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American Mineralogist; December 1978; v. 63; no. 11-12; p. 1100-1106
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Omphacite studies; I, The P2/n-->C2/c transformation

M. E. Fleet, C. T. Herzberg, G. M. Bancroft, and L. P. Aldridge

Univ. West. Ont., Dep Geol. and Chem., London, Ont., Canada

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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