Epidote
| Epidote | |
|---|---|
 (size: 9.2 × 8.1 × 5.6 cm) |
|
| General and classification | |
| chemical formula | Ca 2 (Fe 3+ , Al) Al 2 [O | OH | SiO 4 | Si 2 O 7 ] |
|
Mineral class (and possibly department) |
Silicates and Germanates - group silicates |
|
System no. to Strunz and to Dana |
9.BG.05 ( 8th edition : VIII / C.23) 58.02.01a.02 |
| Similar minerals | Allanite , diopside , dravite , clinozoisite , piedmontite , vesuvianite , zoisite |
| Crystallographic Data | |
| Crystal system | monoclinic |
| Crystal class ; symbol | monoclinic prismatic; 2 / m |
| Space group | P 2 1 / m (No. 11) |
| Lattice parameters |
a = 8.89 Å ; b = 5.63 Å; c = 10.15 Å β = 115.4 ° |
| Formula units | Z = 2 |
| Frequent crystal faces | (100), (001), (10 1 ), (111) |
| Twinning | lamellar after (100) |
| Physical Properties | |
| Mohs hardness | 6 to 7 |
| Density (g / cm 3 ) | measured: 3.38 to 3.49; calculated: 3.391 to 3.464 |
| Cleavage | perfect after {001}, imperfect after {100} |
| Break ; Tenacity | shell-like uneven |
| colour | variable, often dark green, yellow-brown |
| Line color | White |
| transparency | transparent to opaque |
| shine | Glass gloss |
| Crystal optics | |
| Refractive indices |
n α = 1.715 to 1.751 n β = 1.725 to 1.784 n γ = 1.734 to 1.797 |
| Birefringence | δ = 0.019 to 0.046 |
| Optical character | biaxial negative |
| Axis angle | 2V = measured: 90 ° to 116 °; calculated: 62 ° to 84 ° |
| Pleochroism | strong: green E. almost colorless-yellow-green-yellow- brown green-brown E. green-brown-yellow Some Tawmawites: strong green- strong red in the ideal angle with a sharp transition, further deviating from this yellow-green - red-brown |
Epidote is a frequently occurring mineral from the mineral class of " silicates and germanates". It crystallizes in the monoclinic crystal system with the general chemical composition Ca 2 (Fe 3+ , Al) Al 2 [O | OH | SiO 4 | Si 2 O 7 ]. The brackets in the chemical formula mean that although iron and aluminum can represent each other to a limited extent ( substitution ), they are always in the same proportion to the other components. Epidote is also the eponymous representative of the epidote group , a group of chemically similar minerals.
Epidote usually forms prismatic crystals up to 35 cm long, which can be parallel [010] stretched and striped and, depending on their purity, are transparent to opaque. In addition, the mineral also occurs in the form of fibrous, granular to massive and radial-radial aggregates . The color of epidote is variable, but often dark green, sometimes yellow-brown, while the stroke color is white. With a Mohs hardness between 6 and 7, depending on the purity of the mineral, Epidote is one of the medium-hard to hard minerals. At least a steel file is required to scratch it, but it is able to scratch simple window glass itself .
Epidote and clinozoisite (Ca 2 AlAl 2 [O | OH | SiO 4 | Si 2 O 7 ]) form a continuous mixed crystal row .
Etymology and history
Epidote was first discovered in 1782 at Le Bourg-d'Oisans in the French department of Isère. However, the samples found there were initially incorrectly referred to as tourmaline. Only the French mineralogist René-Just Haüy recognized in 1801 that this material was an independent mineral, which he named after the Greek word epidosis for "addition", "increase" or "expansion", as an allusion to the crystal shape characteristic of epidotes, in which one of the sides on the underside of the prism is longer than the other.
classification
In the meanwhile outdated, but still in use 8th edition of the mineral classification according to Strunz , the epidote belonged to the mineral class of "silicates and germanates" and there to the department of " group silicates (sorosilicates)", where together with allanite (Ce) , allanite (La) , Allanit- (Y) , Dissakisit- (Ce) , Dissakisit- (La) , Dollaseit- (Ce) , epidote (Pb) , Ferriallanit- (Ce) , Gatelit- (Ce) , Khristovit- (Ce ) , Clinozoisite , clinozoisite (Sr) , manganiandrosite (Ce) , manganiandrosite (La) , tweddillite , mukhinite , piedmontite , piemontite (Sr) , uedaite (Ce) , vanadoandrosite (Ce) , västmanlandite (Ce ) and Zoisite formed the "epidote group" named after him.
The 9th edition of Strunz's mineral systematics , which has been in effect since 2001 and is used by the International Mineralogical Association (IMA), also assigns the epidote to the class of "silicates and germanates" and there in the department of "group silicates (sorosilicates)". This department is, however, further subdivided according to the type of group formation of the silicate complexes and the coordination of the cations , so that the mineral is classified according to its composition and structure in the subdivision of “group silicates with mixed SiO 4 and Si 2 O 7 groups; Cations in octahedral [6] and greater coordination ”can be found, where the“ epidote group ”with the system no. 9.BG.05 and the other members Allanit- (Ce), Allanit- (La), Allanit- (Y), Dissakisit- (Ce), Dissakisit- (La), Dollaseit- (Ce), Epidote- (Pb) , Epidote- (Sr) , Ferriallanite- (Ce), Khristovite- (Ce), Klinozoisite- (Sr), Klinozoisite, Manganiandrosite- (Ce), Manganiandrosite- (La), Tweddillite, Mukhinite, Piemontite- (Sr), Piedmontite , Uedaite (Ce), vanadoandrosite (Ce) and vanadoepidote .
The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns the epidote to the class of "silicates and Germanates", but there in the more finely divided division of "group silicates: insular, mixed, individual and larger tetrahedral groups". Here he is also the namesake of the "epidote group (clinozoisite subgroup)" with the system no. 58.02.01a and the other members clinozoisite, epidote (Pb), mukhinite, piemontite, piemontite (Sr), tweddillite, gatelite (Ce), clinozoisite (Sr), uedaite (Ce) and epidote (Sr) within the subdivision of “ group silicates: insular, mixed, single and larger tetrahedral groups with cations in [6] and higher coordination; Single and double groups (n = 1,2) ”.
Crystal structure
Epidote crystallizes monoclinically in the space group P 2 1 / m (space group no. 11) with the lattice parameters a = 8.89 Å ; b = 5.63 Å; c = 10.15 Å and β = 115.4 ° and 2 formula units per unit cell .
Modifications and varieties
So far, the following varieties of the epidote are known:
- Allanite-Epidot (also Epidot-Orthit) - mixed crystal of allanites and epidote.
- Beryllium epidote and chromium epidote contain the respective metal mentioned as admixtures.
- Pistacite or pistacite - microcrystalline, colored green ( pistachio !) To black-green by adding iron .
- Pushkinite - green, red and yellow, a variety from the Urals .
- Tawmawit - strong green, light green to light yellow, brown to red due to the addition of chrome, special colors, named after its type locality Tawmaw . The colors can change depending on the direction due to strong pleochroism and thus all can be seen in one piece.
- Withamite - opaque, contains manganese and therefore pink to red in color.
Allanite (with the appendices - (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 separate minerals; Allanit, however, only in connection with the attached elements to distinguish the end links of this mixed series .
Education and Locations
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Epidote forms as a common component of metamorphic rocks , especially those that are formed from igneous rocks such as basalt , e.g. B. amphibolite derived. It is also more common in marble (metamorphic limestone ). In addition, it occurs in hydrothermal veins as a decomposition product of other silicate minerals. Accompanying minerals include various amphiboles , garnets , plagioclase , pyroxenes and zeolites as well as actinolite , albite , calcite , glaucophane , lawsonite , omphacite , prehnite , pumpellyite , quartz , riebeckite , scapolite , talc , vesuvianite and wollastonite .
Epidote becomes unstable with increasing temperature and disintegrates at approx. 650 to 700 ° C. In the rocks of the amphibolite facies this results, after a complex series of reactions, in a change in the rock composition from hornblende, albite, epidote and quartz to hornblende, plagioclase and quartz. The anorthite content therefore increases with increasing temperature.
In total, epidote has so far (as of 2011) been found at around 5600 sites. Sobotín in the Czech Republic, where up to 14 cm long, green crystals emerged, is worth mentioning due to the excellent mineral finds. Crystals about 10 cm long were found on the Knappenwand in Untersulzbachtal , Austria . According to older reports from the 19th century , much larger (50 to 70 cm) crystals are said to have been found on the Knappenwand, but this information cannot be proven by relevant evidence.
In Austria, besides the Knappenwand, the mineral was found in many other places in Salzburg , Carinthia , Styria and Tyrol ; near Badersdorf and Sieggraben in Burgenland as well as near Trattenbach , Aggsbach and in the Waldviertel in Lower Austria; Liebenau in Upper Austria and Montafon in Vorarlberg.
In Germany, epidote is found in the Black Forest in Baden-Württemberg; in the Fichtel Mountains , Swabian-Franconian Forest , Bavarian Forest and in the Spessart in Bavaria; in the Odenwald and Taunus in Hesse; near Egestorf in Lower Saxony; near Silbach in North Rhine-Westphalia; in several places in the Rhineland-Palatinate Eifel ; near Nonnweiler in Saarland; in several places in the Harz Mountains from Lower Saxony to Saxony-Anhalt; in the Ore Mountains , Upper Lusatia and Vogtland in Saxony as well as in several places in Schleswig-Holstein and Thuringia . Particularly rich occurrences are known in the area of the upper hollow , which were especially enriched by leaching.
In Switzerland, the mineral has so far been found in the Bernese municipality of Guttannen , Entlebuch LU in Lucerne and in several places in the cantons of Graubünden , Ticino , Uri and Valais .
Other locations are Afghanistan , Egypt , Algeria , the Antarctic , Argentina , Armenia , Azerbaijan , Ethiopia , Australia , Bangladesh , Belgium , Bolivia , Botswana , Brazil , Bulgaria , Burkina Faso , Chile , China , Costa Rica , the Dominican Republic , the Democratic Republic Republic of the 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 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 , the Philippines , Poland , Portugal , Puerto Rico , Rwanda , Romania , Russia , Zambia , Saudi Arabia , Sweden , Senega l , Serbia , Zimbabwe , Slovakia , Slovenia , the Solomon Islands Guadalcanal , Spain , Sri Lanka , Sudan , South Africa , Swaziland , Taiwan , Tajikistan , Thailand , other sites in Czech Bohemia and Moravia , Turkey , Uganda , Ukraine , Hungary , Uzbekistan , the US Virgin Islands , the United Kingdom (Great Britain), the United States of America (USA), and Venezuela .
Epidote was also found in rock samples from the Mid-Atlantic Ridge and the Central Indian Ridge .
Use as a gem stone
Because it can be completely split, epidote is difficult to work with and is therefore rarely made into gemstone . Clear varieties and a shine-enhancing facet cut can, however, look confusingly similar to a dark citrine or topaz with a yellow-brown color and various green gemstones such as emerald , green tourmaline ( verdelite ) or diopside with dark green color .
Heinrich Harrer reports when crossing West Papua in 1962 that the Dani in the Mulia area prefer the green epidote as the best stone for making stone axes . Fires were lit in selected places in the quarry and, hours later, rocks were broken off with rubble stones, wedges and poles and brought to safety with wooden tongs.
See also
literature
- Heinrich Harrer: I come from the Stone Age . Ullstein, Frankfurt / Main 1963, ISBN 3-524-00331-1
Web links
- Mineralienatlas: Epidote and Mineralienatlas: Mineralienportrait / Epidot (Wiki)
- Webmineral - Epidote (English)
- realgems.org - Epidote (with examples of polished epidotes)
Individual evidence
- ↑ a b c d e Hugo Strunz , Ernest H. Nickel : Strunz Mineralogical Tables. Chemical-structural Mineral Classification System . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p. 585 .
- ↑ a b c epidotes . In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America . 2001 ( handbookofmineralogy.org [PDF; 79 kB ]).
- ↑ a b c d Mindat - Epidote (English)
- ↑ a b c d e f Mineralienatlas: Mineralienportrait / Epidot (Wiki)
- ^ IMA / CNMNC List of Minerals; 2009 (PDF, English; 1.8 MB)
- ↑ Petr Korbel, Milan Novák: Mineral Encyclopedia (= Dörfler Natur ). Nebel Verlag, Eggolsheim 2002, ISBN 978-3-89555-076-8 , p. 214 .
- ^ Lothar Eißmann : The Quaternary Ice Age in Saxony and Northeast Thuringia: Landscape change on the southern edge of the Scandinavian glaciation area . Altenburg: Mauritianum, Frankfurt / Leipzig 1997, p. 98 .
- ↑ a b List of locations for the epidote in the Mineralienatlas and Mindat