Epidote

Epidote is a common mineral from the mineral class of " silicates and Germanates ". It crystallizes in the monoclinic crystal system with the general chemical composition Ca2 ( Fe3 , Al) Al 2 [O | OH | SiO4 | Si2O7 ]. The brackets mean in the chemical formula that iron and aluminum can, although to a minor extent (substitution ), but always stand in the same relation to the other constituents. Epidote is also the namesake representatives of Epidotgruppe, a group of chemically similar minerals.

Epidote usually forms prismatic crystals up to 35 cm in length that can be stretched and streaked in parallel and depending on purity are translucent to opaque. In addition, however, the mineral also comes in the form of fibrous, granular to massive and radialstrahliger aggregates before. The color of epidote is variable, but often dark green, sometimes yellow-brown, while the stroke color is white. With a Mohs hardness, which is depending on the purity of the mineral 6-7, epidote belongs to the medium-hard to hard minerals. To carve it, it needs at least a steel file, but he himself is able to carve simple window glass.

Epidote together with clinozoisite ( Ca2AlAl2 [O | OH | SiO4 | Si2O7 ] ) a complete solid solution series.

Etymology and history

Epidote 1782 was first discovered in Le Bourg d'Oisans in the French department of Isère. However, the specimens found there were initially inaccurately called tourmaline. Only the French mineralogist René -Just Haiiy recognized in 1801 that it was in this material is a different mineral, which he called after the Greek word [ epidosis ] for addition, improvement or expansion, as an allusion to the characteristic of epidote crystal form wherein one of the sides is longer than the other at the bottom of the prism.

Classification

In the now outdated but still in use 8th edition of the mineral classification by Strunz the epidote belonged to the mineral class of " silicates and Germanates " and then to the Department of " group silicates ( Sorosilikate ) ", where he, together with allanite - (Ce), allanite - (La), allanite - (Y), Dissakisit - (Ce), Dissakisit - (La), Dollaseit - (Ce), epidote - (Pb), Ferriallanit - (Ce), Gatelit - (Ce), Khristovit - (Ce ), clinozoisite, clinozoisite - ( Sr), Manganiandrosit - (Ce), Manganiandrosit - (La), Manganipiemontit - ( Sr), Mukhinit, piemontite, piemontite - ( Sr), Uedait - (Ce), Vanadoandrosit - (Ce), Västmanlandit - (Ce) and zoisite named after him " Epidotgruppe " was formed.

The 9th edition used since 2001 and valid by the International Mineralogical Association (IMA ) of the Strunz'schen Mineral classification assigns the epidote also included in the category of " silicates and Germanates " and there in the department of " group silicates ( Sorosilikate ) ". This division, however, is further subdivided by the type of group formation of silicate complexes and the coordination of the cations, so that the mineral according to its composition and structure in the sub-division of " group silicates with mixed SiO4 and Si2O7 groups; Cations "can be found, where it is also the namesake " in octahedral coordination and greater Epidotgruppe "with the system no. 9.BG.05 and the other members of allanite - (Ce), allanite - (La), allanite - (Y), Dissakisit - (Ce), Dissakisit - (La), Dollaseit - (Ce), epidote - (Pb), epidote - ( Sr), Ferriallanit - (Ce), Khristovit - (Ce), clinozoisite - ( Sr), clinozoisite, Manganiandrosit - (Ce), Manganiandrosit - (La), Manganipiemontit - ( Sr), Mukhinit, piemontite ( Sr), piemontite, Uedait - (Ce), Vanadoandrosit - (Ce) and Vanadoepidot forms.

The mainly common in English-speaking classification of minerals according to Dana assigns the epidote in the class of " silicates and Germanates " there, however, in the already fine divided department of " group silicates: Insular, Mixed, individual and larger tetrahedral groups." Here it is also named the " Epidotgruppe ( clinozoisite - subgroup ) " with the system no. 58.02.01a and the other members clinozoisite, epidote - (Pb), Mukhinit, piemontite, piemontite - ( Sr), Manganipiemontit - ( Sr), Gatelit - (Ce), clinozoisite - ( Sr), Uedait - (Ce) and epidote - (Sr ) within the sub-division of " group silicates: Insular, mixed, individual and larger groups with cations in tetrahedral and higher coordination; Single and double groups (n = 1,2) to find ".

Modifications and varieties

So far the following varieties of Epidots are known:

• Allanite - epidote ( epidote - allanite also ) - mixed crystal of Allaniten and epidote
• Beryllium - epidote and epidote Chrome - included as admixtures the respective metals mentioned
• Pistacite - microcrystalline, colored to black-green by impurities of iron green ( Pistazie! ).
• Puschkinit - green, red and yellow variety from the Urals
• Tawmawit - strong green, light green to light yellow, brown to red by admixtures of chromium special shades, named after its type locality Tawmaw. The colors can change through strong pleochroism direction-dependent with each other and thus all be seen in one piece.
• Withamit - opaque, manganhaltig and therefore colored pink to red

Allanite ( with Annexes - (Ce ), - ( La) and - ( Y) ) and clinozoisite are also identified by some sources as a variety of epidote. However, they are recognized by the International Mineralogical Association (IMA ) as independent minerals; Allanite but only in conjunction with their attached elements to distinguish between the end members of this mixed batch.

Education and Locations

Epidote forms as a common component of metamorphic rocks, particularly those derived from igneous rocks such as basalt, eg amphibolite, derived. Also in marble ( metamorphic limestone) it is more common. In hydrothermal veins He also occurs as a decomposition product of other silicate minerals. Accompanying minerals include various amphiboles, garnets, plagioclase, pyroxene and zeolites and actinolite, albite, calcite, glaucophane, lawsonite, omphacite, prehnite, pumpellyite, quartz, riebeckite, scapolite, talc, wollastonite and vesuvianite.

Epidote becomes unstable as the temperature rises and decays at about 650 to 700 ° C. In the rocks of the amphibolite facies result that, after a complex series of reactions in a change in the composition of the rock of hornblende, albite, epidote and quartz to hornblende, plagioclase and quartz. Thus the anorthite content increases with increasing temperature.

Overall, epidote so far (as of 2011) are detected at about 5,600 localities. Noteworthy due to excellent mineral finds, among other things Sobotín in the Czech Republic, where up to 14 cm long, green crystals were revealed. At the Knappenwand in the Austrian Untersulzbachtal crystals were about 10 cm in length have been found. Older reports from the 19th century, according to have been found at the Knappenwand still much larger (50 to 70 cm) crystals, however, these data do not prove by appropriate pieces of evidence.

In Austria, the mineral was out of the guild wall nor in many other places in Salzburg, Carinthia, Styria and Tyrol; at Badersdorf and Sieggraben in Burgenland and in Trattenbach, Aggsbach and in the Waldviertel in Lower Austria; Liebenau in Upper Austria and Montafon in Vorarlberg.

In Germany, epidote found inter alia in the Black Forest in Baden- Württemberg; in the Fichtelgebirge, the Swabian-Franconian Forest, the Bavarian Forest, the Spessart in Bavaria; in the Odenwald and Taunus in Hesse; to Egestorf in Lower Saxony; at Silbach in North Rhine-Westphalia; several places in the Rhineland-Palatinate Eifel; at Nonnweiler Saarland; at several places in the Harz Mountains of Lower Saxony to Saxony -Anhalt; in the Ore Mountains, Upper Lusatia and in Vogtland in Saxony, and in several places in Schleswig -Holstein and Thuringia.

In Switzerland, the mineral so far could be found in the Bernese community Guttannen, Entlebuch LU in Lucerne and at several places in the cantons of Graubünden, Ticino, Uri and Wallis.

More Locations are Afghanistan, Algeria, Egypt, Antarctica, Argentina, Armenia, Azerbaijan, Ethiopia, Australia, Bangladesh, Belgium, Bolivia, Botswana, Brazil, Bulgaria, Burkina Faso, Chile, China, Costa Rica, the Dominican Republic, the Democratic Republic of Congo, Ecuador, Ivory Coast, Fiji, Finland, France, Ghana, Greece, Greenland, Guatemala, Guyana, Haiti, Iceland, India, Indonesia, Iran, Ireland, Italy, Jamaica, Japan, Cambodia, Canada, the channel Island of jersey, Kazakhstan, Kenya, Kyrgyzstan, Colombia, Korea, Korea, Cuba, Lithuania, Madagascar, Malawi, Malaysia, Mali, Morocco, Macedonia, Mexico, Mongolia, Myanmar, Namibia, Nepal, New Caledonia, New Zealand, Nicaragua, Norway, Oman, Pakistan, Panama, Papua New Guinea, Paraguay, Peru, Philippines, Poland, Portugal, Puerto Rico, Rwanda, Romania, Russia, Zambia, Saudi Arabia, Sweden, Senegal, Serbia, Zimbabwe, Slovakia, Slovenia, the Solomon island of Guadalcanal, Spain, Sri Lanka, Sudan, South Africa, Swaziland, Taiwan, Tajikistan, Thailand, other localities in the Czech Republic Bohemia and Moravia, Turkey, Uganda, Ukraine, Uzbekistan, the U.S. Virgin Islands, the United Kingdom ( UK), the United States of America ( USA) and Venezuela.

Even in rock samples of the Mid-Atlantic Ridge and the Central Indian Ridge epidote could be detected.

Crystal structure

Epidote crystallizes in the monoclinic space group P21 / m ( Raumgruppen-Nr. 11) with the lattice parameters a = 8.89 Å; b = 5.63 Å; c = 10.15 Å and β = 115.4 ° and two formula units per unit cell.

Use as a gemstone

Epidote is difficult to process because of its perfect cleavage and is therefore processed fairly rare gemstone. However, Clear varieties, and a gloss -enhancing facet cut can look confusingly similar with yellow-brown color quite a dark citrine or topaz and different with dark green color green gemstones like emerald, green tourmaline ( verdelite ) or diopside.

Heinrich Harrer reported at the crossing of West Papua in 1962 that the Dani prefer in the area around Mulia the green epidote as the best stone for the manufacture of stone axes. At selected points in the quarry fires were kindled and canceled hours later with boulders, rock wedges and bars and brought to safety with wooden tongs.