Stone of the Week - Calcite

Calcite is one of the most abundant minerals on Earth, and is found in a wide variety of geologic formations. The beautiful formations called stalactites and stalagmites that are found in caves are perhaps the most spectacular. Calcite is commonly associated with limestone, as well as hydrothermal veins, hot spring and evaporite deposits, and some igneous & metamorphic rocks. It can grow in a variety of crystal forms – more so than any other mineral. Calcite is made of calcium carbonate, or CaCO3, and is one of two common polymorphs (crystal forms) of calcium carbonate, the other being Aragonite.

Calcite occurs in a vareity of forms as seen in these samples of calcite.


Calcite can be found in a wide range of color, the most common being white and yellow, but also occurring in shades of green, blue, orange, gray, purple, red, and brown. One way to test for calcite is by using acid, such as very dilute hydrochloric acid; when dropped onto a sample containing calcite it will fizz vigorously. Some have also reported using vinegar, although it produces a weaker fizzing action. [Always use caution with acids regardless of how ‘weak’ or dilute they are.]

Some of the more unusual colors of calcite include blue, green, and peach/orange.

A neat property of calcite is its ability to produce a double refraction. This is best seen in calcite that is in a clear rhombohedral form (at one time specimens like these were called Iceland spar). Calcite is also the main constituent in shells for marine organisms, and found in many fossils. Calcite is said to have been valued by ancient peoples for building material once they realized it could be crushed and mixed with water to form cement and plaster. Calcite is sometimes used in decoration or jewelry, but must be handled with care (if not enhanced) as it is more fragile.

Calcite as it appears in its perfect rhombohedral shape (left) and the very clear optical calcite showing double refraction (right).


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Calcite Facts:
Chemical composition: CaCO3
Crystal System: Trigonal
Color: Commonly white & yellow, also peach/orange, gray, blue, purple, green, red, and brown.
Habit: Crystalline, prismatic, tabular, scalenohedral, rhomobohedral, stalactitic, massive, concretions
Fracture: Conchoidal
Cleavage: Perfect
Luster: Vitreous to pearly
Hardness: 3
Specific Gravity: ~2.7
Streak: White
Occurrence: Worldwide, though the UK is known for very excellent crystals, and optical calcite is collected in Iceland.


References
A Guide to Rocks and Fossils by B. Busbey III, R. R. Coenraads, P. Willis, and D. Roots. Published 2002 by Fog City Press. ISBN: 1877019518

Rocks, Minerals, & Fossils of the World by C. Pellant and R. Phillips. Published 1990 by Little, Brown and Co. ISBN: 0316697966

Wikipedia – Calcite. http://en.wikipedia.org/wiki/Calcite

Wikipedia – Aragonite. http://en.wikipedia.org/wiki/Aragonite