Molar elastic strain energy arising from dislocations in andalusite and sillimanite were calculated using equations derived from a non-core linear elasticity model. For perfect (unit) c screw dislocations, minimum dislocation densities of about 1010/cm2 were required in order to perturb significantly the andalusite/sillimanite equilibrium boundary in P-T space. Compared to unit c dislocations, smaller energy perturbations arose from dissociated c screw dislocations, which were commonly observed in kyanite and sillimanite. A low (30mJ/m2) computed value of the stacking-fault energy in these polymorphs was compatible with the large separations of dissociated dislocations in the phases. Dislocation densities in naturally occurring Al2SiO5 polymorphs were typically less than 108/cm2. Assuming that these densities were representative of those existing during metamorphism (as supported by a lack of microtextures indicative of strong recovery) it was concluded that molar strain energies corresponding to observed dislocation densities (<108/cm2) resulted in insignificant perturbation of the P-T phase equilibrium boundaries of the Al2SiO5 polymorphs.

Dislocation Strain Energy in the Al2SiO5 Polymorphs. D.M.Kerrick: Physics and Chemistry of Minerals, 1986, 13[4], 221-6