mineral

Optical Calcite (Iceland Spar)

Calcite (CaCO3)

Hardness: 3 (Mohs scale); Color: Colorless to white-grey; Luster: Vitreous to pearly; Crystal structure: Trigonal (rhombohedral cleavage); Specific gravity: 2.71

Hardness: 3 (Mohs scale)
Color: Colorless to white-grey
Luster: Vitreous to pearly

Identified March 20, 2026More mineral →

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Physical properties

Hardness: 3 (Mohs scale); Color: Colorless to white-grey; Luster: Vitreous to pearly; Crystal structure: Trigonal (rhombohedral cleavage); Specific gravity: 2.71

Formation & geological history

Formed through chemical precipitation from calcium-rich solutions in hydrothermal veins or sedimentary environments. It is a fundamental constituent of limestone and marble.

Uses & applications

Used historically in optical instruments like gun sights and polarizing microscopes due to its double refraction properties. Today, it is primarily used in the production of cement, in chemical industries, and as a popular collector's specimen.

Geological facts

Optical calcite is famous for 'double refraction' (birefringence), where light passing through a clear piece splits into two rays, making text appear doubled when viewed through the crystal. Vikings may have used it to navigate on cloudy days.

Field identification & locations

Identify in the field by its rhombohedral cleavage (it naturally breaks into slanted cubes) and its reaction to dilute hydrochloric acid (it will fizz). Found globally, with notable deposits in Iceland, Mexico, and the USA.