DEFINITION OF CARBONATE MINERALS

Minerals that contain the carbonateanion (CO3

–2). The two dominant carbonate minerals are calcite

and aragonite, which are pseudomorphs (minerals with

the same chemical formula but different crystal structures)

with the formula CaCO3. Calcite has a trigonal crystal form,

typically forming white, pink, colorless, yellow, or gray crystals

with a variety of shapes, including rhombohedrons and

clusters of small angular crystals. Distinct crystal faces and

striations, or cleavage traces, on the crystal faces make many

varieties of calcite easily distinguished from quartz. Aragonite

has orthorhombic symmetry, is denser and harder than calcite,

has a less distinct cleavage, and typically forms as

fibrous aggregates with gypsum in hot springs and in shallow

marine muds and coral reefs. Aragonite is less stable than calcite

and typically reverts to calcite in rock sequences. Most

aragonite is relatively pure, but several varieties are formed

by substitution. Dolomite is another common carbonate mineral,

made of (Ca,Mg)CO3. In strontianite (SrCO3), the strontium

ion substitutes for calcite, and in witherite (BaCO3),

barium substitutes for calcium. Other common carbonates

include magnesite (MgCO3), rhodochrosite (MnCO3), and

siderite (CO3). Other orthorhombic carbonate minerals

include alstonite (Ca,Ba[CO3]2) and cerussite (PbCO3), which

is an alteration product of galena. Vaterite is a rare hexagonal

form of CaCO3.

See also CALCITE; CARBONATE; MINERALOGY.

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