San Diego Natural History Museum--Your Nature ConnectionSDNHM Field Guide
[Calcite. Collection of the San Diego Natural History Museum.]

Calcite

From the Greek word, chalx, meaning "lime."

Description and Occurrence

Calcite crystals occur in over 300 forms -- more than any other mineral. Its more common forms are rhombohedron, scalenohedron, and prism. It can also appear tabular, acicular, fibrous, powdery, granular, compact, stalctitic, oolitic, or earthy. It occurs in various colors: white, pale shades of gray, yellow, red, green blue, brown to black.

In the building industry, calcite is used as a component in cement, a filler in paint, and as a structural and ornamental stone. Crushed calcite (lime) is used to mark the lines on a playing field. Clear crystals were once used to make polarizing prisms, and they are still used today in optical instruments.

In San Diego County, calcite mines can be found in the badlands of the Anza-Borrego Desert State Park and near the Salton Sea in Imperial County. During World War II, calcite was considered for use in targeting instrumentation, and mines were opened near the Salton Sea. Mining was halted when a synthetic material was invented, replacing calcite.

Field Notes: Calcite breaks into perfect, six-sided crystals (rhombohedrons). Placing a drop of vinegar or dilute hydrochloric acid on a sample will cause it to fizz. A clear specimen shows a double image when looked through.


Physical Properties

Color Streak Transparency Luster Hardness Cleavage Fracture Specific gravity Crystal form
colorless, white, pale shades of gray, yellow, red, green blue, brown to black white transparent to opaque vitreous, dull 3 perfect conchoidal 2.7 hexagonal

Photo: Calcite. Collection of the San Diego Natural History Museum
Photo credit: Linda West

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