mineral

Optical Calcite (Iceland Spar)

Calcite (Calcium Carbonate, CaCO3)

Hardness: 3 on Mohs scale; Color: Colorless or white; Luster: Vitreous to pearly; Crystal structure: Trigonal (Rhombohedral cleavage); Specific Gravity: 2.71

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

Identified April 8, 2026More mineral →

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Physical properties

Hardness: 3 on Mohs scale; Color: Colorless or white; Luster: Vitreous to pearly; Crystal structure: Trigonal (Rhombohedral cleavage); Specific Gravity: 2.71

Formation & geological history

Formed through chemical precipitation in sedimentary environments or hydrothermal veins. This rhombohedral shape is often the result of cleavage from larger crystal masses.

Uses & applications

Used historically in optical instruments like polarizing microscopes and gun sights (due to double refraction). Also used as an acid neutralizer and in the manufacturing of cement and lime.

Geological facts

Optical calcite is famous for the property of double refraction (birefringence); if you place it over a line on a piece of paper, the line will appear doubled. It was historically discovered in Iceland, hence the name 'Iceland Spar'.

Field identification & locations

Identify in the field by its perfect rhombohedral cleavage and its strong reaction (effervescence) when touched with dilute hydrochloric acid. It is very soft and can be scratched by a copper coin.

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