Sparingly soluble salt minerals have been used by many researchers as model systems to study mineralization and dissolution. To date, most research has focused on calcite and these studies have significantly enriched our understanding in mineral-water interactions. For example, extensive investigations have revealed the formation of etch pits on calcite cleavage surfaces, pit morphology, and anisotropy in step velocities.  In comparison, however, another common salt mineral, gypsum, has not received enough attention. For example, we have not acquired detailed knowledge in how monolayers of gypsum respond to solution chemistry change. The purpose of this study is to explore the surface process of gypsum during dissolution.

This study investigates the behavior of gypsum {010} cleavage faces during dissolution at near and far from equilibrium condition. Dissolution experiments were conducted on both the (010) and (0ī0) surfaces, and the surface process and step motion of gypsum were monitored by in situ fluid cell atomic force microscopy. Experimental results reveal that: (i) gypsum dissolution on the {010} faces is not characterized by the formation of deep etch pits, even at far from equilibrium conditions, because the lateral step speed far exceeds the vertical dissolution rate; (ii) the only type of etch pits on the {010} cleavage faces is that embraced by the [100] and [001] steps; previously reported pits formed by the [101] and [001] steps are likely incorrect due to neglecting the atomic structure differences of the (010) and the (0ī0) cleavage faces; (iii) compared to calcite, the anisotropy in step velocity is much more pronounced, presumably due to the significant difference between the atomic structures of the [100] and [001] steps on gypsum.

These findings are complementary to what we learned from studying other salt minerals, but further suggest that etch pit formation may not be universally significant to mineral dissolution and weathering.