Calcite

CaCO3

Calcite is the stable form of the widely distributed mineral calcium carbonate (CaCO3). It is noted for its beautiful varieties and its perfect crystals. Calcite is the primary mineral component of limestone and its metamorphic form, marble. It is very soluble in groundwater and is common in the shells of invertebrates like clams and oysters.

A large percentage of the calcite in rocks was deposited in sedimentary environments of both chemical (limestone) and biochemical (chalk) origin. Calcite clastic rocks consist of transported fragments from previously deposited, biologic materials (calcarenite). A small amount of calcite is of igneous origin and is the primary component of the rare rock called calcite carbonatite.

Calcite dissolves when chemical weathering takes place. It does so in surface-water and groundwater solutions containing organic acids or dissolved carbon dioxide (CO2) and is deposited when the CO2 escapes. The formation of limestone caves or sinkholes, and the subsequent deposit of speleothems are a result of this process. Precipitated calcite forms stalactites and stalagmites in caves, as well as deposits of travertine around hot springs and calcareous tufa deposits around other springs and streams.

Calcite is polymorphous (having the same chemical formula but different crystal structure) with the minerals aragonite and vaterite as well as with several other forms that apparently exist only under extreme experimental conditions.

Calcite, in its many varieties, has more than 600 distinct crystal forms. It ranges from white or colorless through a great variety of colors -- pink, red, yellow, greenish, bluish, lavender, brown or black -- depending on its impurities. It is found in tens of thousands of localities and has become a specialty of mineral collectors, many of whom belong to the International Calcite Collectors Association.

Uses of Calcite

The limestone form of calcite has been extensively used since ancient times. It is burned to quicklime (CaO), slaked to hydrated lime [Ca(OH)2] and mixed with sand to make mortar. Limestone is one of the primary ingredients in the manufacture of portland cement. It is employed in metallurgical processes and as aggregate for both concrete and asphalt mixes.

Travertine
Sample of Travertine

Marble has long been used for statuary and carvings, and as polished slabs for facing stone. Travertine is also a favorite interior facing stone, usually for interior use. Iceland spar, a transparent form, has been used for optical and crystallography research for two centuries.

Physical Properties of Calcite

Calcite effervesces vigorously with dilute hydrochloric (muriatic) acid, which easily distinguishes calcite from dolomite. Calcite is brittle, often flourescent, sometimes thermo luminescent.

Chemistry

Crystal System

Hardness

CaCO3

Scalenohedron /Rhombohedron

3 (Mohs' scale)

Cleavage

 Fracture

Specific Gravity

Rhombohedral

Conchoidal

2.71

Color, Transparency
& Luster

 VARIETIES 
- Color white, transparent
- Luster glassy
  • Limestone
  • Marble
  • Chalk
  • Travertine
  • Iceland Spar

-- Bob Katz

 


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