KYANITE-SILLIMANITE EQUILIBRIUM AT 900 C BY CRYSTAL WEIGHT CHANGE

The sillimanite to kyanite transition was redetermined at 900 C in piston cylinder apparatus with NaCl pressure medium using a hydrothermal weight change method and very pure natural sillimanite and kyanite single crystals. Weight changes of the kyanite crystals in experiments of 24 hr. duration traversed the sillimanite-to-kyanite equilibrium boundary in 0.5 kbar increments from 10.0 to 15.0 kbar. Kyanite crystals were weighed before and after the experiments on an ultramicrobalance to a precision of 0.2 microg and water was weighed to a precision of 2 microg by difference after sealing without weight loss. The weight change data yield a null point of 11.7+-0.2 kbar. We thus confirm that the equilibrium transition lies in the upper one-half of the approximately one-kbar-wide interval of uncertainty found by previous investigators at that temperature.