Recent studies of topographical changes during dissolution at gypsum crystal surfaces focus on the formation and growth of etch pits because dissolution both parallel and normal to the surface is much faster at etch pits than at defect-free portions of the surface. In this study, the in situ absolute retreat rates of gypsum (010) cleavage surfaces are measured in flowing water by reflection digital holographic microscopy. Observations made on randomly sampled fields of view on seven different cleavage surfaces reveal a range of local dissolution rates, the local rate being determined by the topographical features at which material is removed. Four characteristic types of topographical activity are observed: 1) smooth regions, free of etch pits or other noticeable defects, where dissolution rates are relatively low; 2) shallow, wide etch pits bounded by faceted walls which grow gradually at rates somewhat greater than in smooth regions; 3) narrow, deep etch pits which form and grow throughout the observation period at rates that exceed those at the shallow etch pits; and 4) relatively few, submicrometer cleavage steps which move in a wave-like manner and yield local dissolution fluxes that are about five times greater than at etch pits. Molar dissolution rates at all topographical features except submicrometer steps can be aggregated into a continuous, mildly bimodal distribution with a mean of 3.0 umol/m2/s and a standard deviation of 0.7 umol/m2/s.

In situ nanoscale observations of gypsum dissolution by digital holographic microscopy

253rd National Meeting (2017)

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