Mineralogia della calcite

Introduction
Description Twinning
Physical properties
Optical properties
Uses
Localities

Calcite

Ca (CO3)
Carbonate of calcium

Polymorphs: Aragonite, Vaterite

Environment: sedimentary, hydrothermal, intrusive, igneus

Calcite is truly one of the best collection type minerals. There are lots of interesting forms and varieties as well as colorful and beautiful specimens to collect. It is generally easy to identify using its rhomohedral cleavage, reaction to acid and double refraction and makes for a great classroom example of these properties. If it is not the significant mineral on a specimen, it might be an accessory to other wonderful minerals and only enhancing their attractiveness. With its many different forms, environments, associations and colors, a collector could never have all possible combinations of calcite covered.

DESCRIption:

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(a)

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(b)

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(c)

The crystals of calcite can form literally a thousand different shapes by combining the basic forms of the positive rhombohedron, negative rhombohedron, steeply, moderately and slightly inclined rhombohedrons, various scalahedrons, prism and pinacoid to name a few of the more common forms. There are more than 300 crystal forms identified in calcite and these forms can combine to produce the thousand different crystal variations. Crystal Habits are extremely variable with almost any trigonal form possible. Common among calcite crystals are the scalenohedron (a), rhombohedron (c), hexagonal prism (b), and pinacoid. Combinations of these and over three hundred other forms can make a multitude of crystal shapes, but always trigonal or pseudo-hexagonal. Pseudomorphs after many minerals are known, but easily identified as calcite. Also massive, fibrous, concretionary, stalactitic, nodular, oolitic, stellate, dendritic, granular, layered, etc. etc.

	Twinning:
	Calcite produces many twin varieties. Twinning is often seen and results in crystals with blocky chevrons, right angled prisms, heart shapes or dipyramidal shapes. A notch in the middle of a doubly terminated scalenohedron is a sure sign of a twinned crystal. lamellar twinning also seen resulting in striated cleavage surfaces.
	phisycal properties:
	Hardness: 3 Density: 2.70 to 3 Color: white, yellow, brown, blue, colourless, pink, green, red Streak: white, gray Luster: transparent, translucent, glassy, pearly Crystal system: trigonal Cristal forms: massive, fibrous, granular, stalactitic Cleavage: perfect (10-10) Fracture: brittle, conchoidal Fusibility: 7 Flame color: yellow Effervesces easily with dilute acids and may be fluorescent, phosphorescent, thermoluminescence and triboluminescent.

optical properties:

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Refractive indices of 1.49 and 1.66 causing a significant double refraction effect. Double refraction occurs when a ray of light enters the crystal and due to calcite's unique optical properties, the ray is split into fast and slow beams. As these two beams exit the crystal they are bent into two different angles (known as angles of refraction) because the angle is affected by the speed of the beams. A person viewing into the crystal will see two images ... of everything

	Uses:
	In cements and mortars, production of lime, limestone is used in the steel industry; glass industry, ornamental stone, chemical and optical uses and as mineral specimens.
	LOCALITies:
	Potosí, Bolivia Sao Gabriel, Rio Grande do Sul, Brazil Chenzou mine, Hunan province, China Boltsburn mine, Rockhope, Co. Durham, England Cammock Eals mine,Stanhope,Weardale, Co.Durham, England Herodsfoot mine, Lanreath, Cornwall, England Dulcoate quarry, Somerset, England Waterbank mine, Ecton, Staffordshire, England Judkins quarry, Nuneaton, Warwickshire, England Bramham, Yorkshire, England Bellecroix, Fontainebleau, France Cap Garonne, Var, France Johann mine, Wittichen, Black Forest, Germany Andreasberg, Harz Mts, Germany Mashamba West mine, Shaba, Zaire (Var: Cobaltian Calcite )
Ó Stefano Zizzi (1998)