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

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 (001) surface and fast growth of etch pits from the resulting steps, which represent proxies for the {hk0} surface. Measurements of dissolution rates as a function of temperature allow a determination of an apparent activation energy (Ea,app) for the process, via mass-loss calculations from image analysis. We obtain a value of Ea,app = 49 ± 2 kJ mol , which is consistent with separate calculations based on planar area etch pit growth, and measurements of etch pit perimeters, indicating that this value of Ea,app is representative of {hk0} 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 Ea,app = 140 kJ mol , on extrapolation towards a pristine surface with no defects. We suggest that this higher value of Ea,app represents the slow formation of etch pits into the (001) surface. 2011 Elsevier Ltd. All rights reserved.