Cochromite

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Cochromite
General and classification
other names

IMA 1978-049

chemical formula
  • CoCr 2 O 4
  • (Co, Ni, Fe 2+ ) (Cr 3+ , Al) 2 O 4
Mineral class
(and possibly department) 		Oxides and hydroxides
System no. to Strunz
and to Dana 		4.BB.05
07.02.03.05
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  = 8.29  Å
Formula units Z  = 8
Physical Properties
Mohs hardness 7 to 7.5 ( VHN 50 = 1218)
Density (g / cm 3 ) calculated: ≈ 4.99
Cleavage indistinct
Break ; Tenacity shell-like
colour black
Line color greenish gray
transparency opaque
shine Metallic luster

Cochromite is a very seldom occurring mineral from the mineral class of "oxides and hydroxides" with the idealized chemical composition CoCr 2 O 4 and thus, chemically speaking, a cobalt - chromium - oxide . However, since part of the cobalt in natural cochromite samples has been replaced ( substituted ) by nickel and / or iron and part of the chromium by aluminum , the formula is also used in various sources with (Co, Ni, Fe 2+ ) (Cr 3+ , Al) 2 O 4 given.

Cochromite crystallizes in the cubic crystal system in the structure of spinel , but has so far only been found in the form of irregular grains up to about 20 μm in size. The mineral is in any form opaque ( opaque ) and displays on the surfaces of black grains have a metallic luster . Cochromite leaves a greenish-gray line on the marking board .

Etymology and history

Was first discovered Cochromit in a small nickel - deposit in the area of Farm Bon Accord north of Barberton in the South African province of Mpumalanga . It was first described in 1978 by SA de Waal, who named the mineral based on its cobalt content and its chemical relationship to chromite .

Type material for this mineral is not defined.

classification

The current classification of the International Mineralogical Association (IMA) counts the cochromite to the spinel supergroup , where together with chromite , coulsonite , cuprospinel , dellagiustaite , deltalumite , franklinite , gahnite , 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.

Since the cochromite was only discovered in 1978 and recognized as an independent mineral, it is not yet listed in the outdated 8th edition of the mineral classification according to Strunz . Only in the Lapis mineral directory , which was revised and updated in 2018 by Stefan Weiß, which is still based on this classic system from Karl Hugo Strunz out of consideration for private collectors and institutional collections , the mineral received the system and mineral number. IV / B.03-40 . In the "Lapis system" this corresponds to the class of "oxides and hydroxides" and there the section "Oxides with a metal: oxygen ratio = 3: 4 (spinel type M 3 O 4 and related compounds)", where cochromite together with chromite, Magnesiochromite, manganochromite, nichromite and zincochromite form the group of "chromite spinels".

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 cochromite 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, Coulsonit, Cuprospinell, Filipstadit , Franklinite, Gahnit , 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 the cochromite to the class of "oxides and hydroxides" and there in the "multiple oxides" section. Here it is in the " chrome subgroup " with system no. 07.02.03 can be found in the subsection “Multiple oxides (A + B 2+ ) 2 X 4 , spinel group”.

Chemism

The microprobe analysis of the cochromite samples from the Bon Accord type locality in South Africa showed a composition of 50.38% Cr 2 O 3 , 17.45% CoO , 9.11% Al 2 O 3 , 7.67% NiO , 7.45% FeO , 4.14% Fe 2 O 3 and 1.26% TiO 2 . In addition, traces of MgO (0.95%), MnO (0.84%), ZnO (0.59%) and SiO 2 (0.11%) were found. Based on 4 oxygen atoms, the empirical formula was calculated as (Co 0.50 Ni 0.22 Fe 2+ 0.22 Mg 0.05 Mn 0.03 Zn 0.02 ) Σ = 1.04 (Cr 1.43 Al 0.38 Fe 3+ 0.11 Ti 0.03 ) Σ = 1.95 O 4 .

Another sample from Bo Phloi in Thailand contained fewer foreign admixtures , but in addition to 22.38% CoO and 11.57% Cr 2 O 3 , a significant proportion of Al 2 O 3 of 49.61% and 9.55% FeO , 6.04% MgO and 0.67% TiO 2 . The empirical formula is calculated with this result as (Co 0.52 Mg 0.26 Fe 2+ 0.23 ) Σ = 1.01 (Al 1.70 Cr 0.27 Ti 0.02 ) Σ = 1.99 O 4 . The sample from Thailand has a higher aluminum than chromium content, which contradicts the defined and recognized formula for cochromite. The reason for this can be the fact that the sample was found as inclusion in corundum (Al 2 O 3 ).

Crystal structure

Cochromite crystallizes cubically in the spinel structure with the space group Fd 3 m (space group no. 227) with the lattice parameter a  = 8.29  Å and 8 formula units per unit cell . Template: room group / 227

Education and Locations

Cochromite formed in the small, tabular nickel deposit Bon Accord by displacement of chromite in the contact zone between quartzite and serpentinized ultramafites . It is assumed that the formation conditions of the thermal metamorphosis are at a temperature of about 730 ° C and a pressure of less than 2  kbar .

So far, only two sites for cochromite have been documented worldwide (as of 2018). In addition to its type locality in South Africa, the mineral was also found in the rock samples from the Mutnowski volcano on the Russian peninsula of Kamchatka in the Far East Federal District.

See also

literature

  • SA de Waal: Nickel minerals from Barberton, South Africa: III. Willemseite, a nickel-rich talc . In: The American Mineralogist . tape 55 , no. 1-2 , 1970, pp. 31–42 ( minsocam.org [PDF; 662 kB ; accessed on June 28, 2019]).
  • P. García Casado, I. Rasines: Preparation and crystal data of the spinel series Co 1 + 2s Cr 2 −3 s Sb s O 4 (O ⩽ s ⩽ 23) . In: Polyhedron . tape 5 , no. 3 , 1986, pp. 787-789 , doi : 10.1016 / S0277-5387 (00) 84438-1 .

Web links

Individual evidence

  1. ↑ a b Malcolm Back, William D. Birch, Michel Blondieau and others: The New IMA List of Minerals - A Work in Progress - Updated: March 2019. (PDF 1703 kB) In: cnmnc.main.jp. IMA / CNMNC, Marco Pasero, March 2019, accessed June 28, 2019 .
  2. ↑ 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.  189 (English).
  3. ↑ a b c d e Stefan Weiss: The great 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 .
  4. ↑ a b c d e f g h i j Cochromite . 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; 71  kB ; accessed on June 28, 2019]).
  5. ^ Cochromites. In: mindat.org. Hudson Institute of Mineralogy, accessed June 28, 2019 .
  6. ↑ David Barthelmy: Cochromite Mineral Data. In: webmineral.com. Retrieved June 28, 2019 .
  7. ↑ 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).
  8. ↑ 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 28, 2019 .
  9. ↑ Find location list for cochromite in the Mineralienatlas and Mindat ( accessed on June 28, 2019).