High-resolution imaging of biotite dissolution and measurement of activation energy

We have used a direct imaging technique, in situ atomic force microscopy (AFM) to observe the earliest stages of the dissolution of a biotite surface by oxalic acid at temperatures close to ambient conditions, using a specially designed AFM liquid cell and non-invasive intermittent contact mode of operation. From the nm-resolution data sets in x, y and z dimensions, we have measured dissolution rates and determined activation energies for the process as a function of temperature, via a mass-loss calculation. The value of E_a obtained, 49±2 kJ mol^-1, appears to be too high to indicate a diffusion-controlled process and is more in line with expectations based on a process limited by the rate of ligand-induced metal cation detachment from the (001) surface. This is consistent with visual observations of the relative rates of etch-pit formation and growth, and accepted knowledge of the biotite crystal structure. Separate calculations based on planar area etch-pit growth, and measurements of etch-pit perimeters confirm this result, and also indicate substantially higher activation energy, up to 80 kJ mol^-1, when the edge pits are in an incipient stage.