CALCITE MINERAL FACTS Nevada Turquoise gem stones
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Calcite Mineral Facts:

Chemical Formula: CaCO3

Colors: Colorless and when pure, but often colored by impurities. May be variously tinted, gray, red, green, blue, yellow, or even brown to black when very impure

Hardness: 3.0

Density: 2.72
The density is average for most minerals. 

Cleavage: Perfect cleavage parallel to unit rhombohedron (angle of rhombohedron = 105 and 75). The cleavage of calcite is so perfect that crystals when shattered by a hammer blow usually break into perfect little rhombohedrons.

Crystallography: Hexagonal - Rhobohedral
Crystals are very varied in habit, often highly complex.
Crystals may be granular, coarse to fine. Also fine-grained to compact, earthy. In stalactitic forms, etc.

Luster:. Vitreous to earthy.

Optics: (Refractive Index):  Transparent to opaque. RI: w = 1.658; E = 1.486;  Strong Double Refraction


Composition, Structure and Associated Minerals:
Calcite is one of the most beautifully crystallized minerals known. Its crystals are very common, and sometimes very large. They are usually colorless, though sometimes colored, and are nearly always transparent. Besides occurring in crystals the mineral is often found massive, in granular aggregates, in stalactites, in pulverulent masses, in radial groupings, in fibrous masses and in a variety of other forms. As calcite is soluble in water containing CO2, it has often been found in pseudomorphs after other minerals. The forms that have been observed on calcite crystals are arranged in such a manner as to produce three distinct types of habit, as follows: (1) the rhombohedral type, bounded by the flat rhombohedrons and often blunt scalenohedrons, (2) the prismatic type, with the prism predominating as the principal termination; and (3) dog-tooth spar, containing the same scalenohedrons as on the first type mentioned above with other steeper ones and small steep rhombohedral planes. Nail-head spar contains the flat rhombohedron with the prism. Some of the crystals are very complicated, belonging to no one of the distinct types described above, but forming barrel-shaped or almost round bodies. Over 300 well established forms have been identified on them. Twined forms are common.

Theoretically, calcite contains 56 per cent CaO and 44 per cent CO2, but practically the mineral contains also small quantities of Mg, Fe, Mn, Zn and Pb, metals whose carbonates are isomorphous with Calcite. It is a very poor conductor of electricity.

The principal varieties of the mineral to which distinct names have been given are:
Iceland spar, the transparent variety used in the manufacture of optical instruments.
Satin spar, a fine, fibrous variety with a satiny luster.
Limestone, granular aggregates occurring as rock masses and beds.
Marble, a crystalline metmorphosed limestone, with interlocking crystals.
Stalactites, cylinders or cones of calcite that hang from the roofs of caves. They are formed by the evaporation of dripping water.
Stalagmites, corresponding cones on the floors of caves beneath the stalactites.
Mexican onyx, banded crystalline calcite, often translucent.
Travertine, a deposit of white or yellow porous calcite produced in springs or rivers.

Calcite, Fine Crystals

Identification and Diagnostics
In the closed tube calcite often decrepitates. Before the blowpipe it is infusible. It colors the flame reddish yellow and after heating reacts alkaline toward moistened litmus paper. The mineral dissolves with evolution of CO2 in cold hydrochloric acid. Concentrated solution of the mineral dissolved in HCl gives precipitate of calcium sulfate when a few drops of sulfuric acid are added; no precipitate will form if solution is dilute. Its dissociation temperature at normal pressures  is 898, though it begins to lose CO2 before that point. The reaction with HC1, together with the alkalinity of the mineral after heating, softness (3), its perfect cleavage, light color, vitreous luster, etc. distinguish calcite from all other minerals. Distinguished from dolomite by the fact that fragments of calcite effervesce freely in cold hydrochloric acid, while those of dolomite do not.

Occurrence, Localities and Origins:
Calcite is one of the most common and widely diffused of minerals. It occurs as enormous and widespread sedimentary rock masses, in which it is the predominant, at times practically the only mineral present. Such rocks are the limestones, marbles (metamorphosed limestones), chalks, calcareous marls, calcareous sandstones, etc. Its principal methods of origin are precipitation from solutions, the weathering of calcareous minerals, and secretion by organisms. Calcite commonly forms pseudomorphs after many different minerals and the hard parts of animals. It occurs as a secondary mineral in igneous rocks as a product of decomposition of lime silicates. It is found lining the amygdaloidal cavities in lavas. It occurs in many sedimentary and metamorphic rocks in greater or less proportion. It is the cementing material in the light colored sandstones. Calcite is also one of the most common of vein minerals, occurring as a gangue material, with all sorts of metallic ores.

Calcite is doubly refractive, splitting light in two.


It would be quite impossible to specify all of the important districts for the occurrence of calcite in its various forms. Some of the more notable localities in which finely crystallized calcite is found are as follows: Andreasberg in the Harz Mountains; Freiberg, Schneeberg and other places in Saxony; in Cumberland, Derbyshire, Devonshire, Matlock, Lancashire and the mines of Cornwall, England; Iceland; Guanajuato, Mexico; Keweenaw Point and the Lake Superior copper district, Mich.; the zinc regions of Illinois, Wisconsin and Missouri - especially Joplin, Missouri; Lake; Lockport and Rossie in New York, Nova Scotia, etc. Iceland spar is obtained in the Eskefjord and the Breitifjord in Iceland. Travertine is deposited from the waters of the Mammoth Hot Springs, Yellowstone National Park. It occurs also along the River Arno, near Tivoli, Rome.

Calcite has many important uses. In the form of Iceland spar, on account of its strong double refraction, it is employed in optical instruments for the production of polarized light. Calcite rocks are used as building and ornamental stones. Marbles are used very extensively as ornamental and building material. The most important marble quarries in the United States are found in Vermont, New York, Georgia, Tennessee, etc. Calcite is also  employed as fluxes in smelting operations, as one of the ingredients in glass-making and in the manufacture of lime, and certain ground improving fertilizers. The most important use for calcite is for the manufacture of lime for mortars and cements. Limestone when heated to about 1000 F. loses its carbonic acid, and is converted into quicklime, CaO. This, when mixed with water (staked lime), swells, gives off much heat, and finally by absorption of carbon dioxide from the air hardens, or, as commonly termed, "sets." The chemistry of the process of their hardening is not fully understood, but various silicates of calcium and aluminium are probably formed.

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Green calcite, showing cleavage lines


Please note that the author, Chris Ralph, retains all copyrights to this entire document and it may not be reproduced, quoted or copied without permission.

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