Calcite
- Calcite
- Calcite
Gap angle 75 °
Nw = 1.640 to 1.660 1.658 pure nε = 1.486 In the range of 190 to 1700 nm of about 1.6 N? Falls to about 1.4, while nε falls from about 1.9 to about 1.5.
Purely 0,172; uniaxial negative
2Vx then 4-14 ° ( 25 ° )
Calcite, calcite, calcite or calcite, is a very common mineral and namesake of the Calcitgruppe within the mineral class of " carbonates ( and relatives ) ." It crystallizes in the trigonal crystal system with the chemical composition Ca [ CO3 ] and developed various crystal or aggregate forms ( habitus ) that are either colorless or milky white to gray, by impurities also yellow, pink, red, blue, green, brown or black color can be.
With a Mohs hardness of 3 Calcite is one of the soft minerals and serves as a reference to the to 10 ( diamond) reaching scale after Friedrich Mohs.
Special Features
Characteristic of calcite crystals is particularly high birefringence. Light is not incident along the optical axis of the crystal is split into two light beams, one ordinary and an extraordinary ray. For these two rays are due to different directions of polarization different refractive indices. This is reflected in the fact that at a certain point of view as any observed by a clear crystal object appears twice, one for the identification of calcite very useful feature, hence the common name spar. In Iceland, the most famous occurrence of calcite, it is called a silfurberg ( silver rock).
Compared to other minerals calcite is hardly resistant to weathering. It is much softer than quartz or feldspar and soluble even in acidic water. In cold, dilute acids to calcite dissolves under vigorous evolution of gas.
Depending on the locality may calcite by incorporation of rare earth under UV light, red, blue or yellow, but other colors fluoresce. Furthermore, occur also phosphorescent, kathodo, thermo- and rarely tribolumineszierende Calcite.
Etymology and history
The name calcite is derived from the Greek χάλιξ chálix (m and f; mortar, lime, gravel ) or from Latin calx (lime).
Classification
In the now outdated but still in use 8th edition of the mineral classification by Strunz calcite belongs to the mineral class of " carbonates, nitrates and borates " and then to the Department of " water clear carbonates without foreign anions ", where he served as the eponymous mineral the " Calcitgruppe " forms with the system number V/B.02 and the other members Gaspéit, magnesite, Otavit, rhodochrosite, siderite, smithsonite, Sphärocobaltit and vaterite.
The 9th edition used since 2001 and valid by the International Mineralogical Association (IMA ) of the Strunz'schen Mineral classification assigns the calcite in the class of " carbonates and nitrates " ( borates now form a separate class ) and then in the Department of " carbonates without further anions, without H2O ". This division is further divided according to the element group membership of the cations involved, so that the mineral is found according to its composition in the subsection " alkaline-earth (and other M2 ) carbonates " where there is the still existing " Calcitgruppe " with the system No. 5.AB.05 forms. The vaterite now forms a separate group.
The mostly commonly used in English-speaking classification of minerals according to Dana assigns the calcite as the outdated Strunz classification in the common class of " carbonates, nitrates and borates " and there in the department of " water clear carbonates " one. Here it is also named the " Calcitgruppe ( Trigonal: R- 3c)" with the system number 14:01:01 and the other members Gaspéit, magnesite, Otavit, rhodochrosite, siderite, smithsonite and Sphaerocobaltit within the subdivision 14:01 Anhydrous carbonates with a simple formula A CO3.
Modifications and varieties
Calcite is chemically (such as aragonite and vaterite similar ) a calcium carbonate, a calcium salt of carbonic acid.
As Blätterspat or Papierspat Calcitvarietäten are denoted by thin, leaves like crystals.
Kanonenspat is a Calcitvarietät with langsäulig stretched, pseudohexagonalem habit. As Manganocalcit a manganese-containing variety of calcite is called.
A pseudomorphs of calcite after Ikaite is called Glendonit.
Education and Locations
Made Calcite is after the chemical equilibrium:
The equilibrium of the above reaction shifts with increasing temperature increasing to the right side. In warm waters beings can thus form with less energy expenditure calcareous. In steam boilers and other vessels, in which soft water is heated, formed in this way scale.
Calcite can be both massive and granular, fibrous or occur in crystals and in the latter case shows the highest form of wealth of all minerals. It is a rock -forming mineral of the most common minerals of the earth's crust and occurs in both igneous, for example in carbonatites, in metamorphic (marble) or sedimentary rocks such as limestone. It occurs alone or associated with other minerals in the corridors, but also occurs at the surface. Often created / generated by calcite biomineralization, whether in rock formations, in the base, as in part undesired plaque ( preferably next to calcium hydroxyapatite ), etc.; here but always in very specific micro - environmental conditions.
Calcite dissolves readily in acidic water and is therefore easily leached from limestone cave systems which arise. The dissolved calcite deposits elsewhere again. The typical stalactites, stalagmites and stalactites formed.
By far the largest Calcitvorkommen go back to sea sediments. Here, the calcite skeletons and shells of innumerable small marine animals such as clams, corals and various protists, such as the coccolithophores set, from the seabed. These calcareous algae are smaller than 30 microns and are counted for nanoplankton. They form tiny Kalkschilde, called coccoliths, which sink to the ocean floor after the death of the algae. The White Cliffs of Dover consist of such coccoliths. Also, coral reefs play a prominent role in the calcite.
Inorganic, abiogenic formation of calcite areas are flat, located in Intertidalbereich, tropical marine platforms. There calcite in the form of a millimeter in size beads ( Kalkooiden ) is precipitated. Calcite in the marble goes back to thermal metamorphism of Calcitsedimenten.
From a sea depth of 3500 meters, the so-called calcite compensation depth to calcite dissolves completely in water. Therefore, at this depth neither calcithaltige sediment, mussel shells or skeletons are preserved.
Calcite is used as storage before the Statolithenmembran the macula organs of the inner ear. It plays a major role in the perception of accelerations and the perpendicular direction there.
Known due to exceptional Calcitfunde is mainly Iceland, where in addition to the water-clear calcite crystals were also the largest ever found. On Helgustadir near Reyðarfjörður the largest crystal had dimensions of 7 m × 7 m × 2 m and the heaviest weight of 280 t. In the "Sterling Bush" cave in Lewis County (New York) was 95 cm × 46 cm and found a weight of 500 kg, a calcite rhombohedron of 109 cm ×.
One of the biggest exhibited in museums Calcite is located in the Natural History Museum. It has a weight of 230 kg.
Crystal structure
Calcite exhibits trigonal space group R 3 c ( Raumgruppen-Nr. 167) with the lattice parameters a = 4.99 Å and c = 17.06 Å and 6 formula units per unit cell.
Use
Building materials and raw materials
The calcite- rock marble, limestone and onyx are a high quality decorative and construction materials, along with calcite in limestone is used for the production of cement and fertilizer and as an additive in the smelting of ores. High purity crystals are used because of their optical properties in the optical industry, in particular in the polarization optical system, for example in the form of Glan -Taylor prisms or a retardation plate. In addition, it is used in acidic, basic and rutile electrodes as Schutzgasbildner the manual arc welding.
Gem
Calcite is actually too soft for a commercial gemstone production. Occasionally, he is yet offered in smooth cut as cabochon or tumbled stone. Savvy collectors succeeds beyond even to bring calcite in the form of facets.
Terrarium sand
Due to the acid solubility of calcite is used in the form of " Caltrat " in terrariums. Terrarium animals that had been maintained on conventional ( non-digestible ) flow and / or silica sand, suffered after the inclusion of these sands to often fatal blockages. By the use of ( digestible ) Caltrat this risk is substantially eliminated.