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The Mineral Calcite


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Calcite Crystals
PHYSICAL PROPERTIES:

Chemistry: CaCO3
Composition:
Calcium Carbonate
Class: Carbonates
Group: Calcite
Crystal System: trigonal
Fracture: conchoidal
Hardness: 3 on clevage face
Specific Gravity: 2.7
Refractive Index: 1.49 and 1.66
Luster: Glassy to dull
Streak: white
Cleavage: perfect in three directions
Color: Colorless, white, pale tints
Transparency:
Crystals are transparent to translucent
Associated Minerals:
Some of the more common minerals associated with calcite are. Apatite, barite, biotite, celestite, copper, fluorite, galena, gold, quartz, sphalerite, sulfur, zeolites, several metal sulfides, other carbonates and borates and many other minerals.

Composition:
Calcium Carbonate ( 56% CaO, 44% CO2; Mn, Fe, and Mg may partially replace the Ca).

DISTINGUISHING CHARACTERISTICS:
Fluorescence, phosphorescence, thermoluminescence and triboluminescence are other important properties of calcite.
Calcite is frequently fluorescent, a small amount of manganese is enough to make it glow red under some wave lengths of ultraviolet light. Such as those from Franklin, New Jersey. Some Mexican Iceland spar can fluoresce a nice purple or blue color and unique specimens will even phosphoresce (continue to glow) after the UV source has been removed. Triboluminescence should occur in most specimens, but is not easily demonstrated. In a dark room strike the specimen and it should glow. Calcite is not the only calcium carbonate mineral. There are no less than three minerals or phases of CaCO3. Aragonite and vaterite are polymorphs with calcite, meaning they all have the same chemistry, but different crystal structures and symmetries. Aragonite is orthorhombic, vaterite is hexagonal and calcite is trigonal. Aragonite is a common mineral, Vaterite on the other hand is extremely scarce and rarely seen. Aragonite will over time convert to calcite and calcite pseudomorphs after aragonite are not uncommon. Calcite is much softer on the base than on its cleavage face. It is number 3 of the Mohs scale. But it can be scratched with the fingernail on the basal plane (about 2.5).
 
ENVIRONMENT:
Calcite forms in all types of occurrences, with all classes of rock. Calcite is the primary mineral component of limestone and its metamorphic form is marble. It forms oolitic, fossiliferous and massive limestones in sedimentary environments and even serves as the cements for many sandstone's and shale's. Most calcite is white, though various impurities may tint it almost any color, even black. Calcite is commonly a late vein mineral and may need to be removed with a very dilute hydrochloric acid solution to reveal well-formed crystals of other minerals.

CRYSTAL DESCRIPTION:
Calcite is the stable form of the widely distributed mineral calcium carbonate (CaCO3). It is noted for its beautiful varieties and its perfect crystals. Crystal Habits are extremely variable with almost any trigonal or pseudo-hexagonal form possible. One of it's most well known crystal forms is the scalenohedron or "Dogtooth" Other common crystal forms are rhombohedron, hexagonal prism and pinacoid. There are more than 300 crystal forms identified in calcite and these forms can combine to produce literally a thousand different crystal variations. Calcite also produces many twin varieties that are favorites among twin collectors. There are also phantoms, included crystals, color varieties, pseudomorphs and unique associations. Calcite is polymorphous, (having the same chemical formula but different crystal structure) with the minerals aragonite and vaterite as well as with several other forms that apparently exist only under extreme experimental conditions. Calcite comes in many other forms too. Such as massive, fibrous, concretionary, stalactitic, nodular, oolitic, stellate, dendritic, granular and so on.

TESTS:
Easily scratched, dissolves in cold dilute hydrochloric acid or vinegar with effervescence.
Other carbonates such as dolomite or siderite do not react as easily with these acids as does calcite and this leads to differentiating these somewhat similar minerals more readily. Aragonite dissolves as easily, but has a different crystal form and no cleavage. When heated, aragonite crumbles to powder and loses its fluorescence. Even when not previously fluorescent, calcite usually becomes so after heating

LOCALITIES:

The list of Localities we have seen from our dealers, books and web sites seems to be endless. Some of the older more famous localities are Palm Wash, California; Missouri- Kansas-Oklahoma ( tri state lead district); Iceland (the original Iceland spar). Oolitic calcite sand forms on the shores of Great Salt Lake. Also the Pugh Quarry, Ohio; Rosiclare, Illinois; Franklin, New Jersey; Elmwood, Tennessee; Germany; Brazil; Guanajuato, Mexico; Cornwall, Durham and Lancashire, England; Bombay area of India and many Brazilian and African localities. As well as others around the world with their own unique varieties

USES:
In cements, mortars, and the production of lime. Limestone is used in the steel industry; glass industry and as the ornamental stone marble. Calcite is also used in chemical and optical uses and as mineral specimens. Flawless transparent calcite is used in optical instruments especially in geological (polarizing) microscopes.

FACTS & HISTORY:
Calcite gets its name from "chalix" the Greek word for lime.


It is one of the most common minerals on the face of the Earth, comprising about 4% by weight of the Earth's crust and is formed in many different geological environments.

Calcite is the primary mineral component in cave formations. Stalactites and stalagmites, cave veils, cave pearls, "soda straws" and the many other different cave formations are made of calcite.

Mexican onyx is a variety of calcite that is used extensively for ornamental purposes. It is not the same onyx as the quartz variety of onyx

Another well known variety is "Iceland Spar", which is basically clear cleaved fragments of completely colorless (ice-like) calcite. Originally discovered and named after Eskifjord, Iceland, where the calcite is found in basalt cavities.

Iceland spar is used for optical equipment and during World War II it was a strategic mineral. Iceland spar also best demonstrates the unique property of calcite, called double refraction. Double refraction occurs when light enters the crystal and calcite's unique optical properties, split the light into fast and slow beams. As these two beams exit the crystal they are bent into two different angles. This is the term known as angle of refraction. Because the angle is affected by the speed of the beams. A person looking into the crystal sees two images