mineral

Optical Calcite

Calcium Carbonate (CaCO3)

Hardness: 3 on Mohs scale; Color: Colorless to white; Luster: Vitreous to pearly; Crystal Structure: Trigonal (rhombohedral cleavage); Cleavage: Perfect rhombohedral in three directions; Specific Gravity: 2.71

Hardness: 3 on Mohs scale
Color: Colorless to white
Luster: Vitreous to pearly

Identified June 23, 2026More mineral →

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Physical properties

Formation & geological history

Formed in sedimentary environments as a chemical precipitate from evaporating seawater or in hydrothermal veins and hot springs. It can also form through biological processes in marine organisms.

Uses & applications

Used in the manufacture of high-grade optical instruments (polarizing prisms) due to its double refraction properties. Also used in construction (as limestone/marble), fertilizer, and as a popular collector's specimen.

Geological facts

Also known as Iceland Spar, this mineral exhibits double refraction, where light rays entering the crystal are split into two, making any text viewed through it appear doubled. Vikings may have used it for navigation on cloudy days.

Field identification & locations

Identified in the field by its rhombohedral shape, softness (can be scratched by a copper penny), and its strong reaction (fizzing) to cold dilute hydrochloric acid. Commonly found in Mexico, Iceland, and the United States.