mineral

Calcite (Optical Calcite / Iceland Spar)

Calcium Carbonate (CaCO3)

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

Identified June 1, 2026More mineral →

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Physical properties

Hardness: 3 (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

Forms in sedimentary environments through chemical precipitation, in hydrothermal veins, and in metamorphic rocks like marble. This rhombohedral variety often forms in vugs and cavities where crystals have room to grow.

Uses & applications

Used in optical instruments (polarizing prisms), manufacturing of cement, lime for soil treatment, and as a popular collector specimen for demonstrating double refraction.

Geological facts

Optical calcite, also known as Iceland Spar, was famously used in WWII for gun sights and historically by Vikings for navigation, as it can polarize light and locate the sun on cloudy days. It exhibits double refraction, where text viewed through the crystal appears doubled.

Field identification & locations

Identify in the field by its 3-directional rhombohedral cleavage (slanted cube shape), its reaction (fizzing) with dilute hydrochloric acid, and its double refraction. It is found globally in limestone and marble quarries.

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