Magnesiochromite

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Magnesiochromite
Magnesiochromite-474053.jpg
Magnesiochromite (black) in matrix from the Mistake Mine, Butler Estate Chrome Deposit, Wright Mountain , Diablo Range , Fresno County, California (size 3.2 × 2.5 cm)
General and classification
other names
  • Magnochromite
  • Mitchellite
  • Picrochromite
chemical formula MgCr 3+ 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.01c)
02/07/03/01
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  = 8.36  Å
Formula units Z  = 8
Frequent crystal faces {111}, {100}
Twinning according to the spinel law {111}
Physical Properties
Mohs hardness 5.5
Density (g / cm 3 ) measured: 4.39 to 4.67; calculated: 4.414
Cleavage absent, secretions after {111} possible
Break ; Tenacity uneven to slightly scalloped; brittle
colour deep red to black
Line color dark gray or brown
transparency opaque (opaque); edges translucent
shine Metallic luster
magnetism weakly magnetic
Crystal optics
Refractive index n  = 1.96
Birefringence none, as it is optically isotropic

Magnesiochromite is a rather seldom occurring mineral from the mineral class of "oxides and hydroxides" with the idealized chemical composition MgCr 3+ 2 O 4 and therefore, chemically speaking, a magnesium - chromium - oxide .

Magnesiochromite crystallizes in the cubic crystal system and is mostly found in the form of granular to massive mineral aggregates . It seldom develops octahedral crystals and twins with poorly formed crystal faces up to about 1.5 mm in size.

The mineral is generally opaque, but translucent on thin edges or corners. The surfaces of the deep red to black crystals and aggregates show a metallic sheen . In contrast to the surface color , the line color of magnesiochromite is dark gray to brown.

Etymology and history

The mineral was first discovered near Grochau in the powiat Ząbkowicki (Frankenstein district) in the Polish Voivodeship Lower Silesia and described in 1868 by Georg Max Bock (* 1843) in his dissertation on some Silesian minerals, their constitution and some other analytical results in which he wrote it initially referred to as magnochromite . Alfred Lacroix changed the name in 1910 in his work Minéralogie de la France et de ses colonies to the name magnesiochromite, which is still valid today.

Type material for magnesiochromite is not defined.

classification

The current classification of the International Mineralogical Association (IMA) counts magnesiochromite to the spinel supergroup , where together with chromite , cochromite , coulsonite , cuprospinel , dellagiustaite , deltalumite , franklinite , gahnite , galaxite , guit , hausmannite , hercynite , hetaerolite , jakobsite , maghemite , Magnesiocoulsonit , Magnesioferrit , magnetite , Manganochromit , spinel , Thermaerogenit , Titanomaghemit Trevorit , Vuorelainenit and Zincochromit forming the spinel subgroup within the Oxispinelle.

In the outdated 8th edition of the mineral systematics according to Strunz , the magnesiochromite 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 together with chromite, magnesiochromite and manganochromite and with the now discredited members Chromohercynit and picotite the group of "chrome spinels" with the system no. IV / B.01c .

In the last revised and updated Lapis mineral directory by Stefan Weiß in 2018 , which, out of consideration for private collectors and institutional collections, is still based on this classic system of Karl Hugo Strunz , the mineral was given the system and mineral number. IV / B.03-10 . In the "Lapis system" this also corresponds to the department "Oxides with a metal to oxygen ratio = 3: 4 (spinel type M 3 O 4 and related compounds)", where magnesiochromite together with chromite, cochromite, manganochromite, nichromite and zinc chromite form the group which forms "chromite spinels".

The 9th edition of Strunz's mineral systematics , which has been valid since 2001 and updated by the IMA until 2009, also classifies magnesiochromite 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 , is 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, Gahnite, galaxite, hercynite, jacobsite, magnesiocoulsonite, magnesioferrite, magnetite, manganochromite, nichromite (N), qandilite , spinel, trevorite, ulvöspinell , vuorelainenite and zincochromite 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 magnesiochromite to the class of "oxides and hydroxides" and there in the section "multiple oxides". Here it is together with chromite, cochromite, manganochromite, nichromite and zinc chromite in the " chrome subgroup " with the system no. 07.02.03 can be found in the subsection “Multiple oxides (A + B 2+ ) 2 X 4 , spinel group”.

Chemism

The idealized chemical composition of magnesiochromite (MgCr 3+ 2 O 4 ) contains 12.64% by weight magnesium (Mg), 54.08% by weight chromium (Cr) and 33.28% by weight oxygen (O) . In the oxide form, this corresponds to 20.96% by weight of MgO and 79.04% by weight of Cr 2 O 3 .

Magnesiochromite, however, forms a mixed crystal row with chromite (Fe 2+ Cr 2 O 4 ) and spinel (MgAl 2 O 4 ), which is why part of the magnesium in natural magnesiochromites is usually replaced by iron and part of the chromium by aluminum diadoch ( substituted ) . Smaller amounts of manganese, titanium, vanadium and nickel were also measured in various mineral samples .

Crystal structure

Magnesiochromite crystallizes isostructurally ( isotype ) with chromite and magnetite in the cubic crystal system in the space group Fd 3 m (space group no. 227) with the lattice parameter a  = 8.33  Å and 8 formula units per unit cell . Template: room group / 227

Education and Locations

Magnesiochromite forms as an accessory component in ultramafic rocks such as dunites , serpentinites , kimberlites , lamproites and komatiites . Occasionally it is also found in the form of sprinkles in lamprophyren and mid-ocean basalts . As accompanying minerals may include pyroxene , magnetite , pigeonite and various olivine and plagioclase occur.

As a rather rare mineral formation, magnesiochromite can sometimes be abundant at various sites, but overall it is not very common. Around 300 sites have been documented so far (as of 2018).

In Germany, magnesiochromite was found only at a few sites such as the Clara mine near Oberwolfach in Baden-Württemberg, on the slag heaps of the zinc smelter in Genna near Letmathe in North Rhine-Westphalia and near Schwarzenberg in the Saxon Ore Mountains. In addition, magnesiochromite could be detected as a component of the Erxleben meteorite , which fell in 1812 near the town of the same name in the Börde district of Saxony-Anhalt.

In Austria we know the mineral so far only from a basalt - quarry in Kloch and out of the magnesite mine Breitenau am Hochlantsch in Styria and from a slag heap of Montanwerke Brixlegg in North Tyrol.

The only known site in Switzerland so far is the Val de Moiry in the canton of Valais, more precisely a site under the glacier floor of the Moiry glacier with serpentine rock and the so-called Pointe du Tsaté with rodingite dykes and alpinotypes, metamorphic corridors in serpentinite.

Other locations include Australia, China, France, India, Italy, Japan, Canada, Russia, Slovakia and the USA.

See also

literature

  • GM Bock: About some Silesian minerals, their constitution and some other analytical results . Royal and University Library, Wroclaw November 1868 (Philosophical dissertation).
  • M. Websky : About Grochauite and Magnochromite . In: Journal of the German Geological Society . tape 25 , 1873, pp. 394–398 ( rruff.info [PDF; 264 kB ; accessed on September 23, 2019]).
  • A. Lacroix : Minéralogie de la France et de ses colonies . tape 4 . Librairie Polytechnique, Ch.Beranger, Paris 1910, p. 311–315 (French, available online at archive.org  - Internet Archive [accessed September 23, 2019]).
  • LJ Spencer : A (sixth) list of new mineral names . In: Mineralogical Magazine . tape 16 , 1913, pp. 352–378 (English, rruff.info [PDF; 1,2 MB ; accessed on 23 September 2019] Magnesiochromite from p. 394).
  • Haraldur Sigurðsson , J.-G. Schilling: Spinels in Mid-Atlantic Ridge basalts: Chemistry and occurrence . In: Earth and Planetary Science Letters . tape 29 , no. 1 , 1976, p. 7-20 , doi : 10.1016 / 0012-821X (76) 90021-2 (English).
  • HSC O'Neill, WA Dollase: Crystal structures and cation distributions in simple spinels from Powder XRD structural refinements: MgCr 2 O 4 , ZnCr 2 O 4 , Fe 3 O 4 and the temperature dependence of the cation distribution in ZnAl 2 O 4 . In: Physics and Chemistry of Minerals . tape 20 , 1994, p. 541–555 (English, rruff.info [PDF; 552 kB ; accessed on September 23, 2019]).
  • Davide Lenaz, Francesco Prinicivalle: The crystal chemistry of detrital chromian spinel from the southeastern Alps and outer Dinarides: The discrimination of supplies from areas of similar tectonic setting? In: The Canadian Mineralogist . tape 43 , 2005, p. 1305–1314 (English, rruff.info [PDF; 985 kB ; accessed on September 23, 2019]).

Web links

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

Individual evidence

  1. ↑ a b M. Websky : About Grochauite and Magnochromite . In: Journal of the German Geological Society . tape  25 , 1873, pp. 394–398 ( rruff.info [PDF; accessed August 28, 2018]).
  2. ↑ Mineral Atlas: Magnesiochromite
  3. ↑ 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 , p.  390 .
  4. ↑ 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.  189 (English).
  5. ↑ a b c d e f g h i j k Magnesiochromite . In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America . 2001 (English, handbookofmineralogy.org [PDF; 70  kB ; accessed on August 28, 2018]).
  6. ↑ a b c Webmineral - Magnesiochromite (English)
  7. ↑ a b c Helmut Schrätze , Karl-Ludwig Weiner : Mineralogie. A textbook on a systematic basis . de Gruyter, Berlin; New York 1981, ISBN 3-11-006823-0 , pp.  375 .
  8. ^ Karl Pretzsch : Directory of the Breslau university publications 1811–1885 . Georg Olms Verlag, Hildesheim / New York 1975, ISBN 3-487-05573-2 , p.  216 ( limited preview in Google Book Search - reprint of 1905 edition).
  9. ^ A. Lacroix : Minéralogie de la France et de ses colonies . tape  4 . Librairie Polytechnique, Ch.Beranger, Paris 1910, p. 311–315 (French, available online at archive.org  - Internet Archive [accessed September 23, 2019]).
  10. ↑ Catalog of Type Mineral Specimens - M. (PDF 124 kB) In: docs.wixstatic.com. Commission on Museums (IMA), December 12, 2018, accessed September 23, 2019 .
  11. ↑ 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).
  12. ↑ Stefan Weiß: The large Lapis mineral directory. All minerals from A - Z and their properties. Status 03/2018 . 7th, completely revised and supplemented edition. Weise, Munich 2018, ISBN 978-3-921656-83-9 .
  13. ↑ Ernest H. Nickel, Monte C. Nichols: IMA / CNMNC List of Minerals 2009. (PDF 1703 kB) In: cnmnc.main.jp. IMA / CNMNC, January 2009, accessed September 23, 2009 .
  14. ↑ Localities for Magnesiochromite. In: mindat.org. Hudson Institute of Mineralogy, accessed September 23, 2019 .
  15. ↑ Meteorite Erxleben. In: lpi.usra.edu. Meteoritical Bulletin Database, accessed September 23, 2019 .
  16. ↑ List of locations for magnesiochromite in the Mineralienatlas and Mindat , accessed on September 23, 2019.