Spinel - MgAl2O4
Spinel is the name given to the mineral MgAl2O4. It has a common structural arrangement shared by many oxides of the transition metals with formula AB2O4.
In the normal pattern the oxygens (red) form a cubic close packed (face centred) array and the Mg(II) (A(II)-green) and Al(III) (B(III)-silver) sit in tetrahedral (1/8 occupied) and octahedral (1/2 occupied) sites in the lattice, giving a Unit Cell with 8 Mg's, 16 Al's and 32 O's.
An inverse spinel is an alternative arrangement where half of the trivalent ions swap with the divalent ions so that the M(II) now occupy octahedral sites ie B(AB)O4.

Highlight the ccp (ABCABC) layers of O's.
Show 1st layer of O's Show 2nd layer of O's
Show 3rd layer of O's Show 4th layer of O's
Show 3rd and 4th layer Show 1st and 4th layer
Highlight a Mg site Highlight Al site (s)
Highlight an O site Ball and stick

<a href="spinel.pdb"></a>

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Information is available to show how the individual structural pages were created.
The display shows an idealised unit cell (Space group 227 Fd-3m) with origin 2 chosen. The cubic cell lengths used were a=b=c=8.0832A. This analysis follows the determination by Bragg and uses a centrosymmetric space group Fd3m. An alternative to this, which appears to have been largely overlooked, was described by Grimes et. al. and uses the Space Group F-43m. Both show approximately cubic closed packed arrays of oxygens.
One method of predicting whether a structure will be normal or inverse is to calculate Octahedral Site Preference Energies for both the A(II) and B(III). The ion with the more stable energy is predicted to fit into the octahedral site.
However, see J. Am. Chem. Soc., 1982, 104, 92-95 for a different method that does NOT involve OSPE calculations.
For alternate views of spinels,
pictures of spinel layers

and check out the following sites; Univ. of Iowa, Wellesley College MoleCVUE project, and the University of Le Mans.
Information is available to show how the individual structural pages were created.
Return to the structure of binary salts introductory page.
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Copyright © 2002 by Robert John Lancashire, all rights reserved.

Created and maintained by Dr. Robert J. Lancashire,
The Department of Chemistry, University of the West Indies,
Mona Campus, Kingston 7, Jamaica.
Created March 2000. Links checked and/or last modified 8th July 2002.
URL http://wwwchem.uwimona.edu.jm:1104/courses/spinel.html