CaCO 3 occurs in both the calcite and aragonite structure types. Hence, calcite is the stable form of CaCO 3 at room temperature. Each Ca is surrounded by nine closest oxygens. The cations have an arrangement approximating hexagonal closest packing, which gives rise to marked pseudohexagonal symmetry. This is reflected in the pseudohexagonal twinning which is characteristic of all members of the group.
Pseudohexagonal twins are also common.
These cyclic twins are distinguished from true hexagonal forms by noting that the basal surface is striated in three different. Also found in reniform, columnar, and stalactitic aggregates. Occurrence - Aragonite is less stable than calcite under atmospheric conditions and much less common. Experiments have shown that carbonated waters containing calcium more often deposit aragonite when they are warm and calcite when they are cold. The pearly layer of many shells and pearl itself is aragonite.