Kyanite-Andalusite Equilibrium from 700° to 800°C

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Abstract
The metastable kyaniteandalusite equilibrium in the Al 2 SiO 5 system has been reversed at 700°, 750°, and 800°C at elevated water pressures, with a variety of natural and synthetic kyanites and andalusites as starting materials. Sillimanite, the stable form of Al 2 SiO 5 under these conditions, did not appear. The value of the transition pressure at 750°C is 6.6 ± 0.4 kilobars, several kilobars below pressures given by several convergent previous determinations. The Al 2 SiO 5 pressure-temperature triple point now indicated lies far from the points found by others. The revised aluminum silicate phase diagram indicates that many rocks crystallized at lower pressures than formerly thought possible.