Reply to comment on “Repulsion between calcite crystals and grain detachment during water-rock interaction” by Le Merrer and Colombani, 2017

Abstract

The comment by Le Merrer and Colombani (2017) focuses on the mechanisms that could account for our experiments, in which we observed the detachment of micrometre scale calcite grains from the surface of micritic limestone during contact with a reactive fluid (Levenson and Emmanuel, 2017). They discuss some of the forces acting on the grains and imply that our observations are likely to be artefacts of the experimental method. Furthermore, they suggest that because our measured calcite dissolution rates do not match exactly the dependence on ionic strength predicted by Colombani (2016), an "unidentified phenomenon" could be at play in our experiments. While Le Merrer and Colombani (2017) raise some valid points, we think that an alternative interpretation is possible. We are pleased to be able to discuss these aspects of our paper in greater detail.In experiments carried out under a range of flow conditions, we have observed the detachment of micrometre size grains from the surface of micritic limestone (Emmanuel and Levenson, 2014; Emmanuel, 2016, 2017). Le Merrer and Colombani (2017) show that under our experimental conditions neither thermal agitation nor fluid shearing are strong enough to overcome gravity and remove grains from the surface. Moreover, they assert that although the repulsive forces related to the electric double layer and hydration are much stronger than the gravitational force, repulsion cannot remove grains from the surface because it only operates over the relatively short range of several nanometres. However, by considering the energy gains and losses involved in removing a grain from the surface, it can be shown that this is not necessarily the case. From the relationship describing the effective weight of a grain, F = (r calcite -r water )gV grain , where r calcite represents the density of calcite (2710 kg m -3 ), r water , the density of water (1000 kg m -3 ), g, gravitational acceleration (9.81 m s -2 ), and V grain , the grain volume, it follows that a 1 mm calcite grain surrounded by water would gain ~2 × 10 -20 J in gravitational potential energy for every micrometre it is removed from the surface in the vertical direction. According to the experiments carried out by Røyne et al. (2015), calcite surfaces in close proximity in pure water experience only repulsive forces so a calcite grain in contact with another calcite surface would possess a certain potential energy that could be translated into gravitational or kinetic energy. An estimate of this energy can be made by integrating the area under the force-distance curves, reported by Røyne et al. (2015), and then normalising by the surface area of the crystals used in their experiments. The crystal surface areas were reported to be in the range of 300 to 3500 mm 2 , and accordingly, we estimate the potential energy to be ~0.1-1.7 × 10 -20 J mm -2 . Using these values, a 1 mm calcite grain with an area of 5 mm 2 , in contact with another calcite surface, has ~0.5-8 × 10 -20 J of potential energy. Significantly, the higher end of th...