Calcite is the least soluble polymorph of CaCO 3 and, hence, the most thermodynamically stable. It has a solubility product (K sp ) of 10 -8. . Since the solution will be saturated with calcite, it is unlikely that the pores result from the dissolution of the crystals, particularly not in conditions where the pH ≥ 12. It is more likely that the selective adsorption of additive Ca 2+ ions, in addition to its other effects, also created the pores. This may result from their occlusion or poisoning of growth sites in the crystals. Therefore, as seen in Figure , a proportional increase in the intensity of pores can be expected as the concentration of additive Ca 2+ ions is increased.