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The effect of non-stoichiometry on the high-temperature behaviour of MgAl2O4 spinel

Published online by Cambridge University Press:  05 July 2018

F. Nestola ,
L. Secco ,
M. Bruno ,
M. Prencipe ,
F. Martignago ,
F. Princivalle  and
A. Dal Negro
F. Nestola*
Affiliation:
Dipartimento di Geoscienze, Universitá di Padova, Via Giotto 1, 35137 Padova, Italy I.G.G. Unitá di Padova, CNR, I-35137 Padova, Italy
L. Secco
Affiliation:
Dipartimento di Geoscienze, Universitá di Padova, Via Giotto 1, 35137 Padova, Italy
M. Bruno
Affiliation:
Dipartimento di Scienze Mineralogiche e Petrologiche, Universitá di Torino, Via Valperga Caluso 35, 10125 Torino, Italy
M. Prencipe
Affiliation:
Dipartimento di Scienze Mineralogiche e Petrologiche, Universitá di Torino, Via Valperga Caluso 35, 10125 Torino, Italy
F. Martignago
Affiliation:
Dipartimento di Geoscienze, Universitá di Padova, Via Giotto 1, 35137 Padova, Italy
F. Princivalle
Affiliation:
Dipartimento di Scienze della Terra, Universitá di Trieste, Via Weiss 8, 34127 Trieste, Italy
A. Dal Negro
Affiliation:
Dipartimento di Geoscienze, Universitá di Padova, Via Giotto 1, 35137 Padova, Italy
*
* E-mail: fabrizio.nestola@unipd.it
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Abstract

The effect of cation vacancies upon the thermal expansion and crystal structure of a synthetic defect spinel with composition Mg0.4Al2.4☐0.2O4 was investigated by X-ray diffraction, in situ, at temperatures up to 1273 K. No evidence of symmetry violations from the Fd3m evenat the highest temperature were noted. The volume thermal expansion is markedly less than that of stoichiometric MgAl2O4 spinel, regardless of the degree of inversion. The u oxygen atomic coordinate remains constant throughout the temperature range investigated, with the M–O an dT–O bond lengths showing identical rates of expansivities. An analysis of the evolution of polyhedral volumes with temperature indicates that at 1273 K the octahedron inflates by 0.099 Å3 and the tetrahedron by 0.056 Å3. The expansion of the octahedron is significantly greater than in stoichiometric MgAl2O4 spinel, whereas the expansion of the tetrahedron is similar. The results demonstrate that an excess of Al in the spinel structure accompanied by the formation of cation vacancies strongly affect an important thermodynamic property, in this case, thermal expansion. Such an effect must be considered for those phases stable inthe Earth’s mantle where 4–5 wt.% Al2O3 is thought to be present.

Keywords

spinelsingle crystalhigh temperaturenon-stoichiometry.
Type
Research Article
Information
Mineralogical Magazine , Volume 73 , Issue 2 , April 2009 , pp. 301 - 306
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2009

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