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Raman spectra of ilmenite-type phases on the Mg₄Si₄O₁₂-Mg₃Al₂Si₃O₁₂ join

¹ Laboratoire de Sciences de la Terre, CNRS-UMR 5570, Ecole Normale Supérieure de Lyon, F-69364 Lyon Cedex 07, France
² Department of Chemistry, Gakushuin University, 1–5–1 Mejiro, Toshima-ku, Tokyo 171–8588, Japan

Corresponding author, e-mail: Bruno.Reynard{at}ens-lyon.fr

Raman spectra of ilmenite-type phases on the Mg₄Si₄O₁₂-Mg₃Al₂Si₃O₁₂ join have been collected. With increasing Al content, bandwidths increase while band positions remain nearly constant, consistent with the occurrence of disorder in the solid solution. However, at least eight active Raman bands are observed up to the highest Al content, which is inconsistent with the seven bands expected for an Rc corundum-type structure. It implies that Al, Mg and Si are ordered at short range distance and that the occurrence of a fully disordered corundum-type phase is unlikely within the narrow P-T range of stability of the ilmenite-structure phases in this system, unless ordering occurs during temperature quenching as is the case in Mg-rich majorites. The present Raman spectra also provides a basis for identification of silicate ilmenites in shocked meteorites.

Key-words: ilmenite, Raman spectroscopy, high pressure, phase transition.