Optical Calcite (Iceland Spar)

Physical Properties

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

Formation & Geological History

Formed through chemical precipitation in sedimentary environments, hydrothermal veins, or as a primary mineral in igneous carbonatites. Can be found in rocks of almost any geological age.

Uses & Applications

Used in the manufacture of cement and mortar, as a flux in glass making, in the chemical industry, and historically in optical instruments (like polarizing prisms) due to its double refraction.

Geological Facts

Iceland Spar is famous for its property of double refraction (birefringence); if you place a crystal over a line on a piece of paper, the line will appear double. It was allegedly used by Vikings as a 'sunstone' to navigate on cloudy days.

Field Identification & Locations

Identify by its perfect rhombohedral cleavage (it naturally breaks into slanted box shapes) and its reach with dilute hydrochloric acid (it will fizz). Commonly found in limestone quarries and hydrothermal deposits globally.

Identified on: 4/13/2026

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