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Strain modulation around inclusions in an annealed natural cordierite

¹⁾ Laboratoire Structure et Propriétés de l'Etat Solide, Université des Sciences et Technologies de Lille, ESA 8008, 59655-Villeneuve d'Ascq Cedex, France
²⁾ Institut für Mineralogie, Universität Münster, Corrensstrasse 24, D-48149, Münster, Germany
³⁾ Department of Earth Sciences, University of Cambridge, Downing St., Cambridge, CB2 3EQ, U.K.

^* Present address: Dipartimento di Scienze della Terra, Università degli Studi di Siena, Via Laterina 8, I-53100 Siena, Italy. E-mail: gcapitani{at}unisi.it

A natural ferromagnesian cordierite which had been annealed at 1355°C has been found, by TEM, to contain submicroscopic cross hatched modulated microstructures (tweed) localised near Al₂O₃ inclusions, while most of the sample, apart from some melting features, remained homogeneous and orthorhombic in symmetry. SAED patterns from regions with the tweed textures exhibited splitting parallel to [30]^* and [110]^* of the orthorhombic cell. Such features in cordierite have been interpreted in previous investigations as intermediate metastable states formed during the high-to-low transition associated with Al,Si ordering. Such metastable states appear to be favoured in K-bearing cordierites because the substitution Si⁴⁺ + _channel K⁺ + Al³⁺, which increases the number of the Al-O-Al bonds, hinders the development of long range Al,Si ordering and the high-to-low transition. In the present cordierite studied here, the mechanism of modulated structure formation is most probably similar but the chemical exchange involved is different and of the type R²⁺ + Si⁴⁺ 2Al³⁺ (where R²⁺ is an octahedral cation). This entails the presence of some Al in octahedral coordination. It is suggested that octahedral Al would stabilise the hexagonal form of cordierite.

Key-words: modulated structure, cordierite, TEM.

This article has been cited by other articles:

				G. C. CAPITANI, J. C. DOUKHAN, T. MALCHEREK, and M. CARPENTER "Early partial melting" in annealed natural cordierites European Journal of Mineralogy, October 1, 2002; 14(5): 879 - 890. [Abstract] [Full Text] [PDF]