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

¹ Dipartimento di Geoscienze, Università di Padova, Via Giotto 1, 35137 Padova, Italy
² I.G.G. Unità di Padova, CNR, I-35137 Padova, Italy
³ Dipartimento di Scienze Mineralogiche e Petrologiche, Università di Torino, Via Valperga Caluso 35, 10125 Torino, Italy
⁴ Dipartimento di Scienze della Terra, Università di Trieste, Via Weiss 8, 34127 Trieste, Italy

^* E-mail: fabrizio.nestola{at}unipd.it

The effect of cation vacancies upon the thermal expansion and crystal structure of a synthetic defect spinel with composition Mg_0.4Al_2.40.2O₄ was investigated by X-ray diffraction, in situ, at temperatures up to 1273 K. No evidence of symmetry violations from the Fd_m even at 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 and T-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 ų and the tetrahedron by 0.056 ų. 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 in the Earth's mantle where 4-5 wt.% Al₂O₃ is thought to be present.

KEYWORDS: spinel, single crystal, high temperature, non-stoichiometry