Abstract
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.
Highlights
The study of the reactivity of silicate minerals is essential to understand numerous bio-geochemical processes
In this study we investigate the reactivity of the cleaved biotite (001) surface, at pH 1 and pH ca. 9.5, by using in situ flowthrough LCM-DIM experiments, combined with phase shifting interferometry (PSI)
The same surface reacted for ca. 17 h at pH 1 and 25 °C shows edge retreat, layer swelling and peeling (Figure 1b), the latter processes being a consequence of biotite dissolution
Summary
The study of the reactivity of silicate minerals is essential to understand numerous bio-geochemical processes. Flow-through reactors filled with powdered samples are frequently used to study the reaction mechanisms of mica dissolution and possible formation of new phases [3,4,5,6,7,8,9] In this type of experiment, the full control over the parameters that influence the reactions (e.g., flow rate, pH, temperature and solution composition) allows one to quantify the mineral dissolution rates and the study of the reaction mechanisms under a wide range of experimental conditions. This experimental approach is rather unapt to deal with the reactivity of each crystal face, elucidate the face-specific dissolution–precipitation mechanisms and determine the specific location of the secondary mineral formation. With the progress of these techniques our understanding of the mechanisms of the surface reactivity of phyllosilicates has greatly improved
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