Spinels

The unit cell of nickel aluminate. Blue: oxygen, red: nickel, gray: aluminum

Spinels are chemical compounds of the general type AB ₂ X ₄ , wherein A, B metal cations are, whose oxidation number , the sum 8, and X is predominantly a divalent oxygen - or sulfur - anion (the compound in total an oxide or sulfide ) is. Important examples are the namesake of the compound class (MgAl ₂ O ₄ ) and gahnite (ZnAl ₂ O ₄ ) called magnesium spinel or spinel . In normal spinels, all 16 B atoms are on the octahedral lattice positions, in inverse spinels eight B atoms and eight A atoms are coordinated octahedrally.

history

The spinel structure was one of the first crystal structures on which a successful X-ray structure analysis was performed shortly after X-ray diffraction was discovered in 1912 . The spinel structure was elucidated in 1915 by Shoji Nishikawa (1884-1952) using Laue diagrams, and independently by William Henry Bragg . In 1931 it was recognized that the cations are not distributed completely regularly (Makhachki).

Crystal structure

Spinel structure
green Mg ²⁺ , blue Al ³⁺ , red O ²⁻

Many compounds of the type AB ₂ O ₄ crystallize in the crystal structure of the spinel type, which is one of the most important and most common structure types and is also referred to as the spinel structure after the main mineral.

The O ² - ions thereby form a cubic dense crystal lattice whose tetrahedral to one-eighth of mostly dipositive A-ions such as Mg ²⁺ and its octahedral half of most triply positively charged B ions such as Al ³⁺ are occupied . Other charges are also possible, e.g. B. W ⁶⁺ (Na ⁺ ₂ ) O ₄

Inverse spinels also have the formula AB ₂ O ₄ . They are in the same grid. However, the A ions occupy a quarter of the octahedral holes, the B ions a quarter of the octahedral and one eighth of the tetrahedral holes. This means that half of the octahedron gaps and one eighth of the tetrahedron gaps are again occupied. Examples are magnetite Fe ₃ O ₄ (= Fe (III) ₂ Fe (II)) and TiMg ₂ O ₄ .

calculation

It is possible to predict whether a spinel is normal or inverse. The ligand field stabilization energy (LFSE) in normal spinel is compared with the LFSE in inverse spinel.

example

FeCr ₂ O ₄ :

• Fe ²⁺ :
  • Tetrahedral gap : In the tetrahedral ligand field, the 3 t ₂ orbitals are raised by 4 Dq and the 2 e orbitals are lowered by 6 Dq. These are filled with 6 electrons (Fe ²⁺ is ad ⁶ ion). This includes the LFSE

${\displaystyle 3\cdot 6\,\mathrm {Dq} -3\cdot 4\,\mathrm {Dq} =6\,\mathrm {Dq} =0,6\,\Delta _{\mathrm {T} }}$.

because , ${\displaystyle \Delta _{\mathrm {T} }={\tfrac {4}{9}}\cdot \Delta _{\mathrm {O} }}$

this corresponds to an LFSE of

${\displaystyle \mathrm {LFSE} ={\tfrac {4}{9}}\cdot 0,6\,\Delta _{\mathrm {T} }=0,266\,\Delta _{\mathrm {O} }}$.

• Octahedral gap : In the octahedral ligand field the 2 e _g orbitals are raised by 6 Dq and the 3 t _2g orbitals are lowered by 4 Dq. These are filled with 6 electrons in the high-spin arrangement. This includes the LFSE

${\displaystyle \mathrm {LFSE} =4\cdot 4\,\mathrm {Dq} -2\cdot 6\,\mathrm {Dq} =4\ \mathrm {Dq} =0,4\Delta _{\mathrm {O} }}$.

• Cr ³⁺ :

• Tetrahedral gap: Cr ³⁺ is ad ³ ion. This includes the LFSE

${\displaystyle 2\cdot 6\,\mathrm {Dq} -1\cdot 4\,\mathrm {Dq} =8\,\mathrm {Dq} =0,8\,\Delta _{\mathrm {T} }}$.

because , ${\displaystyle \Delta _{\mathrm {T} }={\tfrac {4}{9}}\cdot \Delta _{\mathrm {O} }}$

this corresponds to an LFSE of

${\displaystyle \mathrm {LFSE} ={\tfrac {4}{9}}\cdot 0,8\,\Delta _{\mathrm {O} }=0,356\,\Delta _{\mathrm {O} }}$.

• Octahedral gap: The LFSE is included

${\displaystyle \mathrm {LFSE} =3\cdot 4\,\mathrm {Dq} -0\cdot 6\,\mathrm {Dq} =12\,\mathrm {Dq} =1,2\,\Delta _{\mathrm {O} }}$.

Normal spinel (Fe ^T Cr ^O Cr ^O O ₄ ): ${\displaystyle \mathrm {LFSE} =0,266\,\Delta _{\mathrm {O} }+2\cdot 1,2\,\Delta _{\mathrm {O} }=2,666\,\Delta _{\mathrm {O} }}$

Inverse spinel (Fe ^O Cr ^T Cr ^O O ₄ ): ${\displaystyle \mathrm {LFSE} =0,4\,\Delta _{\mathrm {O} }+0,356\,\Delta _{\mathrm {O} }+1,2\,\Delta _{\mathrm {O} }=1,956\,\Delta _{\mathrm {O} }}$

The normal spinel thus has a higher ligand field stabilization energy. FeCr ₂ O ₄ exists as normal spinel.

Occurrence

Spinels are extremely important geologically. Many minerals crystallize in the spinel structure, including oxides, sulfides, selenides and silicates. The current spinel supergroup, newly defined by the IMA / CNMNC, currently has 56 minerals (as of 2018). It is believed that the spinel ringwoodite forms a larger part of the earth's mantle .

Minerals and varieties of the spinel supergroup

Franklinite

Hercynite

Red and blue spinel

The following minerals currently belong to the spinel supergroup recognized by the International Mineralogical Association (IMA), which are divided into subgroups according to their composition according to Ferdinando Bosi, Cristian Biagioni and Marco Pasero (as of 2019):

• Oxispinelle
  • Spinel subgroup
    • Chromite Fe ²⁺ Cr ₂ O ₄ ( chrome iron stone )
    • Cochromit CoCr ₂ O ₄
    • Coulsonite Fe ²⁺ V ³⁺ ₂ O ₄
    • Cuprospinell CuFe ³⁺ ₂ O ₄
    • Dellagiustait V ²⁺ Al ₂ O ₄
    • Delta lumite (Al _0.67 ☐ _0.33 ) Al ₂ O ₄
    • Franklinit ZnFe ³⁺ ₂ O ₄ ( zinc ferrite )
    • Gahnit ZnAl ₂ O ₄ ( zinc spinel )
    • Galaxit MnAl ₂ O ₄ ( manganese spinel )
    • Guit Co ²⁺ Co ³⁺ ₂ O ₄
    • Hausmannite Mn ²⁺ Mn ³⁺ ₂ O ₄
    • Hercynite FeAl ₂ O ₄ ( Ferrospinell )
      • Picotite , a variety of Hercynite containing magnesium and chromium, (Fe ²⁺ , Mg) (Al, Cr ³⁺ ) ₂ O ₄
    • Hetaerolite ZnMn ³⁺ ₂ O ₄
    • Jakobsit Mn ²⁺ Fe ³⁺ ₂ O ₄
    • Maghemite (Fe ³⁺ _0.67 ☐ _0.33 ) Fe ³⁺ ₂ O ₄
    • Magnesiochromite MgCr ₂ O ₄
    • Magnesiocoulsonite MgV ₂ O ₄
    • Magnesioferrite MgFe ³⁺ ₂ O ₄ ( Magnoferrite , Magnoferrite )
    • Magnetite Fe ²⁺ (Fe ³⁺ ) ₂ O ₄ ( magnetic iron stone )
      • Titanomagnetite , mixed crystal of the series Magnetite – Ulvöspinell with Fe ²⁺ (Fe ³⁺ , Ti) ₂ O ₄
    • Manganochromite Mn ²⁺ Cr ₂ O ₄
    • Spinel MgAl ₂ O ₄ ( magnesia spinel )
      • Pleonast (Mg, Fe ²⁺ ) (Al, Fe ³⁺ ) O ₄ , ferrous variety of spinel
    • Thermal aerogenite CuAl ₂ O ₄
    • Titanomaghemite (Ti _0.5 ☐ _0.5 ) Fe ³⁺ ₂ O ₄
    • Trevorite NiFe ³⁺ ₂ O ₄
    • Vuorelainenite Mn ²⁺ V ³⁺ ₂ O ₄
    • Zincochromit ZnCr ₂ O ₄
  • Ulvöspinell subgroup
    • Ahrensite Fe ₂ (SiO ₄ )
    • Brunogeierite Fe ²⁺ ₂ Ge ⁴⁺ O ₄
    • Filipstadit (Mn ₂₊ , Mg) ₂ (Sb ⁵⁺ , Fe ³⁺ ) O ₄
    • Qandilite (Mg, Fe ₃₊ ) ₂ (Ti, Fe ³⁺ , Al) O ₄
    • Ringwoodite Mg ₂ (SiO ₄ ) or SiMg ₂ O ₄
    • Tegengrenite (Mn ³⁺ _0.5 Sb ⁵⁺ _0.5 ) Mg ₂ O ₄
    • Ulvöspinell ( Ulvit ) Fe ²⁺ ₂ TiO ₄

• Selenospinels
  • Bornhardtite subgroup
    • Bornhardtite Co ²⁺ Co ³⁺ ₂ Se ₄
    • Trüstedtit Ni ²⁺ Ni ³⁺ ₂ Se ₄
  • Tyrrellite subgroup
    • Tyrrellite Cu (Co, Ni) ₂ Se ₄

• Thiospinels
  • Carrollite subgroup
    • Carrollit CuCo ₂ S ₄
    • Cuproiridsite CuIr ₂ S ₄
    • Cuprocalininite CuCr ₂ S ₄
    • Fletcherit CuNi ₂ S ₄
    • Florensovite Cu (Cr _1.5 Sb _0.5 ) S ₄
    • Malanite Cu ¹⁺ (Ir ³⁺ Pt ⁴⁺ ) S ₄
    • Rhodostannite Cu ¹⁺ (Fe ²⁺ _0.5 Sn ⁴⁺ _1.5 ) S ₄
    • Toyohait Ag ¹⁺ (Fe ²⁺ _0.5 Sn ⁴⁺ _1.5 ) S ₄

Linneity (silvery octahedron) in magnetite matrix

• • Linneit subgroup
    • Cadmoonite CdIn ₂ S ₄
    • Cuprorhodsite (Cu ¹⁺ _0.5 Fe ³⁺ _0.5 ) Rh ³⁺ ₂ S ₄
    • Daubréelith FeCr ₂ S ₄
    • Greigit Fe ²⁺ Fe ³⁺ ₂ S ₄
    • Indit FeIn ₂ S ₄
    • Joegoldsteinite MnCr ₂ S ₄
    • Kalininit ZnCr ₂ S ₄
    • Linneit Co ²⁺ Co ³⁺ ₂ S ₄
    • Polydymit Ni ²⁺ Ni ³⁺ ₂ S ₄
    • Siegenit CoNi ₂ S ₄
    • Violarit FeNi ₂ S ₄
    • Xingzhongit Pb ²⁺ Ir ³⁺ ₂ S ₄

The nichromite described by De Waal in 1978 with the formula NiCr ₂ O ₄ would also be classified among the Oxis spinels. The first description and the chosen name were published without examination by the CNMNC , so nichromite has not yet been recognized as an independent mineral type.

Syntheses

The synthesis of spinels is often achieved by coprecipitation . For example, the chlorides of the respective metal are first dissolved, precipitated as hydroxides and then burned.

The following synthetically produced spinels are known so far:

• the pigment Thénards Blau , a cobalt aluminate with the formula CoAl ₂ O ₄
• Cobalt black , also cobalt (II, III) oxide with the formula Co ²⁺ Co ³⁺ ₂ O ₄ , which is an intermediate product in the production of metallic cobalt
• Zinc cobalt spinel , also zinc cobaltite with the formula ZnCo ₂ O _4, is green-black. Rinman's green has a similar composition and was erroneously often referred to as spinel, but it is a mixed oxide with the composition ZnO * (CoO) * x (x = 5%).

use

The cobalt spinel CoAl ₂ O ₄ cobalt aluminate (Thénards Blue) is known as a color pigment in industry and in classical analytical chemistry as a detection reagent . Other spinels are also used as pigments resistant to light, weather and chemicals . See also mixed-phase oxide pigments . Colored or black spinels are also used as gemstones, especially those of the actual spinel . Iron spinels are used as photocatalysts, cobalt chromite as a catalyst in the breakdown of pollutants.

Magnetite is one of the most important iron ores. It serves as a pigment and has been used in magnetic data storage. Similar to yttrium-iron-garnet , magnetite and related spinels are also used as ferrites in ferrite cores (also suitable for microwaves ). However, they have higher losses.

literature

• Will Kleber , Joachim Bohm , Hans-Joachim Bautsch : Introduction to crystallography . 18th edition. Oldenbourg Wissenschaftsverlag, 1998, ISBN 978-3-486-27319-9 ; P. 160 .
• Paperback of high frequency technology, Lange K. + Locher KH, Springer-Verlag, ISBN 3-540-54715-0 , p. L38 (Spinelle)

Web links

• Steven Dutch: Spinel Structure. Natural and Applied Sciences, University of Wisconsin - Green Bay, September 22, 1997, accessed May 4, 2018 . 
• Mineral Atlas : Spinel Supergroup

Individual evidence

1. ↑ Entry on spinels. In: Römpp Online . Georg Thieme Verlag, accessed on January 10, 2017.
2. ↑ Shoji Nishikawa : Structure of some crystals of the spinel group . In: Proceedings of the Tokyo Mathematico-Physical Society . tape  8 , 1915, p. 199-209 .
3. ^ William Henry Bragg : XXX. The structure of the spinel group of crystals . In: Philosophical Magazine Series 6 . tape  30 , no. 176 , 1915, pp. 305-315 , doi : 10.1080 / 14786440808635400 .
4. ↑ R. Gross, translation and explanation of "WH Bragg: Structure of the spinel group of crystals", in: New Yearbook for Mineralogie, Geology and Paläontologie, 1st volume 1917 online
5. ^ ^{A b} Cristian Biagioni, Marco Pasero: The systematics of the spinel-type minerals: An overview . In: American Mineralogist . tape 99 , no. 7 , 2014, p. 1254–1264 , doi : 10.2138 / am.2014.4816 ( preliminary version online [PDF]).
6. ↑ ^{a b} 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).
7. ↑ ^{a b c d e f} 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 16, 2019 .  
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. ↑ Mindat - Picotite
10. ↑ Kreidezeit Naturfarben GmbH - Pigments, product information (PDF 159 kB)
11. ↑ Christian Suchomski: Structural, optical and magnetic properties of nanocrystalline metal oxide thin films with mesoporous morphology. Inaugural dissertation to obtain the academic degree "Doctor rerum naturalium" . Physico-Chemical Institute of the Justus Liebig University Giessen, Giessen March 2012, p. 7 ( uni-giessen.de [PDF; 11.4 MB ] for use see page 7, chapter 2.1 Chromium- and iron-based spinels).