mineral

Iceland Spar (Optical Calcite)

Calcite (Calcium Carbonate, CaCO3)

Hardness: 3 on Mohs scale; Color: Clear/Colorless; Luster: Vitreous to pearly; Crystal Structure: Trigonal/Rhombohedral; Cleavage: Perfect rhombohedral in three directions; Specific Gravity: 2.71.

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

Identified June 12, 2026More mineral →

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Physical properties

Hardness: 3 on Mohs scale; Color: Clear/Colorless; Luster: Vitreous to pearly; Crystal Structure: Trigonal/Rhombohedral; Cleavage: Perfect rhombohedral in three directions; Specific Gravity: 2.71.

Formation & geological history

Formed in hydrothermal veins, volcanic cavities, and sedimentary environments through the precipitation of calcium-rich solutions. Often found in basaltic rugs or limestone deposits.

Uses & applications

Historically used in optical instruments like polarizing microscopes and gun sights due to its double refraction. Today, it is used for mineral education, research, and as a collector's specimen.

Geological facts

Iceland Spar is famous for 'double refraction' (birefringence), where looking through the crystal makes images appear double. It is hypothesized that Vikings used these 'sunstones' to navigate on cloudy days by locating the sun via polarization.

Field identification & locations

Identified by its perfect rhombohedral cleavage (leaning cube shape) and its ability to double images placed behind it. It will also effervesce (fizz) vigorously when in contact with weak hydrochloric acid.

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