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Complex Calcite Crystal

The Mineral calcite




Calcite is the one of the most common minerals on earth. It occurs in a seemingly unlimited variety of shapes and colors. It constitutes a major portion of many of the earth's rocks. Calcite belongs to the calcite group of minerals, a group of related carbonates that are isomorphous with one another. They are similar in many physical properties, and may partially or fully replace one another, forming a solid solution series. All members of the calcite group crystallize in the trigonal system, have perfect rhombohedral cleavage, and exhibit strong double refraction in transparent rhombohedrons.

Calcite and Aragonite are polymorphous to each other. Although Calcite and Aragonite contain the same chemical composition, they differ in crystal structure. Calcite forms trigonal crystals, whereas Aragonite forms orthorhombic crystals. Sometimes the crystals of Calcite and Aragonite are too small to be detected, and it is only possible to distinguish these two minerals by complex scientific optical tests. Since the true identity of microcrystalline forms of Calcite or Aragonite may not known, they may be mislabeled as the wrong mineral.

A microcrystalline type of Calcite in globular form is common in certain regions. This Calcite forms from precipitating calcium-rich water inside caverns or on limestone cliffs. It exists in the form of stalagmites, stalactites, flowstone, and strange globular growths. These growths constantly accumulate, forming layers. They are frequently impure, trapping in organic matter such as leaves, twigs, and moss as they accumulate. Because of their impure status, they are classified by some as rocks. These calcareous growths have designated names based on their shape, habit, or formation. Most of these growths are Calcite, but some are crystallized as Aragonite. The environment of formation, however, can be a key guide to whether the mineral crystallized as Calcite or Aragonite. Aragonite will generally develop only at hot springs, whereas most other calcareous growths will be Calcite.

Calcite may form as an undesirable coating on top of another mineral. The calcite can be easily burned off by soaking it in acid, which will cause it to effervesce and eventually dissolve, leaving the mineral below exposed.
Chemical Formula CaCO3
Composition Calcium carbonate, sometimes with impurities of iron, magnesium, or manganese, and occasionally zinc and cobalt.
Variable Formula (Ca,Fe,Mg,Mn,Zn,Co)CO3
Color Colorless, white, yellow, brown, orange, pink, red, purple, blue, green, gray, black. May also be multicolored or banded.
Streak White
Hardness 3
Crystal System Hexagonal
3D Crystal Atlas
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Crystal Forms
and Aggregates
Occurs in a great variety of shapes, with the most common forms asrhombohedral and scalenohedral crystals. Crystals may be tabular, acicular, prismatic, flaky, and needle-like. May occur as bundles of scalenohedrons, intergrown rhombohedrons, hair-like masses of acicular crystals, grainy, stalactitic, fibrous, massive, and earthy. Scalenohedral twinning is common.
Transparency Transparent to opaque
Specific Gravity 2.7
Luster Vitreous
Cleavage 1,3 - rhombohedral
Fracture Conchoidal. Rarely observed due to the perfect cleavage.
Tenacity Brittle
Other ID Marks 1) Commonly fluorescent; specimens from different localities fluoresce different colors. Some Calcite is also phosphorescent.
2) Transparent crystals exhibit strong double refraction.
3) May be thermoluminescent.
Complex Tests 1) Effervescent in hydrochloric acid and most other acids.
2) Calcite that doesn't fluoresce usually becomes fluorescent upon heating.
In Group Carbonates; Calcite Group
Striking Features Hardness, cleavage, fluorescence, and effervescence with hydrochloric acid.
Environment Calcite is a constituent of all mineral environment, including sedimentary, igneous, and metamorphic.
Rock Type Igneous, Sedimentary, Metamorphic
Popularity (1-4) 1
Prevalence (1-3) 1
Demand (1-3) 1

Calcite ON EBAY
OTHER NAMES
Calcspar

VARIETIES
 -  Crumbly white Calcite found on cavern floors near stalagmites and stalactites.
 -  Dark gray to black variety of Calcite with a bitumen coating or inclusions.
 -  Lamellar variety of Calcite.
 -  Cobaltocalcite refers to an intermediary mineral between Calcite and Sphaerocobaltite in a solid solution series. It is generally looked at as a cobalt-rich variety of Calcite, but can also be looked as a calcium-rich variety of Sphaerocobaltite. The chemical formula of Cobaltocalcite is (Ca,Co)CO3. (Cobaltocalcite may also be mistakenly used as a synonym for Sphaerocobaltite.)
 -  Calcite aggregate resembling a corn cob with distinct kernels.
 -  Calcite in groupings of thick and pointy scalenohedral crystals.
 -  Calcite formed by mineral-rich water that deposits the dissolved mineral on the walls of caverns and cliffs, forming a smooth and humpy growth.
 -  Calcite with long, sharp, incisor-like crystals.
 -  Large, transparent, colorless to lightly colored, rhombohedral variety of Calcite. Double refraction is especially noted in Iceland Spar crystals. (Iceland Spar may occasionally also be used as a synonym for Calcite.)
 -  Calcite crystals resembling a nail, with a triangular cross section head atop a long and thin prismatic crystal. May also refer to two perpendicular scalenohedral crystals intersecting in the shape of a "T".
 -  Travertine or Tufa in the mineral form of Aragonite or Calcite that exhibits color banding.
 -  Synonym of Iceland Spar.
 -  Calcite grouping of small, white scalenohedral crystals appearing as grains of rice.
 -  Orange-red, "salmon" colored variety of Calcite that is usually opaque.
 -  Calcite that trapped particles of sand in its interior when it formed
 -  Fibrous variety of Gypsum. May occasionally also describe a fibrous form of Calcite or Aragonite.
 -  Icicle-like mineral formation (usually Calcite) found on the roof of caverns, created when mineral-rich water drips down and the dissolved mineral accumulates into the icicle-like formation.
 -  Tall, domed mineral formation (usually Calcite) on the bottom of caverns that from from the build-up of mineral-rich water that deposits the dissolved mineral on the cavern floor.
 -  Mounds of calcium carbonate formed from hot springs that contain calcium-rich water that bubbles up to the earth and cools down, and its capability to hold calcium is reduced. The water eliminates the calcium, and the calcium forms a growing mound of calcium carbonate, which is porous. Travertine is usually Aragonite, although it may also be Calcite.
 -  Aragonite (or Calcite) formed from precipitating water that traps in organic matter, such as leaves, twigs, and moss. Also calcareous mounds formed from deposition of hot springs that trap in organic matter.
POLYMORPHS
Aragonite, Vaterite

USES
Calcite is the primary ore of calcium. Calcite is indispensable in the construction industry, forming the base of cement. Many important chemicals are created from Calcite, as well as useful drugs. It is also crucial in the manufacture of fertilizers, metals, glass, rubber, and paint. The transparent Iceland Spar variety, in which the double refraction is very apparent, was used as prisms for polarizing microscopes and other optical devices.

Calcite also forms rocks that are used for ornamental purposes, such as marble and banded travertine or tufa. Calcite is also the main component of chalk, which is processed for drawing chalk. To collectors, Calcite is one of the best-known and most commonly collected minerals. Most specimens are inexpensive except for those of exceptional size and crystal form, or from classic occurrences.

NOTEWORTHY LOCALITIES
There are literally thousands of excellent Calcite localities, so only a select few of the best and most well known are listed here. The original Iceland Spar variety was described from an old mine in Iceland called the Helgustadir Mine which was known since the 1600's. Two classic European localities are St. Andreasberg, Harz Mts, Germany and Pribram, Bohemia, Czech Republic. Currently, most Iceland Spar on the market comes from Chihuahua, Mexico.

Large, golden-yellow to brown crystals have come from numerous areas in the tri-state mineral region of Kansas, Missouri, and Oklahoma. Excellent prismatic crystals have come from the Sweetwater Mine, Reynolds Co., Missouri, and very large golden crystals from the Elmwood Mine, Carthage, Tennesse. Calcite with a strong blue phosphoresce comes from Terlingua, Brewster Co., Texas. Blue crystal clusters come from Balmat, St. Lawrence Co., New York, and white and orange "Salmon Calcite" that fluoresces bright red occurs associated at Franklin and Ogdensburg, Sussex Co., New Jersey. Prospect Park, Passaic Co., New Jersey has all different types, forms, and colors of Calcite.


DISTINGUISHING SIMILAR MINERALS
Aragonite - Different crystal form.
Dolomite - Crystals are rounded and slightly warped. However, transparent clear Dolomite is indistinguishable from Calcite unless complicated optical tests are performed.
Quartz - Much harder (7).
Gypsum - Softer (2), is sectile and slightly flexible.


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