In situ atomic force microscopy measurements of biotite basal plane reactivity in the presence of oxalic acid

Abstract

We have used a direct imaging technique, in situ atomic force microscopy (AFM), to observe the dissolution of the basal biotite surface by oxalic acid over a range of temperatures close to ambient conditions, using a specially designed AFM liquid cell and non-invasive intermittent contact mode of operation. From the 3-dimensional nanometre-resolution data sets, we observe a process characterised by the slow formation of shallow etch pits in the (0 0 1) surface and fast growth of etch pits from the resulting steps, which represent proxies for the {h k 0} surface. Measurements of dissolution rates as a function of temperature allow a determination of an apparent activation energy ( E a,app ) for the process, via mass-loss calculations from image analysis. We obtain a value of E a,app = 49 ± 2 kJ mol −1, which is consistent with separate calculations based on planar area etch pit growth, and measurements of etch pit perimeters, indicating that this value of E a,app is representative of {h k 0} surface dissolution. The measurement of etch pit perimeters also enables an estimation of apparent activation energy as a function of step density indicating substantially higher apparent activation energy, up to E a,app = 140 kJ mol −1, on extrapolation towards a pristine surface with no defects. We suggest that this higher value of E a,app represents the slow formation of etch pits into the (0 0 1) surface.