other changes is small.