Reactions of Feldspar and Mica with Water at Low Temperature and Pressure1

R. M. Garrels and Peter Howard
Department of Geology, Harvard University, Cambridge, Massachusetts
1 Paper no. 156. Published under the auspices of the Committee on Experimental Geology and Geophysics and the Division of Geological Science at Harvard University.

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 aK+/aH+ exceeds 109–10 or 107–8 respectively. These ratios decrease in the temperature range 25–65°C by a factor of about 100.7. 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+5 H 2 O=3 kaolin+2 K + +2O H − 3 K−feldspar+2 H 2 O=K−mica+6 quartz+2O H − can be considered to result from H+−K+ exchange, and that the energy contribution from other changes is small.

Clays and Clay Minerals; 1959 v. 6; no. 1; p. 68-88; DOI: 10.1346/CCMN.1957.0060107
© 1959, The Clay Minerals Society
Clay Minerals Society (www.clays.org)