Optical Calcite

Optical calcite, also known as Iceland spar, is an exceptionally clear variety of the calcium carbonate mineral calcite (CaCO₃), famous for the double refraction it produces. Optical calcite, also called Iceland spar, is an exceptionally transparent variety of calcite that displays strong double refraction (birefringence). It forms in hydrothermal veins and cavities where calcium carbonate-rich solutions slowly deposit large, perfectly clear crystals.

The remarkable optical properties arise from calcite's trigonal crystal structure, which bends light differently depending on polarization direction. When light enters the crystal, it splits into two rays traveling at different speeds and directions, producing a visible double image of anything viewed through the crystal.

Historically, Iceland's Helgustaðir mine was the world's primary source. Norse sagas reference a 'sunstone' (sólarsteinn) used for navigation. Modern experiments have confirmed that optical calcite can locate the sun's position even through thick clouds by detecting the polarization of scattered sunlight. This may have allowed Vikings to navigate the North Atlantic on overcast days.

Place optical calcite over text or a dot: you'll see a clear double image. This strong birefringence is the definitive test. The rhombohedral crystal shape (like a squashed cube with parallelogram faces) and perfect cleavage in three directions are additional identifiers.

Distinguish from clear quartz (no double refraction, much harder), fluorite (isotropic, no double refraction), and glass (no double refraction, no cleavage). Optical calcite also effervesces readily in dilute hydrochloric acid.