Abstract

Comparisons of plagioclase and microcline taken from different weathering environments show that the mechanisms of feldspar dissolution are independent of the weathering environment. However, microcline and plagioclase do not weather by identical mechanisms. Calcium and sodium can be removed from the plagioclase lattice to depths greater than one or two unit cells, which implies that the “leached layer” may form on weathering plagioclase grains. The larger potassium ions cannot be removed from the microcline lattice deeper than one or two unit cells unless the anionic lattice is distorted or disrupted. Macroscopic solution features on plagioclase parallel the arrangement of calcium and sodium ions within the crystal. Solution features that develop on microcline may begin at dislocations on the crystal surface, or they may be located at inclusions of plagioclase. Scanning electron microscopy of microcline indicates that a precipitate as thick as 1 μm may form at the surface of the mineral. The rate of dissolution of microcline may be related to the cation exchange capacity of this layer and by reactions within the layer.

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