mineral

Optical Calcite (Iceland Spar)

Calcium Carbonate (CaCO3)

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

Hardness: 3 (Mohs scale)
Color: Colorless to white
Luster: Vitreous (glassy)

Identified March 4, 2026More mineral →

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Physical properties

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

Formation & geological history

Formed through chemical precipitation in sedimentary environments or as a secondary mineral in hydrothermal veins. Large rhombohedral crystals are often associated with volcanic basalt cavities or limestone deposits.

Uses & applications

Used historically for optical instruments and gun sights due to its birefringent properties. Today it is used in the manufacture of high-grade cements, as a soil conditioner in agriculture, and as a popular specimen for mineral collectors and metaphysical practitioners.

Geological facts

Famous for the phenomenon of double refraction (birefringence), where a single ray of light entering the crystal is split into two rays, causing objects viewed through it to appear doubled. It was used by the Vikings for navigation (the 'Sunstone') to locate the sun on cloudy days.

Field identification & locations

Identify in the field by its rhombohedral shape (like a leaning box), its softness (can be scratched by a copper penny), and its strong reaction (effervescence) to cold dilute hydrochloric acid. It is commonly found in Mexico, Iceland, and the United States (New Mexico).

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