Calcite
Calcite, also known as calcium carbonate, is a fascinating mineral that is one of the most common minerals on the planet, yet comes in a variety of shapes, sizes, colors, clarity. The mineral also fluoresces, phosphoresces, and may also even be thermoluminescent.
It can stand on its own as a singular mineral or form part of another mineral. Give calcite enough pressure, and it’ll turn into limestone and marble.
Calcite is a polymorph that comes in over 800 forms and is so variable that you could collect it throughout your lifetime and never be able to get your hands on all the forms.
Classification: Carbonate mineral
Chemical Composition: CaCO₃ (calcium carbonate)
Color: Calcite is extremely varied in color, including colorless, white, gray, yellow, green, blue, red, pink, orange, and brown. Some specimens can be multicolored or banded.
Streak: White
Hardness: 3 on the Mohs scale
Cleavage: Perfect in three directions, not at right angles
Fracture: Conchoidal or fibrous
Luster: Vitreous, pearly, or dull
Transparency: Transparent to translucent
Crystal System: Trigonal
Formation and Geology: Calcite forms in a broad spectrum of environments. It is a primary mineral in sedimentary rocks such as limestone and chalk but also occurs in metamorphic marble. It is often deposited in cave environments and forms stalactites and stalagmites. It is also a common component of veins and hydrothermal ores. The mineral’s extensive variety in formation leads to its widespread occurrence globally.
Physical Properties: Calcite’s defining feature is its perfect three-directional cleavage, which means it breaks into rhombohedral pieces. It is relatively soft, being easily scratched by a copper coin. It is also famous for its birefringence; when viewed through a clear piece of calcite, objects appear doubled.
Identification Tips:
- Visual Inspection: Check for the characteristic rhombohedral cleavage.
- Acid Test: Calcite reacts vigorously with dilute hydrochloric acid, even with a cold, weak solution, causing effervescence.
- Hardness Test: Test for softness, as it can be easily scratched by a copper coin.
- Birefringence Test: Look for double refraction when placing a clear specimen over text or a line.
Locations: Calcite is found worldwide, with notable deposits in Iceland, Mexico, Brazil, Romania, and the United States.
Uses and Significance: Beyond its geological importance, calcite is used extensively in the construction sector in the form of limestone and marble. Its optical properties make it valuable in a wide range of applications, including polarizing microscopes and polarizing lenses. Calcite’s diverse colors and forms also make it a popular choice for jewelry and ornamental stone.
