Optical Calcite (Iceland Spar)

Physical Properties

Hardness: 3 on Mohs scale; Color: Colorless/Transparent; Luster: Vitreous; Crystal System: Trigonal; Cleavage: Perfect rhombohedral in three directions; Features double refraction (birefringence).

Formation & Geological History

Formed through chemical precipitation from calcium-rich solutions in sedimentary environments, or within hydrothermal veins. It is a primary constituent of limestone and marble.

Uses & Applications

Used for optical instruments (historically in gun sights and polarizing microscopes), in the manufacturing of cement, as a soil conditioner in agriculture, and as a popular teaching specimen for mineralogy.

Geological Facts

Optical calcite is famous for its 'double refraction' property; when you place the crystal over text, the text appears doubled. Vikings may have used it as a 'sunstone' for navigation on cloudy days.

Field Identification & Locations

Identify in the field by its rhombohedral shape (like a slanted sub-rectangle) and its vigorous reaction (fizzing) when exposed to cold dilute hydrochloric acid. Commonly found in Chihuahua, Mexico and various limestone regions.

Identified on: 4/13/2026

Mode: Standard