• journal_title：American Mineralogist
• Contributor：Ferdinando Bosi ; Ulf Hålenius ; Henrik Skogby
• Publisher：Mineralogical Society of America
• Date：2010-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2010.3336
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：95
• issue：4
• firstpage：602
• section：Articles

Single crystals of spinel and hausmannite having seven different compositions in the MgAl₂O₄-MgMn₂O₄-MnMn₂O₄ system were synthesized and structurally and chemically characterized by X-ray diffraction and electron microprobe techniques. As predicted, tetrahedral and octahedral bond lengths increase with increasing substitutions of Mn²⁺ for Mg and Mn³⁺ for Al, respectively. A transition from cubic to tetragonal symmetry occurs at a critical concentration of Mn³⁺ > 1.4 atoms per formula unit as a result of the Jahn-Teller distortion around octahedrally coordinated Mn³⁺.

The present data in conjunction with data from the literature provide a basis for quantitative analyses of the cation polyhedral-distortion parameters and their variations in spinel- and hausmannite-type structures (Fd3̅m and I4₁/amd, respectively). In contrast to the linear correlation between <λ_M> (octahedral quadratic elongation) and σ²_M (octahedral bond-angle variance) observed for many silicates and isomorphic structures, these two distortion parameters are not correlated in multiple oxides with spinel- and hausmannite-type structures. By using a model of multiple linear regression, it is demonstrated that <λ_M> varies as a function of both σ²_M and Δ_M (octahedral bond-length distortion). The degree of octahedral distortion is significant in the spinel structures and is in fact comparable with that calculated for the hausmannite-type structures. The degree of octahedral distortion is related to steric effects in both spinel- and hausmannite-type structures, whereas the electronic effects caused by Mn³⁺ account for the transition from cubic to tetragonal symmetry.