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Abstract. The effect of ductile deformation (dislocation creep) on the kinetics of the aragonite-calcite transformation has been studied at 1 atm (330°C and 360°C) and 900-1500 MPa (500" C) using undeformed and either previously or simultaneously deformed samples (500' C and a strain rate of s).Deformation enhances the rate of the transformation of calcite to aragonite, but decreases the rate of transformation of aragonite to calcite. The difference results from a dependence of transformation rate on grain size, coupled with a difference in the accommodation mechanisms, climb versus recrystallization, of these minerals during dislocation creep. Dislocation climb is relatively easy in calcite and thus plastic strain results in high dislocation densities without significant grain size reduction. The rate of transformation to aragonite is enhanced primarily because of the increase in nucleation sites at dislocations and subgrain boundaries. In aragonite, on the other hand, dislocation climb is difficult and thus plastic strain produces extensive dynamic recrystallization resulting in a substantial grain size reduction. The transformation of aragonite is inhibited because the increase in calcite nucleation sites at dislocations and/or new grain boundaries is more than offset by the inability of calcite to grow across high angle grain boundaries. Thus the net effect of ductile deformation by dislocation creep on the kinetics of polymorphic phase transformations depends on the details of the accommodation mechanism.

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Average values for NaSi‐CaAl interdiffusion in the compositional interval from An0 to An26 have been determined at 1000°C by the method of lamellar homogenization. At 1500 MPa confining pressure (P), increases 1 order of magnitude (5.0×10−21 to 4.0×10−20 m2/s) for 4 orders of magnitude increase in hydrogen fugacity (0.029, Mn3O4‐Mn2O3 buffer, to 197 MPa, FeO‐Fe3O4 buffer). At constant hydrogen fugacity (fH2), increases rapidly at low pressure and becomes nearly independent of P above 1000 MPa. (For fH2 = 0.1 MPa, = 2.8 × 10−22 m2/s at P = 0.1 MPa, 5.0 × 10−21 at P = 500, 1.3×10−20 at P = 1000, and 1.4×10−20 at P = 1500). The dependence of on increasing pressure, when a hydrogen‐related species is present, is believed to be due to an increase in the concentration of the structural defect associated with increase in the hydrogen impurity. In most crustal igneous rocks, which are internally buffered near quartz‐fayalite‐magnetite, the dependence of on fH2 is relatively minor compared to the effect of confining pressure.