Calcite is a mineral with a hardness of 3 out of 10 on the Mohs scale of mineral hardness [?]. These Trigonally structured gems are made of calcium carbonate, their full chemical compound being CaCO3.
Of all minerals, calcite is by far the richest in forms. It appears as rhombohedral, scalenohedral, or prismatic crystals, often intergrown or twinned. It occurs in masses, colorless or in white, pink, green, or yellow, and often visibly thermoluminescent (emits light when heated).
Like quartz, calcite often has twinned structure caused by temperature and stress changes.
It is semi-hard, with perfect rhombohedral cleavage. It is transparent with vitreous to iridescent pearly luster. Some varieties are fluorescent in ultraviolet light.
It is soluble in cold hydrochloric acid, with brisk effervescence. It is infusible but dissociates in a Bunsen burner flame, giving lime and carbon dioxide.
Calcite is a typical sedimentary mineral formed by chemical precipitation through the evaporation of solutions rich in calcium bicarbonate, as in stalactites and travertines, or by extraction through the action of marine and freshwater organisms. It remains stable under metamorphic conditions up to the highest grades, simply recrystallizing and increasing in grain size as long as the pressure of carbon dioxide remains high.
It is the major constituent of limestone. Calcite is the more stable form of calcium carbonate found in seashells, the other being aragonite (responsible for the attractive aspects of shells such as the iridescence of mother of pearl). But during burial, aragonite in the shells progressively recrystallizes to the more stable calcite. During this process the calcite will tend to grow in the pore spaces, cementing and strengthening the rock.
A colored variety of microcrystalline calcite from Mexico and Pakistan is called onyx.
The famous Carrara marble in Tuscany involved recrystallization of a limestone consisting of calcite to a marble consisting of calcite; the particular quality of the marble is due to the calcite crystals being of even, uniform size, closely intergrown and relatively free of impurities. This makes the rock stronger.
Italy is also famous for translucent rhombohedra from Porretta (Moxena), Sarrabus (Sardinia), the basalts of the Lessini mountains (Vicenza) and large scalenohedra from Passo Molignon (Trento).
Large, very clear rhombohedral forms of calcite were used to make polarizing prisms (the Nicol prism) for petrological microscopes. Material for this was found in basalt cavities in Iceland, the Harz (Germany), Erzgebirge (Bohemia), Saxony (Czechoslovakia), and in Colorado (USA).
Compact masses have a variety of uses, especially in building (cement, lime, structural and ornamental stone), metallurgy (as flux and slag), the manufacture of fertilizers (for soil enrichment), and the chemical industry (in caustic soda, calcium chloride, liquid carbon dioxide, etc.) Other uses include marble for sculpture, lithographic stones for printing, as raw material in the glass and cellulose industry, loose earthy masses as powders for polishing, and as fillers in rubber and paint manufacturing.
Collectors are familiar with the calcite crystals containing dendritic copper from the Keweenaw Peninsula in Michigan (USA), and the crystals covered with sand from Fontainebleau (France).