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The Mineral calcite

Complex Calcite Crystal

Calcite is the one of the most common minerals. It occurs in a great variety of shapes and colors, and 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

Color

Colorless, white, yellow, brown, orange, pink, red, purple, blue, green, gray, black. May also be multicolored or banded.

Crystal System

Hexagonal

Properties

Streak
White
Hardness
3
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.

Crystal Habits

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.

3D Crystal Atlas

Additional Information

Composition
Calcium carbonate, sometimes with impurities of iron, magnesium, or manganese, and occasionally zinc and cobalt.
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

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 most often perceived as a cobalt-rich variety of Calcite with a rich pink color. (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 with manganese impurities resulting in a pink color.
 -   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.

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 thousands of excellent Calcite localities, and a guide such as this cannot practically list all of them. Only a select few of the author's favorites are listed here. The original Iceland Spar variety was described from an old mine on the east coast of Iceland called the Helgustadir Mine, in Reydarfjörder, which was known since the 1600's.

Two classic European Calcite localities are St. Andreasberg, Harz Mountains, Germany; and Pribram, Bohemia, Czech Republic. Prismatic, colorless Calcite crystals are well-known among collectors from Bigrigg and Egremont, Cumbria, England; and Yellow and orange Calcite crystals, sometimes in exceptional "butterfly twins," have come from Malmberget, Lappland, Sweden. Currently, most Iceland Spar on the market comes from Chihuahua, Mexico.

Large, golden-yellow to brown Calcite crystals have come from several areas in the tri-state mineral region of Kansas, Missouri, and Oklahoma. This includes excellent prismatic crystals have come from the Sweetwater Mine, Reynolds Co., Missouri; and large scalenohedral crystals from Joplin, Jasper Co., Missouri. Very large, lustrous brown golden-colored crystals come from the Elmwood Mine, Carthage, Tennessee.

Calcite with a strong blue phosphorescence comes from Terlingua, Brewster Co., Texas. Pink and purple crystals, sometimes very large, have come from Rossie, St. Lawrence Co., New York. White and orange "Salmon Calcite" that fluoresces bright red is found at Franklin and Ogdensburg, Sussex Co., New Jersey. Prospect Park, Passaic Co., New Jersey has produced many different types, forms, and colors of Calcite. Beautiful orange Calcite has come from York, York Co., Pennsylvania.

Distingushing 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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