Calcite Composition,
Crystallization & Structure

Composition. Calcium carbonate, CaCO₃ = Carbon dioxide 44.0, lime 56.0. Small amounts of magnesium, ferrous iron, manganese and zinc may replace the calcium.

Crystallization. Hexagonal-rhombohedral. Crystals are very varied in habit, often highly complex. Over 300 different forms have been described. Three important habits: (1) Prismatic, in which the prism faces are prominent, in long or short prisms with basal plane or rhombohedral terminations; (2) Rhombohedral, in which rhombohedral forms predominate, both low and steep rhombohedrons, the unit (cleavage) form is not common; (3) Scalenohedral, in which the scalenohedrons predominate, often with prism faces and rhombohedral truncations. All possible combinations and variations of these types. Twining according to several different laws frequent. Fig. 22 represents one type of twinning in which the basal plane is the twinning plane.

Structure. Crystallized or crystalline granular, coarse to fine. Also fine-frained to compact, earthy. In stalactitic forms, etc.

Physical Properties. Perfect cleavage parallel to unit rhombohedron (angle of rhombohedron = 105° and 75°). H. = 3. G. = 2.72. Luster vitreous to earthy. Color usually white or colorless. May be variously tinted, gray, red, blue, yellow, etc. Also, when impure, brown to black. Usually transparent to translucent. Opaque when impure. Strong double refraction, hence the name doubly-refracting spar.

Test. infusible. After intense ignition, residue gives alkaline reaction to moistened test paper. Fragment moistened with hydrochloric acid and heated gives orange-red flame. Fragments effervesce freely in cold dilute hydrochloric acid. Concentrates solution gives precipitate of calcium sulphate when a few drops of sulfuric acid are added; no precipitate will form if solution is dilute. Distinguished by its softness (3), its perfect cleavage, light color, vitreous luster, etc. Distinguished form dolomite by the fact that fragment of calcite effervesce freely in cold hydrochloric acid, while those of dolomite do not.

Varieties. 

1. Ordinary. Calcite in cleavable or crystalline masses. When transparent and colorless known as Iceland spar, because of its occurrence in quantity in Iceland.
2. Limestone, Marble Chalk. Calcite exists in enormous quantities in the form f limestone rocks, which form large part of the sedimentary strata of the earth. When these rock masses have been subjected to great heat and pressure they develop a crystalline limestones structure, usually showing cleavage faces of greater or less size. Crystalline limestones are knows as marble. On account of various impurities and through the presence in them of other minerals they assume a wide range of colors, and form a long series of ornamental stones to which various names are given. Chalk is a very fine-grained, pulverulent deposit of calcium carbonate, occurring at times in large beds. It has been formed through the slow accumulation on the sea bottom of fragments of shells and of the skeletons of minute sea animals.
3. Cave Deposits, etc. Calcareous waters often deposit calcite in the form of stalactites, concretions, incrustations, etc. It is usually semitranslucent, of light- yellow colors, Many caves in limestone regions are lined with such deposits. Hot calcareous spring waters may from a deposit of calcite, known as travertine, around their mouths. Such a deposit is being formed at the Mammoth Hot Springs, Yellowstone Park
4. Siliceous Calcites. Calcite crystals may enclosed considerable amounts of quartz sand (up to 60 per cent) and form what are known as sandstone crystals. Such occurrences are found at Fontainebleau, France (Fontainebleau limestone), and in the Bad Lands, South Dakota.