Offretit

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Offretit
Offretite-627123.jpg
Prismatic offretit from Vinařice u Kladna , Okres Kladno , Central Bohemia , Czech Republic (field of view 3 mm)
General and classification
chemical formula KCaMg [Al 5 Si 13 O 36 ] · 15H 2 O
Mineral class
(and possibly department) 		Silicates and Germanates - framework silicates
System no. to Strunz
and to Dana 		9.GD.25 ( 8th edition : VIII / J.26)
77.01.02.04
Crystallographic Data
Crystal system hexagonal
Crystal class ; symbol ditrigonal-dipyramidal; 6 m 2
Room group (no.) P 6 m 2 (No. 187)
Lattice parameters a  = 13.29  Å ; c  = 7.58 Å
Formula units Z  = 1
Physical Properties
Mohs hardness 4th
Density (g / cm 3 ) measured: 2.13; calculated: 2.06
Cleavage clearly after {0001}
Break ; Tenacity uneven; brittle
colour colorless, white
Line color White
transparency transparent to translucent
shine Glass gloss
Crystal optics
Refractive indices n ω  = 1.489 to 1.495
n ε  = 1.486 to 1.492
Birefringence δ = 0.003
Optical character uniaxial negative

Offretite is a rather seldom occurring mineral from the mineral class of "silicates and germanates". It crystallizes in the hexagonal crystal system with the chemical composition KCaMg [Al 5 Si 13 O 36 ] · 15H 2 O and is therefore, chemically speaking, a water-containing potassium - calcium - magnesium - aluminosilicate . Structurally, offretite belongs to the zeolite family within the framework of tectosilicates .

Offretit only develops small hexagonal, prismatic and occasionally longitudinally striped crystals up to about three millimeters in length with a glass-like sheen on the surfaces. Usually these are connected to form radial to spherical mineral aggregates . In its pure form, Offretit is colorless and transparent. However, due to multiple light refraction due to lattice construction defects or polycrystalline formation, it can also appear white, with the transparency decreasing accordingly. Its Mohs hardness of 4 corresponds to that of the reference mineral fluorite , so like this it can be easily scratched with a pocket knife .


Etymology and history

Offretite was first discovered on Mont Semiol near Châtelneuf in the French Loire department and described in 1890 by Ferdinand Gonnard (1833–1923), who named the mineral after the French mineralogist Albert Jules Joseph Offret (1857–1933).

classification

Already in the outdated, but partly still in use 8th edition of the mineral systematics according to Strunz , offretite belonged to the mineral class of "silicates and germanates" and there to the department of "tectosilicates (tectosilicates), with zeolites", where together with bellbergite , chabazite Ca , Chabasit-K , Chabasit-Na , Chabasit-Sr , Erionit-Ca , Erionit-K , Erionit-Na , Gmelinit-Ca , Gmelinit-K , Gmelinit-Na , Lévyn-Ca , Lévyn-Na , Mazzit-Mg , Mazzit-Na , Perlialite , Tschernichit and Willhendersonit the group of "Cube Zeolites I" with the system no. VIII / J.26 .

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 offretite to the category of “tectosilicates with zeolitic H 2 O; Family of zeolites ”. This is, however, further subdivided according to the framework structure, so that the mineral can be found according to its structure in the sub-section “Chains of five rings”, where it forms the unnamed group 9.GD.25 together with Wenkit .

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns offretite to the class of "silicates and germanates" and there in the department of "structural silicates: zeolite group". Here it is in the group " Chabazite and allied species " with the system no. 77.01.02 to be found in the subsection "Real Zeolites".

Education and Locations

Former cavity in basalt from the basalt works near Herbstein , Hesse (field of view 1 cm), completely lined with needle-like offretite crystals

Offretite forms during the weathering of potash-containing basalts and is usually found there in the form of cavity fillings ( drusen ). Furthermore, it can arise together with chabazite through the action of the waters of terrestrial salt lakes on volcanic tuffs .

As a rather rare mineral formation, Offretite can sometimes be abundant at various sites, but overall it is not very common. Around 130 sites are known to date (as of 2014). In addition to its type locality Mont Semiol near Châtelneuf, the mineral also occurred in France at Laveissière , Le Collet-de-Dèze and Le Volamont in the Haute-Loire department (Auvergne); at Deglazines and Le Bousquet d'Orlaguet in the Aveyron department (Midi-Pyrénées) and at Borée and Saint-Jean-le-Centenier in the Ardèche (Rhône-Alpes) department.

In Germany, offretite was found at the Höwenegg quarry near Immendingen and at the Eichert and Limberg quarries near Sasbach in Baden-Württemberg; in some places in the Fichtel Mountains (Großer Teichelberg, Lerchenbühl), on the Zeilberg near Maroldsweisach, in the basalt works near Wiesau and in a basalt quarry on the Kuschberg in Bavaria; in several places in the area around the Vogelsberg (Gedern, Herbstein, Hungen) in Hesse; in the Bramburg quarry near Adelebsen in Lower Saxony and in the Galgenkopf adhesive sand pit near Weitefeld , on the Ettringer Bellerberg near Ettringen , near Arensberg in the Vulkaneifel district, in the Stemmer clay pit near Boden (Westerwald) , on the Bittersberg near Maxsain and on the Ölberg near Hundsangen in Rhineland-Palatinate .

The only known site in Austria so far is a basalt quarry near Klöch in Styria.

Other sites are found in Argentina, Australia, Costa Rica, Finland, Italy, Canada, in the Kerguelen Archipelago (French southern and Antarctic territory), New Zealand, Poland, Spain, Scotland in the United Kingdom, the Czech Republic, Ukraine, Hungary and in different states of the USA.

Crystal structure

Offretite crystallizes hexagonally in the space group P 6 m 2 (space group no. 187) with the lattice parameters a  = 13.29  Å and c  = 7.58 Å as well as one formula unit per unit cell .

See also

literature

Web links

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

Individual evidence

  1. ↑ a b c d e f Hugo Strunz , Ernest H. Nickel: Strunz Mineralogical Tables . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p.  709 .
  2. ↑ Webmineral - offretites
  3. ↑ a b c Offretite . In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (eds.): Handbook of Mineralogy, Mineralogical Society of America , 2001 ( PDF 81.4 kB ).
  4. ↑ a b Mindat - Offretite
  5. ↑ cths.fr: OFFRET Albert, Jules, Joseph
  6. ^ Helmut Schrätze, Karl-Ludwig Weiner: Mineralogie. A textbook on a systematic basis . de Gruyter, Berlin; New York 1981, ISBN 3-11-006823-0 , pp. 918 .
  7. ↑ Mindat - Number of localities for Offretetit
  8. ↑ Find location list for offretite in the Mineralienatlas and in Mindat