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Atomic Positional Parameters
Ca 6b  0.0000 0.0000 0.0000
C  6a  0.0000 0.0000 0.2500
O  18e 0.2593 0.0000 0.2500

Reference
RL Sass, R Vidale and J Donohue (1957) Acta Crystallographica, 10, 567

Mineral Chemistry
CaCO₃ occurs in several forms - calcite, aragonite and vaterite - each with distinct crystal structures. In calcite, calcium ions and carbonate (CO₃) groups are arranged such that calcite may be considered as a distorted NaCl (halite) structure. Thus calcium ions are coordinated by six oxygens; each oxygen is bonded to two calcium ions.

Calcite is one of the most common minerals, occurring in extensive sedimentary deposits - limestones - derived from shells and skeletons of marine organisms. Metamorphosed limestones recrystallize to form marbles. In carbonatites and nepheline syenites, calcite occurs as a primary igneous phase. Carbonates isostructural with calcite include magnesite (MgCO₃), siderite (FeCO₃), rhodochrosite (MnCO₃) and smithsonite (ZnCO₃).

The CHIME figure shows calcium ions as large grey spheres to make clearer the coordination with oxygen.