Calcite

Calcite is the only common non-silicate rock forming mineral, being instead calcium carbonate. It has two refractive indices causing a significant double refraction effect - when a clear calcite crystal is placed on an image, a double image is observed.

Calcite will fizz when dilute hydrochloric acid is placed on it. It may be fluorescent, phosphorescent, thermoluminescent and triboluminescent.

Calcite is one of the most ubiquitous minerals, being an important rock forming mineral in sedimentary environments. It is an essential component of limestones, and occurs in other sedimentary rocks. It also occurs in metamorphic and igneous rocks, and is common in hydrothermal environments. Calcite is a common vein filling mineral in many rock types.

Chemical composition - CaCO3

Hardness - 3

Specific gravity - 2.7

Transparency - Transparent to translucent

Colour - Generally white or colourless, but also with light shades of yellow, orange, blue, pink, red, brown, green, black and grey

Streak - White

Lustre - Vitreous to resinous

Cleavage/fracture - Perfect in three directions at oblique angles / conchoidal

Crystal habit/mode of occurrence - Prismatic (rhombohedral crystals) / granular, massive