mineral

Optical Calcite

Calcium Carbonate (CaCO3)

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

Identified May 24, 2026More mineral →

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Physical properties

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

Formation & geological history

Formed primarily through chemical precipitation in sedimentary environments or as a secondary mineral in igneous and metamorphic rocks. Often found in hydrothermal veins or as mass deposits in limestone and marble.

Uses & applications

Used in optical instruments for its polarizing properties, as an acid neutralizer in chemical industries, and as a popular specimen for mineral collectors and educational purposes.

Geological facts

Optical calcite, also known as Iceland Spar, was famously used by Vikings for navigation, as its polarizing effect allowed them to locate the sun even through thick clouds or fog. It exhibits double refraction, where an image viewed through it appears doubled.

Field identification & locations

Identified in the field by its rhombohedral shape, relative softness (can be scratched by a copper coin), and strong reaction to cold dilute hydrochloric acid. It is commonly found in Mexico, Iceland, and the United States.