Strain modulation around inclusions in an annealed natural cordierite

Abstract

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 Al 2 O 3 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 [310] * 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 Si 4+ + □ channel → K + + Al 3+ , 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 R 2+ + Si 4+ → 2Al 3+ (where R 2+ 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.