Atomic Positional Parameters
Ca 4c 0.2500 0.4150  0.7597
C  4c 0.2500 0.7622 -0.0862
O1 4c 0.2500 0.9225 -0.0962
O2 8d 0.4736 0.6810 -0.0862

Reference
JPR De Villiers (1971) American Mineralogist, 56, 758-767

Mineral Chemistry
Aragonite, in contrast to calcite, has calcium ions coordinated by nine oxygens; each oxygen is bonded to three calciums. Aragonite has a more compact atomic arrangement than calcite and is the stable phase at high pressures and low temperatures. Shells of aragonite are formed by many living organisms - the pearl has the aragonite structure. Over time, aragonite deposited in sediments will transform to calcite.

Carbonates isostructural with aragonite include witherite (BaCO₃), strontianite (SrCO₃) and cerussite (PbCO₃).

The CHIME figure shows calcium ions as large grey spheres, oxygens as small red spheres, and carbons as small light grey spheres. The unit cell is outlined.