The thermochemistry of the kyanite-sillimanite equilibrium

The enthalpies of solution of kyanite and sillimanite in a lead borate solvent (2PbOXB ~2~ O ~3~ ) have been determined calorimetrically at 701 + or - 2 degrees C. From the observed values for one sample of kyanite and two different samples of sillimanite the enthalpies of transformation kyanite = sillimanite were redetermined to be 1.42 + or - 0.18 kcal/g.f.w. and 1.52 + or - 0.23 kcal/g.f.w., respectively at 701 degrees C. From these values we have recalculated the P-T equilibrium curves for the kyanite-sillimanite equilibrium for pressures below 12 kb. Within the indicated errors both the calculated curves are consistent with recent experimental determinations.