Aragonite

• Arragonischer apatite

Aragonite is a common mineral from the mineral class of " carbonates and nitrates ." It crystallizes in the orthorhombic crystal system with the composition Ca [ CO3 ], and is therefore a chemical point of calcium carbonate.

Aragonite crystallizes in different forms. Prismatic crystals occur as well as mineral aggregates, which may be ( tree- like) and parallelfaserig, radialstrahlig acicular or spherical or oolitic, banded, columnar and dendritic. Uninjured crystal surfaces have a glass-like luster, whereas cleavage and fracture surfaces show greasy.

In pure form, aragonite is colorless and transparent. But it may also appear white and accept by foreign admixtures a gray, yellow, red, green, violet or blue color through multiple light refraction due to lattice defects or polycrystalline training, the transparency decreases accordingly.

Aragonite is the eponymous mineral of a group of minerals of similar structure and / or composition of the Aragonitgruppe.

Special Features

Pure aragonite is either colorless or white. It can also be colored gray to brownish, yellowish, reddish, greenish, bluish or violet by foreign admixtures or impurities. Aragonite has a density of 2.95 g / cm ³ and a Mohs hardness of 3.5 to 4.5. Apart from a variety of Aragonite are all opaque.

Aragonite has luminescence, while the minerals behave differently depending on the locality. In Agrigento Aragonite is found, the rosy light up under UV light and then fluoresce green. Aragonite from Tsumeb glow bright yellow to greenish.

Aragonite dissolves readily in acids with evolution of carbon dioxide. Even from weak acids such as boric acid, acetic acid or citric acid crystal is damaged. Aragonite is more soluble than calcite in kohlendioxidgesättigtem water.

Etymology and history

The mineral was described by Abraham Gottlob Werner in 1796 and named by him after his location in Aragon in northeast Spain.

Classification

In the now outdated but still in use 8th edition of the mineral classification by Karl Hugo Strunz the aragonite common to the class " carbonates, nitrates and borates " and then to the Department of " water clear carbonates without foreign anions " belonged. The mineral is there namesake of the " Aragonitgruppe " with the system no. V/B.04 and the other members Alstonite, Barytocalcit, Cerussite, Olekminskit, Paralstonit, strontianite and Witherite.

The revision of the Strunz'schen Mineral classification in the 9th edition the one hand, the borates were moved to a separate class and on the other hand divides the now reduced class of " carbonates and nitrates " precisely on the nature of the cations involved further. The aragonite is therefore now in accordance with the subsection "B. Alkaline-earth (and other M2 ) carbonates ", where he remains a member of the eponymous " Aragonitgruppe "with the system no. 5.AB.15 is, however, only contains the other members Cerussite, strontianite and Witherite. The minerals Alstonite, Barytocalcit, Olekminskit and Paralstonit were put into separate groups within the subdivision.

The commonly used in English-speaking classification of minerals according to Dana assigns the aragonite also in the class of " carbonates, nitrates and borates ", there, however, in the department of " water clear carbonates with a simple formula A CO3 ", where he also along with Cerussite, strontianite and witherite the " Aragonitgruppe ( Orthorhombic: Pmcn ) " with the system no. 14.01.03.01 forms.

Modifications and varieties

Aragonite is a total of three naturally occurring modifications of calcium carbonate (Ca [ CO3 ] ). The other modifications are calcite ( calcite ) and vaterite. The most stable of the three modifications is the calcite, which occurs mainly as limestone, chalk, marble and travertine ( onyx ) in nature.

From aragonite are several color and Formvarietäten:

• Iron Flower: coralline growth in predominantly white or white- grayish color; rarely pale blue to pale blue - green iron flowers are found
• Pea stone or Pisolith: mostly concentric shells or radial- balls at predominantly white - grayish color
• Nicholsonit: by admixtures of Smithsonite (Zn [ CO3 ] ) usually white, yellowish, greenish or pale pink
• Mother of Pearl: variable
• Sprudelstein: wavy, usually white, gray, yellowish or reddish banded travertine
• Sr - aragonite: strontiumhaltiger aragonite
• Tarnowitzite (English: Plumboan Aragonite ): by finely dispersed inclusions of Cerussite (Pb [ CO3 ] ) usually white, gray or black or yellow colored
• Zeiringit: by Aurichalcit turquoise blue to turquoise green colored aragonite

There are many aragonite similar minerals such as barite, gypsum, calcite and quartz.

Education and Locations

Aragonite is under normal circumstances (20 ° C and atmospheric pressure) actually only metastable, the stable phase is calcite. Are sufficient for the presence of a solvent or the exercise of minimum pressure, for example in a mortar, to convert into calcite aragonite. At higher temperatures the conversion is very fast. Only at high pressure, under conditions of a Hochdruck-/Niedrigtemperaturmetamorphose, aragonite is stable. Is rare to find, therefore, a stone with aragonite, forms an exception Aragonitmarmor. The incorporation of other foreign elements into the crystal lattice gives the structure of the aragonite stability. A well-known example is the incorporation of strontium.

The mineral forms either hydrothermally or formation in decomposing Ca - bearing rocks (so-called " iron bloom "). In nature, aragonite occurs frequently at sintering formation in the calcareous environment and hot springs on. Also in cavities of volcanic effusions ( volcanics ) it comes out as " iron bloom " before, in thermal springs as " air stone " and " pea stone ". Deposits in water pipes, water pipes and water boiler are often made of aragonite.

Aragonite is also the main component of nacre and, therefore, the pearls, which are formed from the shell of the mussels. Even the skeleton of hard corals consists of aragonite.

As a frequent mineral aragonite formation is found in many localities, where so far (as of 2013) 2,800 localities as are known.

Locations of aragonite include the municipality corocoro in Bolivia, the Erzberg in Austria, Špania Dolina and Podrečany in Slovakia, Chianciano in Italy, Tarnobrzeg in Poland and Molina de Aragón in Spain.

In the Czech Republic aragonite comes the one hand, micro- crystalline form as air stone at the outlets of sources in Karlovy Vary, but also in the form of larger crystals at Číčov in the Bohemian Central Mountains. The karl bader aragonite deposits have been described by Johann Wolfgang von Goethe.

Synthetic production

When Biorock technology aragonite is obtained together with brucite by Mineralakkretion to weak current-carrying steel from the sea.

Crystal structure

Aragonite crystallizes in the orthorhombic space group Pmcn ( Raumgruppen-Nr. 62) with the lattice parameters a = 4.96 Å, b = 7.97 Å and c = 5.74 Å and four formula units per unit cell.

Use

Aragonite is used as a gemstone, which is, however, sensitive to its brittleness and good cleavage.

The property that aragonite glows under UV light, using the Filipino artist Edd Aragon to paint images that can only be seen in darkness with UV lamps.

Manipulation and imitation

Aragonite gemstones are using stabilized synthetic resin, since aragonite is quite susceptible to acids and machining, as well as to thereby increase its brilliance can be achieved. From fashion reasons Aragonite are also colored. From a consumer protection reasons, both methods must be specified.

Aragonite is often used as a basis for imitation chalcedony, calcite and jade.

Banded aragonite is offered under misleading trade names mostly as California onyx Mexican onyx or Turkish Onyx (see onyx marble ).

Esoteric

As described already aragonite and calcite deposits in hot water installations, form or pipes. Through the application of magnetic fields, the aragonite will not form on the wall or can not be deposited. The operation of such equipment for descaling may be regarded as questionable after the present state of knowledge, since neither carbonate nor calcium ions are paramagnetic or ferromagnetic. From the physicochemical point of view, the postulated mechanisms of action are not possible ( see also Physical water softening ).