In situ observation of biotite (001) surface dissolution at pH 1 and 9.5 by advanced optical microscopy

Laser confocal differential interference contrast microscopy (LCM-DIM) allows for the study of the reactivity of surface minerals with slow dissolution rates (e.g., phyllosilicates). With this technique, it is possible to carry out in situ inspection of the reacting surface in a broad range of pH, ionic strength and temperature providing useful information to help unravel the dissolution mechanisms of phyllosilicates. In this work, LCM-DIM was used to study the mechanisms controlling the biotite (001) surface dissolution at pH 1 (11 and 25 °C) and pH 9.5 (50 °C). Step edges are the preferential sites of dissolution and lead to step retreat, regardless of the solution pH. At pH 1, layer swelling and peeling takes place, whereas at pH 9.5 fibrous structures (streaks) form at the step edges. Confocal Raman spectroscopy characterization of the reacted surface could not confirm if the formation of a secondary phase was responsible for the presence of these structures.