II.

Composition

Calcite contains the carbonate ion (CO3-2), making it part of the carbonate family of minerals, which also includes dolomite, azurite, and aragonite. Calcite differs from dolomite and azurite in chemistry and from aragonite in internal structure. In addition to the carbonate ion, calcite contains calcium whereas dolomite contains an equal mixture of calcium and magnesium and azurite contains copper.

Calcite and aragonite have identical chemical compositions, but the molecules are stacked differently in the crystals. When two minerals have identical chemical compositions but different crystal structures, they are polymorphs of each other. Calcite is generally more stable than aragonite. Over several years, most aragonite will change to calcite. Thus, newly created calcium carbonate deposits may contain some or even a lot of aragonite, but ancient deposits contain little or no aragonite.

Calcite is only slightly soluble in pure cold water. However, rainwater or other water in contact with air will absorb carbon dioxide from the air. Carbon dioxide reacts with water to form carbonic acid. Calcite reacts with carbonic acid to form calcium bicarbonate. Calcium bicarbonate is quite soluble in water. Thus, the solubility of calcite is sensitive to the acidity of the water. The solubility of calcite is also sensitive to the temperature of the water, being quite soluble in cold water but not in hot water. Calcite frequently dissolves in one location and precipitates in another location when the water becomes less acidic or cools.