calcite, variety Icelandic Spar

Description: Calcite is a rock-forming mineral with a chemical formula of CaCO3. One variety, clear, “Iceland Spar” is used for optical purposes. It occurs in large readily cleavable crystals, is easily divisible into rhombuses, and is remarkable for its birefringence. This means that the index of refraction of the crystal is different for light of different polarization. A ray of unpolarized light passing through the crystal divides into two rays of perpendicular polarization directed at different angles It is speculated that Vikings used its light-polarizing property to tell the direction of the sun on cloudy days for navigational purposes.

Chemical Formula: CaCO3

Crystal Form: Rhombohedral crystals.

Crystal System: Hexagonal

Color: Transparent crystals.

Streak: White

Luster: Vitreous (glass-like).

Fracture: Conchoidal

Cleavage: Perfect, rhombohedral, three directions

Hardness: 3 on Moh's scale

Density/Specific Gravity: 2.7 g/cm3

Magnetism: N/A

Taste: N/A

Hydrochloric acid: Reacts readily with acid to ‘effervesce’, producing small bubbles of CO2.

Radioactivity: N/A

Fluorescence: May fluoresce.

Distinguishing Physical Properties:

  • Clear, transparent

  • Hardness (scratched with a nail)

  • Effervesces in dilute HCl

  • Double refraction, check with printed paper.

Photograph Attribution: Calcite, variety Icelandic spar specimen photograph by Sean C. Murphy, 2020.