From "loose" to "dense" crystalline phases of calcium carbonate through "repulsive" interactions: an experimental charge-density study.

Anion-anion interactions in an eggshell: experimental electron density analysis for two polymorphs of calcium carbonate revealed why the less stable form, aragonite, has higher density than the most stable form, calcite. Although believed to be exclusively repulsive, the interactions between anions cause them to bind more tightly in a crystal and thus make the aragonite phase denser than its calcite counterpart.