Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate. The other polymorphs are the minerals aragonite and vaterite. Calcite crystals are trigonal-rhombohedral, though actual calcite rhombohedra are rare as natural crystals. However, they show a remarkable variety of habits including acute to obtuse rhombohedra, tabular forms, and prisms. Calcite exhibits several twinning types adding to the variety of observed forms. It may occur as fibrous, granular, lamellar, or compact. Cleavage is usually in three directions parallel to the rhombohedron form.
Calcite is quite abundant and can be found all around the world. It can be found in sedimentary, igneous and metamorphic rock formations, with crystals ranging in size from microscopic to massive. It is a key component in carbonate-rich sedimentary rock and following years of metamorphism and recrystallization, partakes in the formation of limestone and later marble. It is also found as solid veins and as well formed crystals lining cavities in rock.
What Is Calcite Used For?
The properties of calcite make it a very useful and necessary mineral for a wide variety of applications. These applications can be for construction purposes, farming, decoration and in some cases even medical.
In construction, calcite in the form of limestone or marble is used in the process of making cement and works as the aggregate in some concretes. This cement is then used in buildings, bridges, roads, runways and other structures. Marble has been used for centuries to create statues and a variety of other decorative structures and monuments.
It's also used as an acid neutralization agent in a variety of different situations. In farming, calcite in the form of limestone and marble has been ground to a powder and used to neutralize the acidity of the soil. High quality marble and limestone has been processed and combined with sugar to create chewable tablets (antacids) that can neutralize the acidity of the stomach. It has also been used to neutralize the acidity of water run-off from mining locations where streams are plagued with acidity as a bi-product of mining.
Calcite that has been ground into a fine powder (most often white in color) has been used for centuries as a white paint pigment.
Types Of Calcite.
Iceland Spar - Also known by the names "optical calcite" or "Iceland crystal", this type of calcite is transparent and has light refractive properties. When looking through this form of calcite, the image you see through the crystal will appear to be doubled, and in some cases different colors. This light ray division is called double refraction.
Cobaltoan Calcite - Also known as "cobaltocalcite", this variety of calcite is pink in color. It owes its color to the presence of cobalt(Co) in place of calcite(Ca) in the crystal lattice.
Calcite Stalactites/Stalagmites - These form in cavities in rock where water has leached through the surrounding rock, picking up minerals (in this case carbonates from limestone) along the way. If the atmospheric conditions are just right, this carbonate-rich water then drips from the ceiling of the cavity, resulting in deposition of calcium carbonate (calcite) in the form of a stalactite. The floor that the precipitate drips on can also accumulate calcite. This structure that rises from the ground is known as a stalagmite.
Dogtooth Spar - A variety of calcite crystal formation in which the crystals display a scalenohedral crystal structure. A compact aggregation of these crystals can give a crystal filled rock cavity the appearance of being filled with sharp dog teeth, hence the name.
Twinned Calcite - A formation of calcite in which both ends of the crystal have well defined terminations.
White Calcite - Calcite that's typically white in color and has no optical properties. While often translucent, this type of calcite is not transparent.
Does Calcite Fluoresce?
Not all calcite will fluoresce, however the calcite specimens that do, can fluoresce a variety of different colors. While orange is a very common color for calcite to fluoresce, it can also be red, blue, white, pink and green. This color can also vary based on the wavelength of UV lighting used.
How Can You Tell The Difference Between Calcite And Quartz?
Visually, calcite and quartz can be difficult to tell the difference between. The structure of individual crystals of calcite are different than the structure of quartz crystals, so identifying these characteristics can assist in crystal identification. When these structures aren't present, which happens in solid crystal filled veins, identification can be done using hydrochloric acid. When a small drop is placed onto calcite, the calcite will begin to effervesce (produce bubbles), while quartz will not have this reaction. That being said, other mineralizations can also have this reaction, possibly leading to false identification.
Fun Facts About Calcite
Trilobites had a system of calcite lenses on their eyes that are believed to have not only allowed them to sense movement of light, but allowed for exceptional 360 degree vision in many species. This unique characteristic of trilobites preserved beautifully, allowing us to study and possibly learn from their vision hundreds of millions of years following their existence.
Calcite plays a role in the natural regulation of CO2 in our atmosphere, via the formation and breakdown of limestone.
Many marine organisms use calcite as a key component of their shells.
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