Prehnite

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Prehnite
Prehnite-Quartz-117742.jpg
Leafy prehnite (bluish yellow) on quartz (colorless) from Imilchil in the Atlas Mountains , Morocco (size: 10 × 6.1 × 4.1 cm)
General and classification
chemical formula Ca 2 Al [(OH) 2 | AlSi 3 O 10 ]
Mineral class
(and possibly department) 		Silicates and germanates - chain and band silicates (inosilicates)
System no. to Strunz
and to Dana 		9.DP.20 ( 8th edition : VIII / G.07)
72.01.03.01
Crystallographic Data
Crystal system orthorhombic
Crystal class ; symbol orthorhombic-pyramidal; mm 2
Space group P 2 cm (No. 28, position 5)Template: room group / 28.5
Lattice parameters a  = 4.65  Å ; b  = 5.48 Å; c  = 18.49 Å
Formula units Z  = 2
Twinning fine lamellar
Physical Properties
Mohs hardness 6.0 to 6.5
Density (g / cm 3 ) measured: 2.80 to 2.95; calculated: [2.90]
Cleavage good after {001}; indistinct after {110}
Break ; Tenacity uneven
colour colorless, white, gray, yellowish, pink, light to dark green
Line color White
transparency transparent to translucent
shine Glass gloss, faint pearlescent gloss
Crystal optics
Refractive indices n α  = 1.611 to 1.632
n β  = 1.615 to 1.642
n γ  = 1.632 to 1.665
Birefringence δ = 0.021 to 0.033
Optical character biaxial positive
Axis angle 2V = 64 to 70 ° (measured); 58 to 68 ° (calculated)

Prehnite , also known as aedelite , chiltonite , coupholite and edelite or under the trade names Cape Chrysolite and Cape Smaragd , is a common mineral from the mineral class of " silicates and germanates " with the chemical composition Ca 2 Al [(OH) 2 | AlSi 3 O 10 ]. Prehnit thus chemically seen a is calcium - aluminum silicate with additional hydroxide .

The mineral crystallizes in the orthorhombic crystal system and develops mostly tabular to prismatic crystals , but also occurs in the form of leafy, fan-shaped or grape to spherical mineral aggregates . In its pure form, prehnite is colorless and transparent. However, due to multiple refraction due to lattice construction defects or polycrystalline formation, it can also appear white and take on a gray or yellowish to greenish color due to foreign admixtures. Prehnite cat eyes are also known.

Etymology and history

Reddish prehnite needles on xonotlite from the “N'Chwaning II Mine” near Kuruman , South Africa

Prehnite is the first known mineral that is named after a person. The Dutch baron and Colonel Hendrik von Prehn (1733–1785) brought some samples from the Cape Province (Cape of Good Hope) with him in 1783 and had them analyzed by Abraham Gottlob Werner . He recognized the material as a new type of mineral and named it after the person who found it.

The dolerite deposit of Karoo near Cradock in the Eastern Cape Province of South Africa is now considered the exact type locality .

classification

Already in the outdated, but still in use 8th edition of the mineral classification according to Strunz , the prehnite belonged to the mineral class of "silicates and germanates" and there to the department of "transition structures between chain and layered silicates", where together with Bavenit it was named "Prehnit-Bavenit -Group "with the system no. VIII / G.07 and the other members Amstallit , Bavenit, Chiavennit , Rudenkoit and Tvedalit .

The 9th edition of Strunz's mineral systematics , which has been in effect since 2001 and is used by the International Mineralogical Association (IMA), initially classifies prehnite in the category of "chain and band silicates (inosilicates)". This is, however, further subdivided according to the structure of the chains or bands, so that the mineral can be found according to its structure in the sub-section “Transitional structures chain and band silicates - sheet silicates”, where it is the only member of the unnamed group 9.DP.20 forms.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns prehnite to the class of "silicates and Germanates" and there to the department of "layered silicates: two-dimensional, unlimited layers with rings other than six-membered". Here he is to be found as the only member of the unnamed group 72.01.03 within the sub-section " Layered silicates: two-dimensional unlimited layers with rings other than six-membered: 4-membered rings".

Crystal structure

Water-clear, ideally orthorhombic-pyramidal prehnite with a pearlescent surface

Prehnite crystallizes orthorhombically in the space group P 2 cm (space group no. 28, position 5) with the lattice parameters a  = 4.65  Å ; b  = 5.48 Å and c  = 18.49 Å as well as two formula units per unit cell . Template: room group / 28.5

properties

Prehnitz twins according to (100) are pyroelectric in the direction of the a-axis. In front of the soldering tube , prehnite melts easily and flakes or puffs up. Only at high temperatures does water also form.

Prehnite is relatively insensitive to acids and is only completely destroyed by them after glowing, but it reacts very sensitively to even the slightest influence of heat.

Occasionally, prehnite shows blue-white or peach-colored fluorescence with short - wave UV light and yellow with long-wave UV light .

Education and Locations

Globular prehnite on epidote

Prehnite arises either secondarily from other minerals or through hydrothermal processes in fissures, passages and geodes within igneous or metamorphic rocks (e.g. gabbro , diabase , melaphyre , crystalline slate). Prehnite is often found in paragenesis with apophyllite , calcite , datolite , epidote , solid copper , pectolite and various zeolites . He also forms pseudomorphoses according to analcime , laumontite and natrolite, among others .

As a frequent mineral formation, prehnite can be found in many localities. Worldwide, around 1400 sites (as of 2011) are known to date. In Germany the mineral could be found especially in Bavaria, more precisely in the Black Forest , Fichtel Mountains , Spessart , Bavarian Forest , in Swabia and the Upper Palatinate ; in the Hessian Lahn-Dill-Kreis and Odenwald ; in the Harz (Lower Saxony, Saxony-Anhalt); in the Sauerland (North Rhine-Westphalia); in the Rhineland-Palatinate Hunsrück , Niederkirchen and Wolfstein ; in the Ore Mountains and Upper Lusatia in Saxony; near Elmshorn and Lübeck in Schleswig-Holstein as well as in the Thuringian Forest and near Wurzbach .

In Austria, prehnite was found in many regions of Carinthia , Lower Austria , Salzburg , Styria , Tyrol , Upper Austria and Vorarlberg . In Switzerland , prehnite was found in several regions of the cantons of Graubünden , Ticino , Uri and Valais .

Other locations are Egypt , Antarctica , Argentina , many regions in Australia , Belgium , Bolivia , Brazil , Bulgaria , Chile , China , Costa Rica , Ecuador , Finland , several regions in France , Greece , Greenland , India , Indonesia , Ireland , Iceland , Israel , many regions in Italy , Japan , several regions of Canada , the Channel Islands , Kazakhstan , Kenya , Colombia , Croatia , Madagascar , Mali , Morocco , Mexico , Mongolia , Namibia , New Zealand , North Korea , many regions in Norway , Oman , Pakistan , Papua New Guinea , Peru , Philippines , Poland , Portugal , Réunion , some regions in Russia , several regions in Sweden , Slovakia , some regions in Spain , several regions in South Africa , South Korea , Tanzania , Trinidad and Tobago , several Bohemian and Moravian regions in Czech Republic, Turkey , Ukraine , Hungary , the United States Virgin Islands (American Virgin Islands), many regions of the United Kingdom (Großbr Italy) and many regions in the United States of America (USA).

Prehnite could also be detected in rock samples of the Mid-Atlantic Ridge , more precisely the “Markov Depth” within the Sierra Leone rupture zone, as well as the Indian Ocean, more precisely the Central Indian Ridge .

Use as a gem stone

Prehnite in the so-called " emerald cut"

So far, prehnite has only been processed into gemstones , although not very often, and offered in different types and shapes of cuts depending on the quality . Due to the similarity in color, there is a risk of confusion with apatite , brazilianite , chrysoprase , jade , peridot , periclase and serpentine .

See also

literature

  • Mineral system of Mr. Inspector Werner with his permission . In: Alexander Willhelm Köhler, CAS Hoffmann (Ed.): Bergmannisches Journal . Crazische Buchhandlung, Freiberg 1789, p. 369–398 ( rruff.info [PDF; 1.9 MB ; accessed on July 28, 2017]).
  • Petr Korbel, Milan Novák: Mineral Encyclopedia (=  Villager Nature ). Nebel Verlag, Eggolsheim 2002, ISBN 978-3-89555-076-8 , p. 248 .

Web links

Commons : Prehnite  - collection of images, videos and audio files

Individual evidence

  1. ↑ 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.  654 .
  2. ↑ a b Webmineral - Prehnite (English)
  3. ↑ a b c d Prehnite . 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; 69  kB ]).
  4. ↑ a b c d e Mindat - Prehnite (English)
  5. ^ Mineral Atlas: Prehnite
  6. ↑ Name search. Trade names and what they mean. EPI - Institute for Gemstone Testing, accessed on April 4, 2018 (entry of Cape Chrysolite or Cape Smaragd required).
  7. ^ A b Walter Schumann: Precious stones and gemstones. All kinds and varieties. 1900 unique pieces . 16th revised edition. BLV Verlag, Munich 2014, ISBN 978-3-8354-1171-5 , pp. 204 .
  8. ↑ Thomas Witzke : naming the minerals, after people at www.strahl.org
  9. ^ Albert Huntington Chester: A dictionary of the names of minerals including their history and etymology . 1st edition. John Wiley & Sons, New York 1896, p. 217 ( available online at archive.org  - Internet Archive ).
  10. ↑ Mindat - Mineral List of the Karoo dolerites locality, Cradock District, Eastern Cape Province, South Africa
  11. ↑ a b c Friedrich Klockmann : Klockmanns textbook of mineralogy . Ed .: Paul Ramdohr , Hugo Strunz . 16th edition. Enke, Stuttgart 1978, ISBN 3-432-82986-8 , pp.  737-738 (first edition: 1891).
  12. ^ Gemstone etiquette from Prof. Leopold Rössler: Prehnit
  13. ↑ Mindat - Number of localities for prehnite
  14. ↑ a b List of localities for prehnite in the Mineralienatlas and Mindat