Andalusite
The mineral Andalusite is a common island silicate from the group of aluminosilicates and has the chemical composition Al 2 [O | SiO4 ]. Andalusite crystallizes in the orthorhombic crystal system and most developed prismatic crystals with a square cross section, but also fibrous, granular or massive aggregates in varying colors such as red, pink, gray-brown, yellow or green. Its Mohs hardness is between 6.5 and 7.5, its density is about 3.2 g / cc and its stroke color is white.
Etymology and history
Was first described in 1798 by Jean -Claude mineral Delamétherie. It was named after his first, but later identified as atypical location - the Serranía de Ronda in Málaga in the Spanish province of Andalusia.
Classification
In the old ( 8th edition ) and new classification of minerals ( 9th Edition ) by Strunz belongs to the Department of Andalusite " island silicates with tetrahedral foreign anions ( Neso - Subsilikate ) ". The new Strunz'sche Mineral classification divided here but now more specific on the location of the cations in the crystal, so that the mineral " in, island silicates with additional anions and cations - and / or only coordination " now the subdivision is assigned and there with Kanonait own group forms.
The commonly used in English-speaking classification of minerals according to Dana assigns the andalusite in the department of " island silicates: SiO4 Groups and O, OH, F, and H 2 O with cations in and > coordination " and there along with Kanonait and Yoderit in the " Al2SiO5 ( andalusite - subgroup ) " a.
Modifications and varieties
Besides andalusite are kyanite ( kyanite ), which forms primarily at high pressures, and sillimanite as high-temperature phase, further modifications of the aluminosilicate.
On varieties are mainly of gray-black chiastolite that really stand out for its graphite inclusions and carbonaceous inclusions existing black cross, and the green colored by incorporation of iron and manganese ions viridin.
Contrast, chrysanthemums stone is a flower -shaped leaves up like mineral aggregate, the bright andalusite crystals are embedded in a dark (mostly black ) matrix.
Education and Locations
Andalusite formed under low pressure by thermal metamorphism in metamorphic rocks such as hornfels. In addition, the mineral is also found in pegmatites, sometimes even as a gem, and occasionally as a mineral soaps in river sediments. Accompanying minerals include kyanite, sillimanite, cordierite, corundum, garnets, tourmalines and various mica.
Common Locations for andalusite include Bimbowrie in Australia, Morro do chapeu / Bahia in Brazil, Darmstadt and Gefrees the Fichtel mountains in Germany and the Alpe Lisens at Sellrain in Austria.
Crystal structure
Andalusite crystallizes in the orthorhombic crystal system, space group Pnnm with the lattice parameters a = 7.7980 Å, b = 7.9031 Å and c = 5.5566 Å and four formula units per unit cell
Use
The raw material
Andalusite is in the porcelain manufacture and in the production of heat-resistant materials use.
As a gemstone
Transparent Andalusite of gem quality are rarely found and are thus correspondingly valuable. In the choice of the section be clearer to strong pleochroism must be observed in order to achieve an optimal color results.