Digital holographic microscopy in reflection mode is used to track in situ, real time nanoscale evolution of topography of cleaved (104) calcite surfaces exposed to flowing or static deionized water. The method utilizes conventional immersion objective lenses and is able to capture full-field holograms of the surface at frame rates of up to 12.5 s-1. Numerical reconstruction of the holograms provides 3D surface topography with vertical resolution of a few nanometers, and enables measurement of the time-dependent dissolution rate. A statistical distribution, or spectrum, of dissolution rates is generated by sampling multiple area domains on multiple crystals. The data show that dissolution is not described well by a single or average rate value, although the modal dissolution rate agrees well with published mean dissolution rates (e.g., 0.1 µmol m-2 s-1 to 0.3 µmol m-2 s-1). Etch pits and other sources of rapidly dissolving material appear at different times and have a heterogeneous spatial distribution across the surface. This makes the distribution in rates measured on a single crystal dependent both on the length scale of observation and on time, even under conditions of nominally constant undersaturation. Statistical analysis of the inherent noise in the measurements indicates that the methods for obtaining them are robust, and that the rate spectra reflect fundamental variability of the material instead of measurement artifacts. The same method can be applied to study other dissolution or growth processes at other mineral surfaces to develop a better understanding of the kinetics in terms of rate spectra.

Calcite dissolution rate spectra measured by digital holographic microscopy

253rd National Meeting (2017)

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