Alteration of trioctahedral micas in the presence of inorganic and organic acids

The alteration of two trioctahedral micas, biotite and phlogopite, was investigated at the meso, micro, and nanoscale using three complementary microscopy techniques to better understand mica surface reactivity. In situ and ex situ experiments were performed to monitor the mineral interface during dissolution in acidic solutions (nitric and oxalic acid, pH -1-2), over a temperature range of 25-100 degrees C. The inorganic acid was used as a benchmark condition to elucidate the effect of the organic acid on the dissolution behavior. The observed topographical changes that arose during mineral alteration revealed the simultaneous occurrence of different processes that heterogeneously shaped the mica surface: 1) the retreat of pre-existing and newly formed steps (edge surface reactivity). In the case of biotite, layer curling and peeling-off occurred in the presence of nitric acid whereas dendritic-shaped step edges resulted from the effect of oxalic acid; 2) the nucleation of etch pits and the formation of dissolution channels on the (001) surface. Oxalic acid promoted the growth of the pits to such an extent that they were discernible at each scale and resolution investigated; and 3) precipitation of secondary phases. Overall, a multi-scale approach offers new insights into the dissolution behavior of biotite versus phlogopite and provides and enhances understanding of the effect that oxalic acid has on the surface reactivity of mica.

Automated summary

The alteration of biotite and phlogopite micas was investigated using three microscopy techniques to understand mica surface reactivity better. Acidic solutions were used to dissolve the minerals, and in situ and ex situ experiments were performed. The findings revealed different processes shaping the mica surface, including the retreat of steps, the formation of etch pits and dissolution channels, and the precipitation of secondary phases. Overall, this multi-scale approach enhanced understanding of mica dissolution behavior and the effect of oxalic acid on mica surface reactivity.