This note reports a high-pressure phase transition in cordierite detected with a volumetric technique in isothermal compression experiments carried out in a piston-cylinder high-pressure apparatus. Transition data at 25, 510, 600, 730 and 900°C were obtained yielding a boundary following the relation: P = 0.0075T + 2 (kbar, °C). Measurements at room temperature suggest a very small volume change (ΔV/Vo = 0.0025) at the transition. The data are in remarkable agreement with a recently proposed boundary between two hydration mechanisms in cordierite (Mirward et al., 1979). Supporting evidence for the high-pressure transition is obtained from X-ray data on quenched samples of hydrous Mg-cordierite, which indicate no change in crystal symmetry but significant changes in the lattice parameters. Because up to 2 mol H2O per formula unit have been determined in quenched high-pressure cordierite, a new hydration model is proposed involving a side by side accommodation of H2O molecules in the channel cavities of the high-pressure phase. An appraisal of the thermodynamic quantities yields only small changes: ΔV = 0.6 cm3/mol and ΔS = 0.5 J/mol K. The available observations suggest that this high-pressure transition in cordierite is likely to be of higher order.

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