Calcite

CALCITE is a carbonate of lime (CaCo3). It crystallizes in the rhombohedron class. It is also known by the name calc-spar. It is white in color 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 rhombohedral cleavage and hardness of 3 on 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 the 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 calcite.

Hardness
3 (only on the basal pinacoidal faces,
calcite has a hardness of less than
2.5 and can be scratched by a fingernail).
Associated MineralsNumerous 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.
Chemical/Typical compositionCaO56.03 %
CO243.97 %
ColourExtremely variable but generally white or
colorless or with light shades of yellow,
orange, blue, pink, red, brown, green,
black and gray. Occasionally iridescent.
CharacteristicsRefractive 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.
LusterVitreous to resinous to dull in
massive forms.
Field IndicatorsCrystal habit, reaction to acid,
abundance, hardness, double
refraction and especially cleavage.

Application