mineral

Optical Calcite (Iceland Spar)

Calcite (Calcium Carbonate, CaCO3)

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

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

Identified April 10, 2026More mineral →

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Physical properties

Formation & geological history

Formed through chemical precipitation in sedimentary environments or as a primary mineral in igneous and metamorphic rocks (like marble). This specific transparent variety often forms in hydrothermal veins.

Uses & applications

Used in high-quality optical instruments (polarizing prisms) due to its double refraction properties. Common calcite is used in construction (acid-neutralizer, cement source) and as a collector specimen.

Geological facts

Famous for the optical phenomenon of double refraction (birefringence), where light passing through it is split into two rays. If you place it over text, you will see a double image.

Field identification & locations

Identify by its classic rhombohedral 'leaning cube' shape and its reaction to weak acid (it will effervesce/fizz). Commonly found globally, with famous deposits in Iceland and Mexico.

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