mineral

Calcite (Iceland Spar)

Calcite (Calcium Carbonate, CaCO3)

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

Identified February 5, 2026More mineral →

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Physical properties

Formation & geological history

Formed primarily through sedimentary precipitation in marine environments or hydrothermal veins. This rhombohedral form often crystallizes in cavities within limestone or through metamorphism in marble.

Uses & applications

Used in the manufacture of cement and mortar, as a soil conditioner in agriculture, and high-clarity 'Iceland Spar' is used in optical instruments for polarizing light.

Geological facts

Iceland Spar is famous for its double refraction (birefringence); if you place it over a line on a paper, the line will appear double. It is theorized to be the 'sunstone' used by Vikings for navigation.

Field identification & locations

Identified in the field by its rhombohedral shape and its reaction to dilute hydrochloric acid (it will fizz). It is much softer than quartz and can be scratched by a copper penny or a knife.

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