Calcite

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). The Mohs scale of mineral hardness, based on scratch hardness comparison, defines value 3 as 'calcite'.Calcite with mottramiteTrilobite eyes employed calciteCalcite crystals inside a test of the cystoid Echinosphaerites aurantium (Middle Ordovician, northeastern Estonia)Rhombohedrons of calcite that appear almost as books of petals, piled up 3-dimensionally on the matrixCalcite crystal canted at an angle, with little balls of hematite and crystals of chalcopyrite both on its surface and included just inside the surface of the crystalThin section Calcite crystals inside a recrystallized bivalve shell in a biopelspariteSeveral well formed milky white casts, made up of many small sharp calcite crystals, from the sulfur mines at Agrigento, SicilyManganoan calciteReddish rhombohedral calcite crystals from China. Its red color is due to the presence of ironCalcite (var.: Cobaltoan calcite)Sand calcites (calcites heavily included with desert sand) in South DakotaCalcite from Ojuela Mine, Mapimí, Mapimí Municipality, Durango, MexicoCalcite, butterfly twin, 4,0 × 3,3 × 1,6 cm. José María Patoni, San Juan del Río, Durango (México)Calcite on Quartz, Rio Grande do Sul Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). The Mohs scale of mineral hardness, based on scratch hardness comparison, defines value 3 as 'calcite'. Other polymorphs of calcium carbonate are the minerals aragonite and vaterite. Aragonite will change to calcite over timescales of days or less at temperatures exceeding 300 °C, and vaterite is even less stable. Calcite is derived from the German Calcit, a term coined in the 19th century from the Latin word for lime, calx (genitive calcis) with the suffix -ite used to name minerals. It is thus etymologically related to chalk. When applied by archaeologists and stone trade professionals, the term alabaster is used not just as in geology and mineralogy, where it is reserved for a variety of gypsum; but also for a similar-looking, translucent variety of fine-grained banded deposit of calcite. In publications, two different sets of Miller indices are used to describe directions in calcite crystals - the hexagonal system with three indices h, k, l and the rhombohedral system with four indices h, k, l, i. To add to the complications, there are also two definitions of unit cell for calcite. One, an older 'morphological' unit cell, was inferred by measuring angles between faces of crystals and looking for the smallest numbers that fit. Later, a 'structural' unit cell was determined using X-ray crystallography. The morphological unit cell has approximate dimensions a = 10 Å and c = 8.5 Å, while for the structural unit cell they are a = 5 Å and c = 17 Å. For the same orientation, c must be multiplied by 4 to convert from morphological to structural units. As an example, the cleavage is given as 'perfect on {1 0 1 1}' in morphological coordinates and 'perfect on {1 0 1 4}' in structural units. (In hexagonal indices, these are {1 0 1} and {1 0 4}.) Twinning, cleavage and crystal forms are always given in morphological units. Over 800 forms of calcite crystals have been identified. Most common are scalenohedra, with faces in the hexagonal {2 1 1} directions (morphological unit cell) or {2 1 4} directions (structural unit cell); and rhombohedral, with faces in the {1 0 1} or {1 0 4} directions (the most common cleavage plane). Habits include acute to obtuse rhombohedra, tabular forms, prisms, or various scalenohedra. Calcite exhibits several twinning types adding to the variety of observed forms. It may occur as fibrous, granular, lamellar, or compact. A fibrous, efflorescent form is known as lublinite. Cleavage is usually in three directions parallel to the rhombohedron form. Its fracture is conchoidal, but difficult to obtain. Scalenohedral faces are chiral and come in pairs with mirror-image symmetry; their growth can be influenced by interaction with chiral biomolecules such as L- and D-amino acids. Rhombohedral faces are achiral. It has a defining Mohs hardness of 3, a specific gravity of 2.71, and its luster is vitreous in crystallized varieties. Color is white or none, though shades of gray, red, orange, yellow, green, blue, violet, brown, or even black can occur when the mineral is charged with impurities. Calcite is transparent to opaque and may occasionally show phosphorescence or fluorescence. A transparent variety called Iceland spar is used for optical purposes. Acute scalenohedral crystals are sometimes referred to as 'dogtooth spar' while the rhombohedral form is sometimes referred to as 'nailhead spar'.