mineral

Optical Calcite

Calcium Carbonate (CaCO3)

Hardness: 3 (Mohs scale); Color: Colorless/Transparent; Luster: Vitreous to Pearly; Crystal Structure: Trigonal/Rhombohedral; Cleavage: Perfect rhombohedral in three directions; Specific Gravity: 2.71

Hardness: 3 (Mohs scale)
Color: Colorless/Transparent
Luster: Vitreous to Pearly

Identified June 12, 2026More mineral →

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Physical properties

Formation & geological history

Formed in sedimentary environments, hydrothermal veins, and as a precipitate in limestone caves. It can also form during metamorphism of limestone into marble. Many optical-grade specimens come from volcanic cavities.

Uses & applications

Historically used in optical instruments like polarizing microscopes and gun sights (due to birefringence). Today used for mineral collecting, manufacturing cement, and neutralizing acids.

Geological facts

Also known as Iceland Spar. It exhibits a optical property called double refraction (birefringence), where light passing through is split into two rays, making objects viewed through it appear double.

Field identification & locations

Identified by its rhombohedral shape and strong double refraction. It will vigorously effervesce (fizz) when exposed to a drop of dilute hydrochloric acid. Common in Iceland, Mexico, and the USA.