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Chemical Formula | CaCO3 | |
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Composition | Calcium carbonate, commonly with some impurities of either iron, magnesium, manganese, and occasionally with zinc and cobalt. Although all impurities do not exist together, they are all listed together in the "Variable Formula" field to avoid confusion. (See the Brownspar Group for more information.) | |
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Variable Formula | (Ca,Fe,Mg,Mn,Zn,Co)CO3 | |
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Color | Occurs in all colors, sometimes even multicolored | |
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Streak | White | |
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Hardness | 3 on a rhombohedron or cleavage face. 2 - 2½ on the base of a crystal. | |
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Crystal Forms and Aggregates |
(Hexagonal) Occurs in a great variety of shapes, the commonest forms are simple rhombohedral 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. | |
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Transparency | Transparent to translucent | |
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Specific Gravity | 2.7 | |
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Luster | Vitreous | |
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Cleavage | 1,3 - rhombohedral | |
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Fracture | Conchoidal. Hardly observed because of the perfect cleavage. | |
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Tenacity | Brittle | |
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Other ID Marks | 1) Commonly fluorescent; specimens from different
localities fluoresce different colors. Some Calcite is
also phosphorescent. 2) Exhibits a strong double refraction 3) Sometimes thermoluminescent |
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Other Names |  Calcspar |
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Varieties | Commonly used variety names Aphrite - Lamellar variety of Calcite. Cobaltocalcite, Cobalt Calcite, or Cobaltian Calcite - Calcite containing cobalt. It is the intermediary form between Calcite and Sphaerocobaltite in a solid solution series. Formula = (Co,Ca)CO3. (See Cobaltocalcite for more information.) Corn Spar - Calcite aggregate resembling a corn cob with distinct kernels Dogtooth Calcite or Dog-toothed Spar - Calcite in groupings of thick, scalenohedral crystals. Hog-toothed Spar - Calcite with long, sharp, incisor-like crystals Iceland Spar or Optical Calcite - Large, transparent, colorless to lightly colored, rhombohedral crystals of Calcite. Double refraction is especially noted in these crystals. NOTE: Iceland Spar is also used as a synonym of Calcite. Nail Head Spar - Calcite crystals resembling a nail -- long and thin with a wide, flat head. Such crystals are formed by two perpendicular scalenohedral crystals intersecting like a "T". Rice Grain Spar - Calcite grouping of small, white scalenohedral crystals appearing as grains of rice. Salmon Calcite - Orange-red Calcite variety Sand Calcite - Calcite that trapped particles of sand in its interior when it formed Microcrystalline forms of Calcite * Stalactite - Icicle-like form of Calcite on the roof of caverns created when mineral-rich water drips down and the dissolved mineral accumulates into the icicle-like formation. Stalagmite - Tall-domed Calcite formations on the bottom of caverns built up from mineral-rich water that deposits the dissolved mineral on the cavern floor. Flowstone - Calcite formed by mineral-rich water that deposits the dissolved mineral on the walls of caverns and cliffs, forming a smooth and humpy growth. Agaric Mineral or Rock Meal - Crumbly white Calcite found on cavern floors near stalagmites and stalactites. Tufa, Calcareous Tufa, or Calcareous Sinter - Calcite (or Aragonite) 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. Travertine - Mounds of calcium carbonate formed from hot springs that contain calcium rich water. When the water cools, the calcium precipitates from the water, formng porous mounds of calcium carbonate. Travertine usually crystallizes as Aragonite. Onyx Marble, Suisan Marble, California Onyx, Oriental Alabaster, and Mexican Onyx - Travertine or Tufa with impurities causing it to exhibit color banding. Rocks formed from Calcite Limestone - Rock composed of at least 50% calcium carbonate, containing impurities such as Quartz, Dolomite, clay, and iron oxides. Marble - Rock formed from calcium carbonate that recrystallized through metamorphism. Chalk - Rock composed of Limestone combined with the skeletal remains of tiny marine organisms. Variety names usually applied to other minerals Alabaster Calcite connotation = translucent, banded, Travertine or Tufa Usual connotation = massive, fine-grained variety of Gypsum Satin Spar Calcite connotation = fibrous Calcite (or Aragonite) Usual connotation = fibrous Gypsum Onyx Calcite connotation = translucent, banded, Travertine or Tufa with black and white bands Usual connotation = Agate banded with black and white bands |
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Polymorphs | Aragonite, Vaterite | |
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In Group | Carbonates ; Calcite group | |
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All About | Calcite belongs to the calcite group,
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 partial solid solution series. All members of the calcite group crystallize in the trigonal subdivision of the hexagonal system (as rhombohedrons and scalenohedrons), have perfect rhombohedral cleavage, and exhibit a strong double refraction in transparent rhombohedrons. 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 and Aragonite are polymorphous to each other. Although Calcite and Aragonite contain the same elements, 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 tests. Since the true identity of these microcrystalline forms is not known, they are simply labeled as "Calcium Carbonate". This definition accurately includes both Calcite and Aragonite. 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, 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 (see the variety field for a list of the names). 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 only form these growths by hot springs; all other growths are Calcite. |
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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 Nicol prisms for polarizing microscopes and other optical devices. Calcite 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 as drawing chalk. To collectors, Calcite is one of the best-known and most extensively collected minerals. | |
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Striking Features | Hardness, cleavage, fluorescence, and effervescence with hydrochloric acid | |
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Complex Tests | 1) Effervesces in hydrochloric acid and most
other acids 2) Calcite that doesn't fluoresce usually becomes fluorescent after heating. |
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Popularity (1-4) | 1 | |
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Prevalence (1-3) | 1 | |
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Demand (1-3) | 1 | |
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Distinguishing Similar Minerals |
Aragonite - different crystal form Dolomite - transparent clear Dolomite is indistinguishable from Calcite unless complicated optical tests are performed. It can also be distinguished by the fact that Dolomite only effervesces with warm hydrochloric acid, and Calcite even with cold, but accurate results of this test are hard to get. Quartz - much harder (7) Gypsum - softer (2), is sectile and slightly flexible. |
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Commonly Occurs With |
Many minerals, including Fluorite, Apatite, Barite, Albite, Gypsum, Dolomite, Quartz, and Wollastonite. | |
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Noteworthy Localities |
There are hundreds of excellent Calcite
localities, so only a select few will be mentioned here: The original Iceland Spar variety came from a large deposit in Iceland, as its name suggest. Currently, most Iceland Spar comes from numerous places in Mexico, where it is found in great abundance. Large, golden-yellow to brown crystals have come from numerous areas in the tri-state mineral region of Kansas, Missouri, and Oklahoma. Colorless, transparent rhombohedrons are found associated with Copper on the Keweenaw Peninsula of Michigan. Calcite with a strong blue phosphoresce comes from Terlingua, Brewster Co., Texas. Blue crystal clusters come from Balmat, St. Lawrence Co., New York, and Calcite fluorescing bright red occurs associated with Franklinite, Willemite, and Zincite at Franklin and Ogdensburg, Sussex Co., New Jersey. |
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