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CALCITE is a carbonate of lime (CaCo3). It
crystallizes in rhombohedron class. It is also known by the name
calc-spar. It is white in colour but is also found in various
shades like pink, brown etc., depending upon the impurities
present. It is identified easily from other lime minerals by its
well defined rhombohdral cleavage and hardness 3 of Moh's scale.
Its composition is identical to that of limestone but the latter
occurs as sedimentary beds. There is another mineral, aragonite,
of the same chemical composition. It crystallizes in
orthorhombic system. It is, however, an unstable mineral, found
mainly associated with gypsum beds and the tests of
reef-building corals. It is not found in the same large quantity
as that of calcite. |
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Hardness |
3 (only on the basal pinacoidal faces, calcite
has a hardness of less than 2.5 and can be scratched by a
fingernail). |
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Associated Minerals |
Numerous but include these classic associations: Fluorite,
quartz, barite, sphalerite, galena, celestite, sulfur, gold,
copper, emerald, apatite, biotite, zeolites, several metal
sulfides, other carbonates, and borates and many other. |
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Chemical/Typical composition |
CaO56.03 %
CO243.97 % |
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Colour |
Extremely variable but generally white or colorless or with
light shades of yellow, orange, blue, pink, red, brown,
green, black and gray. Occasionally iridescent. |
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Characteristics |
Refractive indices of 1.49 and 1.66 causing a
significant double refraction effect (when a clear crystal
is placed on a single line, two lines can then be observed),
effervesces easily with dilute acids and may be fluorescent,
phosphorescent, thermoluminescence and triboluminescent. |
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Luster |
Vitreous to resinous to dull in massive forms. |
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Field Indicators |
Crystal habit, reaction to acid, abundance,
hardness, double refraction and especially cleavage. |
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Industrial Applications
Calcite in pulverized form finds use mostly as filler in rubber
and textile goods. A considerable quantity is also used in the
ceramics industry for the preparation of glazes. Super white
quality of calcite in powder form (-300 mesh) is used in the
manufacture of paint and distemper. Calcite is now insecticides.
It is also used in the manufacture of cement, calcium carbide,
metal polish and sometimes also as a fluxing agent. The
transparent and clear crystals are used in the optical industry,
for the manufacture of nicol prism. Principal characteristics
which make iceland-spar of special value for use in optical
instruments are its very high birefringence, high degree of
purity, perfect crystalline structure and transparency. The
mineral must be in pieces at least 25mm. long, 12mm. thick,
colourless, transparent and free from cloudy inclusions,
cavities, and foreign substances. |
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