Gahnite

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Gahnite
Gahnite-542888.jpg
Dark green gahnite from the Rosendal pegmatites, Kimito , southwest Finland (field of view 15 mm)
General and classification
other names
  • Automolit
  • Fahlunit
  • Zinc spinel
chemical formula ZnAl 2 O 4
Mineral class
(and possibly department) 		Oxides and hydroxides
System no. to Strunz
and to Dana 		4.BB.05 ( 8th edition : IV / B.01a)
02/07/01/04
Crystallographic Data
Crystal system cubic
Crystal class ; symbol cubic hexakisoctahedral; 4 / m  3  2 / m
Space group Fd 3 m (No. 227)Template: room group / 227
Lattice parameters a  = 8.09  Å
Formula units Z  = 8
Frequent crystal faces {111}, {110}, {211}, occasionally {100}
Twinning according to the spinel law (111)
Physical Properties
Mohs hardness 7.5 to 8
Density (g / cm 3 ) measured: 4.38 to 4.60; calculated: 4.607
Cleavage indistinct; Secretions after {111}
Break ; Tenacity shell-like to uneven; brittle
colour blue-green, yellow-brown, greenish-black
Line color gray-white
transparency translucent to opaque
shine greasy glass gloss
Crystal optics
Refractive index n  = 1.79 to 1.80
Birefringence none, as it is optically isotropic

Gahnite , also known as zinc spinel or automolite , is a rarely occurring mineral from the group of spinels within the mineral class of "oxides and hydroxides" with the chemical composition ZnAl 2 O 4 . From a chemical point of view, the mineral is therefore a zinc - aluminum - oxide or zinc aluminate .

Gahnite crystallizes in the cubic crystal system and develops mostly octahedral , cube-shaped or rhombic dodecahedral crystals and twins up to about 12 cm in size. It can also be found in the form of granular to massive mineral aggregates .

The mineral is translucent to opaque and has a greasy glass sheen on the surfaces of the blue-green, yellow-brown or greenish-black crystals .

Gahnite forms mixed series with spinel (MgAl 2 O 4 ) and hercynite (Fe 2+ Al 2 O 4 ).

Etymology and history

Johan Gottlieb Gahn (1745-1818)

The mineral was first discovered in the Eric Matts mine near Falun (also Fahlun ) in Sweden. The first description was in 1807 by Carl Erenbert Freiherr von Moll after a chemical analysis by Anders Gustaf Ekeberg . The latter suggested naming the newly discovered mineral automolit after the Greek word αὐτόμολος [automolos] for defector . It should be an allusion to the unexpected zinc content, "... because this fossil, despite the similarity to the spinel, approaches the ores because of the zinc content." (Ekeberg, after Scherer). However, von Moll disliked these “figurative names” in natural history, so he chose the name Gahnite in honor of the Swedish chemist Johan Gottlieb Gahn .

The synonymous name zinc spinel has a rather descriptive function, as it contains zinc instead of magnesium, in contrast to (magnesio) spinel . Various modifications of this designation are also known. This is what René-Just Haüy called the mineral Spinelle Zincifère and August Breithaupt Zinc Spinel . By Dietrich Ludwig Gustav Karsten the other synonyms is also Fahl Unit handed.

Type material for the mineral is not defined. Since its discovery and first description took place before the founding of the International Mineralogical Association (IMA) in 1959, the scientific recognition of gahnite as an independent mineral species was taken over by the IMA under the identifier grandfathered .

classification

The current classification of the International Mineralogical Association (IMA) counts gahnite to the spinel supergroup , where together with cochromite , coulsonite , cuprospinel , dellagiustaite , delta lumite , franklinite , chromite , galaxite , guit , hausmannite , hercynite , hetaerolite , jakobsite , maghemite , Magnesiochromite , magnesiocoulsonite , magnesioferrite , magnetite , manganochromite , spinel , thermoerogenite , titanomaghemite , trevorite , vuorelainenite and zincochromite form the spinel subgroup within the oxispinelle.

In the outdated, but still in use 8th edition of the mineral classification according to Strunz , the chromite belonged to the mineral class of "oxides and hydroxides" and there to the department of "compounds with M 3 O 4 - and related compounds", where it, together with galaxite, hercynite and spinel the group of "aluminum spinels" with the system no. IV / B.01a .

The 9th edition of Strunz's mineral systematics , which has been in effect since 2001 and was updated by the IMA until 2009, also classifies Gahnit in the oxides division with a substance ratio of "metal: oxygen = 3: 4 and comparable". This, however, is further divided according to the relative size of the participating cations , so that the mineral according to its composition in the subdivision to find "With only medium-sized cations" where it along with Brunogeierit , chromite, Cochromit, Coulsonit, Cuprospinell, Filipstadit , Franklinite , Galaxite, hercynite, jacobsite, magnesiochromite, magnesiocoulsonite, magnesioferrite, magnetite, manganochromite, nichromite (N), qandilite , spinel, trevorite, ulvöspinell , vuorelainenite and zinc chromite the "spinel group" with the system no. 4.BB.05 forms.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns chromite to the class of "oxides and hydroxides" and there in the section "multiple oxides". Here it is together with Galaxit, Hercynit and Spinell in the " aluminum subgroup " with the system no. 07.02.01 to be found in the subsection “Multiple oxides (A + B 2+ ) 2 X 4 , spinel group”.

Crystal structure

Gahnite crystallizes cubically in the structure of spinel with the space group Fd 3 m (space group no. 227) and the lattice parameter a  = 8.09  Å as well as eight formula units per unit cell . Template: room group / 227

properties

In front of the soldering tube , gahnite is infusible and is not attacked by either acids or alkalis.

Modifications and varieties

As Kreittonit (from Greek : κρειττωυ for more), a gahnite variety designated by their higher iron content higher in relation to some other spinels density has.

Education and Locations

Dark green gahnite crystal (size 6 mm) in matrix from the Näverberg mining area, Sweden
Very rare, transparent-green formation of gahnite from Carnaúba dos Dantas, mineral province Borborema, Rio Grande do Norte , Brazil (size 42 mm × 38 mm × 5 mm)
Octahedral gahnite crystal twin from Amity (City of Warwick), Orange County , New York (size 3.0 cm × 2.8 cm × 2.5 cm)

Gahnit forms the one hand as an accessory component in igneous rocks such as granites and granite pegmatite , but can on the other hand regionalmetamorph from the sphalerite moiety in medium to highly metamorphosed sulfide - deposits arise. In low-grade metamorphosed bauxites Gahnit can also share from its mixture Diaspor arise. Gahnite is also found as a detritic mineral in soap deposits.

As a rather rare mineral formation, gahnite can sometimes be abundant at different sites, but overall it is not very common. So far, more than 400 sites for Gahnit have been documented (as of 2018). Depending on the location, gahnite can occur in paragenesis with andradite , calcite , chalcopyrite , cordierite , franklinite , hogbomite , corundum , nigerite , phlogopite , pyrite , pyrrhotite , rhodonite , staurolite and / or willemite .

Besides its type locality was in Fa (h) lun the mineral nor in other places in the province of Dalarna be found, such as lying in close Näverberg field and the Skyttgruben ( Skyttgruvan ) and in several mines in Garpenberg in the municipality Hedemora in Leksand and in the Öster Silvberg mining district in the municipality of Säter . Other well-known sites in Sweden include the metamorphosed manganese-iron deposit Långban in Värmland, the zinc pits ( Zinkgruvan ) near Åmmeberg in the province of Örebro län and in the silver pits near Fröderyd in the province of Jönköpings län .

In Germany, gahnite is known so far in the form of inclusions in quartz and violet fluorite from the Clara mine near Oberwolfach in Baden-Württemberg; Silberberg in Bodenmais , from the graphite mines in Kropfmühl and marble - quarry Wimhof in Vilshofen and from the metamorphosed sulphide deposit Midsummer mine in enamel (Lam) in the district of Cham in Bavaria; from the copper slag heaps of the metal works near Frankfurt-Heddernheim in Hesse and from the Genna zinc works near Letmathe in North Rhine-Westphalia.

In Austria, the mineral was found, among other things, as a by- product of phosphorite nodules, which occur in the soil near Falkenstein Castle in the Weinviertel of Lower Austria; in the pegmatite quarry Steiningerbruch in the municipality of Luftenberg on the Danube and the kassiterite-containing pegmatites near Meitschenhof (municipality of Pregarten ) in Upper Austria; in the Kaiserer and Lohninger quarries in the Hüttwinkltal ( Raurisertal ) in Salzburg, which were formerly quarried on slab gneiss , but now only carry scree ; in the nephelinite - basalts on Stradner Kogel near Wilhelmsdorf (town of Bad Gleichenberg) in Styria and grown into the shales of Rotbachlspitze on Pfitscherjoch and on the Grawandalm in Zemmgrund in North Tyrol .

In Switzerland, gahnite has so far only been found in the pegmatites in the Valle di Ponte ( Pontetal ) near Brissago TI in the canton of Ticino and in granular aggregates from the Lammaschlucht gorge near Fürgangen and in the metabauxites of the Brunegghorn in the canton of Valais.

Also known for their extraordinary gahnite finds are Franklin and Sterling Hill in Sussex County of the US state New Jersey, where crystals up to 12 cm in size are exposed. Gahnite crystals up to 3 cm in size are known from Broken Hill in the Australian state of New South Wales.

Other locations include Argentina, Australia, Brazil, China, Canada, Namibia, Norway, Russia and the Czech Republic.

In addition, gahnite can be produced synthetically during zinc production in muffle furnaces .

use

Cut gahnite, 1.34 ct, from Argentina

As an ore, gahnite is of no economic importance.

Even if gahnite belongs to the group of spinels known as gemstones and also occurs in clear and transparent varieties, it is rarely used as such. Occasionally it is cut into a facet shape by experienced collectors .

See also

literature

  • Carl Erenbert Freiherrn von Moll: Efemeriden of the mountain and metallurgy . Steinische Buchhandlung, Nuremberg 1807, p. 78–80 ( rruff.info [PDF; 1.6 MB ; accessed on August 21, 2018] Section Gahnit, from Fahlun ).
  • H. Saalfeld: structural data from Gahnite, ZnAl 2 O 4 . In: Journal of Crystallography . tape 120 , 1964, pp. 476–478 ( rruff.info [PDF; 114 kB ; accessed on August 25, 2018]).
  • Friedrich Klockmann : Klockmann's textbook of mineralogy . Ed .: Paul Ramdohr , Hugo Strunz . 16th edition. Enke, Stuttgart 1978, ISBN 3-432-82986-8 , pp. 503 (first edition: 1891).

Web links

Commons : Gahnite  - collection of images, videos and audio files
Wiktionary: Gahnit  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ^ A b Carl Erenbert Freiherrn von Moll: Efemeriden der Berg- und Metallkunde . Steinische Buchhandlung, Nuremberg 1807, p. 78–80 ( rruff.info [PDF; 1.6 MB ; accessed on August 21, 2018] Section Gahnit, from Fahlun ).
  2. ↑ a b Dietrich Ludwig Gustav Karsten : Mineralogical tables with consideration of the latest discoveries worked out and provided with explanatory notes . 8th, improved and increased edition. Rottmann, Berlin 1808, p.  102 ( limited preview in Google Book search).
  3. ↑ 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.  503 (first edition: 1891).
  4. ↑ a b c d 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.  188 (English).
  5. ↑ Webmineral - Gahnite (English)
  6. ^ A b Helmut Schrätze , Karl-Ludwig Weiner : Mineralogie. A textbook on a systematic basis . de Gruyter, Berlin; New York 1981, ISBN 3-11-006823-0 .
  7. ↑ a b c d e Gahnite . 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; 70  kB ; accessed on August 20, 2018]).
  8. ↑ a b Stefan Weiß: The large Lapis mineral directory. All minerals from A - Z and their properties . 6th completely revised and supplemented edition. Weise, Munich 2014, ISBN 978-3-921656-80-8 .
  9. ↑ Alexander Nicolaus Scherer (Ed.): General Nordic Annals of Chemistry for the Friends of Natural History and Medicinal Science, especially Pharmacy, Drug Science, Physiology, Physics, Mineralogy and Technology in the Russian Empire . tape  2 . Publisher's publishing house, St. Petersburg 1819, p. 48 ( limited preview in Google Book search).
  10. ↑ Hans Lüschen: The names of the stones. The mineral kingdom in the mirror of language . 2nd Edition. Ott Verlag, Thun 1979, ISBN 3-7225-6265-1 , p. 347 .
  11. ↑ IMA - Catalog of Type Mineral Specimens: S ( Memento from August 26, 2016 in the Internet Archive ) (English, PDF)
  12. ^ IMA / CNMNC List of Mineral Names; March 2018 ( Memento from June 11, 2018 in the Internet Archive ) (PDF 1.65 MB)
  13. ↑ Ferdinando Bosi, Cristian Biagioni, Marco Pasero: Nomenclature and classification of the spinel supergroup . In: European Journal of Mineralogy . tape 31 , no. 1 , September 12, 2018, p. 183–192 , doi : 10.1127 / ejm / 2019 / 0031-2788 (English).
  14. ↑ Ernest H. Nickel, Monte C. Nichols: IMA / CNMNC List of Minerals 2009. (PDF 1703 kB) In: cnmnc.main.jp. IMA / CNMNC, January 2009, accessed June 25, 2019 .
  15. ↑ Mindat - Kreittonite
  16. ↑ Mindat - Number of localities for Gahnit
  17. ↑ Petr Korbel, Milan Novák: Mineral Encyclopedia (=  Dörfler Natur ). Edition Dörfler im Nebel-Verlag, Eggolsheim 2002, ISBN 978-3-89555-076-8 , p. 76 .
  18. ↑ Find location list for chromite in the Mineralienatlas and Mindat
  19. ↑ Hans Jürgen Rösler : Textbook of Mineralogy . 4th revised and expanded edition. German publishing house for basic industry (VEB), Leipzig 1987, ISBN 3-342-00288-3 .
  20. ^ 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. 116, 220 .