Mechanism of feldspar weathering: Some observational evidence

Abstract

Scanning electron microscopy (SEM) studies of sodic plagioclase and potassium feldspars isolated from soils indicate that the dissolution of feldspar during weathering occurs primarily at sites of excess energy on the crystal surface (for example, dislocations) and not by uniform attack over the entire surface of the mineral. As a result, distinctive etch pits develop on the feldspar surface. These can be easily reproduced in the laboratory by leaching fresh feldspars with acidic HF solutions. The reaction products (clay) left by the weathering of feldspars in soils do not form a tightly adhering, protective layer. The material remaining on the feldspar surfaces after washing or ultrasonic cleaning, as seen by SEM, is patchy, discontinuous, and sufficiently hydrous that it forms micromudcracks and pulls away from the surface upon drying. Also, feldspar grains from the A horizon of a mountain soil, from which all adhering clay has been removed, do not exhibit a distinctly decationated surface layer thicker than about 30 A, as shown by x-ray photoelectron spectroscopy (XPS). Results of this study suggest that the weathering of feldspar is controlled by chemical reaction at the feldspar-solution interface, and not by diffusion, either through aqueous solution or through a continuous protective surface layer.