Optical Calcite
Calcite (Calcium Carbonate) · Mineral Specimen / Collector Mineral
Crystal System
Trigonal crystal system with a Rhombohedral habit; commonly forms massive or perfectly cleaved rhombohedrons.
Mineral Group
Carbonate group; Calcite sub-group
Chemical Formula
CaCO3
Formation Process
Primarily sedimentary via chemical precipitation in marine environments, or hydrothermal via calcite-rich veins and hot springs.
About This Crystal
Transparent to translucent colorless rhombohedron with a vitreous to pearly luster. Features internal fractures and plane-parallel cleavage.
Physical Characteristics
Colorless body; white streak; perfect rhombohedral cleavage in three directions; conchoidal to uneven fracture; specific gravity 2.71; strong fluorescence under UV.
Optical Properties
Uniaxial negative; extreme birefringence (0.172) causing strong double refraction; refractive indices nω = 1.658 and nε = 1.486; dispersion is low.
Hardness & Durability
Origin Region
Notably sourced from Iceland (as Iceland Spar), Mexico (Chihuahua), and China; forms in sedimentary and metamorphic environments.
Hardness & Durability
Mohs hardness of 3; very soft and brittle; poor durability for jewelry due to easy cleavage and low hardness.
Care & Maintenance
Avoid all acids (effervesces in HCl); clean only with lukewarm soapy water; store separately to avoid scratches from harder minerals.
Rarity & Value
Common; value depends on optical clarity and size; typically inexpensive but museum-grade 'Iceland Spar' is highly valued for optical study.
Special Characteristics
Strong double refraction (birefringence) that splits light into two rays, causing objects viewed through it to appear doubled.
Lore & History
Historically known as 'Iceland Spar'; used by Vikings for navigation (sunstone) and used in WWII for optical gun sights (Gunsight Calcite).