Allophane
Allophane (also Elhuyarit, Ilbait or Riemannit ) is a mineral from the mineral class of " silicates and Germanates ", more specifically, a layer silicate. It is next to the famous Opal one of the few minerals that belong to any crystal system, but solidify amorphous.
Its chemical composition is Al2O3 x · y SiO2 · z H2O, where the variables x and y in the ratio 1:1. The molecular groups alumina and silica and the crystal water contained occur in different amounts.
Allophane forms grape-like, stalactitic or earthy mineral aggregates of white, gray, bluish, greenish and brown in color with white stroke color.
Special Features
Due to its loose structure with many cavities and the high water content of allophane has a very low density ( oven-dry soil allophanhaltiger <0.9 g / cm ³). It also has a very high reactive surface area (~ 800 m / g ), which is mainly formed by "pores" in the hollow spheres (actually defective points, which interrupt the hollow spheres ). Dehydration and water loss lead to irreversible structure destruction.
Before the blowpipe allophane is infusible.
The two main types of alumina-rich allophane (Al: Si 2:1) and silicon-rich allophane (Al: Si 1:1) arise depending on the availability of the substances in the soil solution.
Etymology and history
The name allophane is a combination of the Greek words allos ἄλλος for " other " and φαίνω phainestai appear for and refers to the often existing similarity with other copper minerals.
Allophane was first discovered in Grafenthal in the district Saalfeld- Rudolstadt, in Thuringia and described in 1816 by JFL Hausmann and F. Stromeyer: About Silver chalcocite and allophane, Göttingische scholars ads 2, 1251-1253
Classification
In the now outdated but still in use 8th edition of the mineral classification by Strunz the allophane belonged to the mineral class of " silicates and Germanates " and then to the Department of " layer silicates ( phyllosilicates ) ," where he had a together with Hisingerite, Imogolith, Neotokit and Odinit independent group was formed.
The 9th edition used since 2001 and valid by the International Mineralogical Association (IMA ) of the Strunz'schen Mineral classification assigns the allophane also included in the category of " silicates and Germanates " and there in the department of " layer silicates ( phyllosilicates ) ". This division, however, is further subdivided according to the crystal structure, so that the mineral according to its construction in the subdivision of " layer silicates ( phyllosilicates ) with kaolinite layers composed of tetrahedral or octahedral nets " can be found, where it, along with Bismutoferrit, Chapmanit, chrysocolla Imogolith and Neotokit the " Chapmanit group " with the system no. 9.ED.20 forms.
The mainly common in English-speaking classification of minerals according to Dana assigns the allophane in the class of " silicates and Germanates " and there in the department of " layer silicate minerals " one. Here he is as the namesake of the " allophane " with the system no. ": With 1-1 layers of layers of six-membered rings phyllosilicates " to find 71.01.04 and the other members Hisingerite, Imogolith, Neotocit and Zinalsit within the subdivision.
Education and Locations
Allophane is mainly produced hydrothermally in columns of sedimentary rocks. When bottoming it arises, in particular by the chemical weathering of volcanic glasses and then forms the so-called loose brown earth. Finding also is allophane in coal and ore deposits. Accompanying minerals include chrysocolla, cristobalite, gibbsite, Imogolith, limonite, quartz and vermiculite.
Overall, allophane has so far (as of 2011) are detected at around 370 localities. Apart from its type locality Grafenthal the mineral in Germany nor in several places in the Black Forest in Baden- Württemberg; at Pechbrunn and Kropfmühl in Bavaria; Goldhausen and Kirschhausen in Hesse; at several places in the Harz Mountains of Lower Saxony to Saxony -Anhalt; at Untermaubach, Wülfrath and in several places of the Sauerland and the winning country in North Rhine -Westphalia; in Antweiler in Rhineland-Palatinate; at Chemnitz marker village and Mechelgrün and at several locations in the Ore Mountains in Saxony and in Gera, Garnsdorf ( Saalfeld / Saale), are found on the mountain man head in Gräfenroda in Brotterode - Trusetal and Weckersdorf in Thuringia.
In Austria allophane joined, among others, on Pauliberg in Burgenland; in Leoben in Styria, and in Freistadt in Upper Austria as well as in several locations of the regions Carinthia, Salzburg and Tyrol.
In Switzerland, the mineral is found on Cavloc Lake in Fornotal (Valley of the Fornogletschers ) in the canton Graubünden and in Ayer (Val d' Anniviers ), Saint- Luc VS and Siders ( Granges ) in the canton of Valais.
Other localities are Argentina, Australia, Moldova, Belgium, Bolivia, China, Ecuador, France, Greece, Ireland, Israel, Italy, Japan, Kazakhstan, Kyrgyzstan, Canada, Colombia, the Democratic Republic of Congo, Mexico, New Zealand, the Netherlands, Norway, Poland, Romania, Russia, Zambia, Serbia, Zimbabwe, Slovakia, Spain, South Africa, Taiwan, the Czech Republic, Turkmenistan, Ukraine, the United Kingdom ( UK ) and the United States of America (USA).
Even in rock samples from the East Pacific Rise allophane could be detected.
Crystal structure
Allophane solidified amorphous or semi- crystalline with aluminum - and coordination.