Natural cordierite crystals with a range of Mg:Fe ratios have been annealed at a range of temperatures between 1200 and 1430°C (under controlledfo2), after previously being annealed at 900 °C to drive off any volatiles contained within their channels. Complex microstructures were observed in these samples by transmission electron microscopy. No evidence was found for a transformation from orthorhombic symmetry to hexagonal symmetry in the bulk of the crystals. Rather, all the heat-treated samples contained pervasive glassy needles parallel to [001], each including a small bubble. These became larger and less regular in shape with increasing annealing temperature, and are interpreted in terms of early partial melting of cordierite at temperatures which are well below the macroscopic melting point. Larger glassy veins were also found, and these contained crystals of mullite. In two samples annealed at temperatures of ∼ 1400 °C or below, pockets of glass with prismatic spinel crystals were observed. Chemical analysis in the electron microscope showed that the glass phase was generally enriched in SiO2 and depleted in Al2O3 with respect to the cordierite matrix. Attempts to rationalise the chemical compositions of all the phases in terms of mass balance were not entirely successful, and it is concluded that these areas of more extensive melting probably occurred in regions of altered cordierite or around inclusions.

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