Heating experiments on natural omphacite at 15 to 18 kbar, using a piston-cylinder solid-media apparatus, have established the critical temperature for the order-disorder P2/n = C2/c transition at 725 plus or minus 20 degrees C. The disordering reaction P2/n-C2/c is quite abrupt, and the observed transformation temperature is independent of run duration for one-day and one-week experiments. Reaction rate data give an activation energy for disordering of about 12 kcal.mol (super -1) , which suggests that cation disorder in omphacite is a cooperative process involving both M1-type and M2- type cations. It appears that the antiphase domain structure in blueschist-facies omphacite is probably a growth feature, and does not form through inversion on cooling of earlier C2/c omphacite. In general, the geothermometric applications of the omphacite phase relations are somewhat limited.

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