The effect of the Ca2+ to activity ratio on spiral growth at the calcite surface

Abstract

Variation in the Ca2+ to CO32- activity ratio of natural waters is rarely considered in models intended to describe calcite growth. Atomic force microscopy (AFM) and differential interference contrast (DIC) microscopy were used to examine spiral growth on calcite {101¯4} surfaces from solutions in which the Ca2+:CO32- activity ratio ranged from 0.1 to 100, at constant supersaturation. In general, growth velocity decreased with increasing Ca2+:CO32- activity ratio and acute steps were more affected by changes in solution composition than obtuse steps. At high Ca2+:CO32- activity ratios, obtuse steps grow faster than acute steps but this trend reverses at low activity ratios. This is reflected in the morphology of growth pyramids. The reversal in the inequivalent step growth velocity indicates that the hydrated carbonate ion preferentially incorporates at kink sites along the more structurally open obtuse step edges, whereas the hydrated calcium ion is more easily accommodated at the more confined acute step kink sites. Furthermore, the experimental data demonstrate that velocity is maximum for obtuse steps when the activities of Ca2+ and CO32- are equal, whereas maximum acute step velocity is achieved at higher relative CO32- activity. The obtuse step velocity data fit the ‘kinetic ionic ratio’ model of Zhang and Nancollas (1998) well, but acute step velocities cannot be described by this model. This is attributed to dissimilar dehydration frequencies for Ca2+ and CO32- and differences in kink geometry at obtuse and acute step edges, which, in turn, affects the frequency of ion incorporation.