Acidic dissolution of plagioclase: In-situ observations by hydrothermal atomic force microscopy

Hydrothermal atomic force microscopy (HAFM) provides in situ access to the surfaces of dissolving crystals at temperatures above the ambient boiling point of water. Here, we applied HAFM to the (001) surfaces of labradorite and anorthite at temperatures up to 125 degrees C. In HCl solutions (pH 2) we observed the formation of a rough and soft surface layer on both minerals. By applying high loading forces to the scanning tip: the soft layer can be removed and the underlying interface (between the fresh solid and the altered layer) can be observed. In this way, in situ information about the thickness of the altered layer on plagioclase and the morphology of the underlying interface can be obtained. On labradorite, the thickness of this layer does not exceed about 30 nm within the first 5 hr of exposure to acidic solution at 125 degrees C, but on anorthite thicknesses of up to about 300 nm were observed. The uncovered interface on anorthite shows a nonuniform morphology and either appears rough in AFM images or shows a step-like pattern. On anorthite, etch pits spread underneath the altered layer. This suggests that material must be released and transported through the layer without obvious changes in morphology of the layer's surface. Based on the rate of spreading of etch pits, the dissolution rate was calculated to be about 2 x 10(-6) mol m(-2) s(-1) at 125 degrees C. This value agrees reasonably well with literature data and supports the suggestion that dissolution mainly takes place underneath the altered layer and not on its surface. Copyright (C) 1999 Elsevier Science Ltd.