Description: Calcite is a rock-forming mineral with a chemical formula of CaCO3. It is extremely common and found throughout the world in sedimentary, metamorphic, and igneous rocks. Some geologists consider it to be a "ubiquitous mineral" - one that is found almost everywhere. It is estimated that 15% of all sedimentary rocks contain calcite.
Chemical Formula: CaCO3
Crystal Form: Over 800 forms of calcite crystals have been identified. Most common are rhombohedrons or prismatic crystals with scalenohedral terminations.
Crystal System: Trigonal
Color: Transparent to translucent calcite crystals are colorless, white or light-colored; massive forms of calcite are typically buff, gray, or white.
Streak: White
Luster: Crystals can be vitreous (glass-like), massive form is dull
Fracture: Conchoidal
Cleavage: Perfect, rhombohedral, three directions
Hardness: 3 on Moh's scale
Density/Specific Gravity: 2.7 g/cm3
Magnetism: N/A
Taste: N/A
Hydrochloric acid: Reacts readily with acid to ‘effervesce’, producing small bubbles of CO2.
Radioactivity: N/A
Fluorescence: May fluoresce red, blue, yellow, and other colors under either shortwave or longwave ultraviolet light.
Distinguishing Physical Properties:
Since calcite can have so many appearances, the simplest way to determine whether a sample contains calcite is to test its reaction to a dilute acid like household vinegar. Calcite will readily react with acid to ‘effervesce’, producing small bubbles of CO2 similar to those formed when you open a bottle of soda and pour it into a glass.
Soft enough to be easily scratched by a nail,
Calcite crystals can also be identified by their rhombic cleavage.
Photograph Attribution: Calcite specimen photograph by Sean C. Murphy, 2020.