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On the basis of proposals by Cahn [Acta Met. (1962). 10, 179-183] for cubic crystals, a coherent elastic model is developed which makes it possible for the orientations of exsolution boundaries formed either by spinodal decomposition or homogeneous nucleation to be predicted. The elastic energy for the coherent exsolution boundary is calculated for all orientations of the boundary, the minimum value for this energy corresponding to the predicted boundary. A program has been written which can be applied to all crystal systems; it uses as input the lattice parameters of crystals having the same compositions as the exsolution domains and their corresponding elastic stiffnesses. The approximations of the model are discussed, especially the neglect of relaxation of the coherence stresses away from the boundary, and the uncertainties in the calculated results due to errors in or lack of knowledge of the input data. The coherent elastic model has been used to determine the orientations of exsolution boundaries in the feldspars. The agreement between calculated and observed exsolution boundaries is remarkable, considering that room-temperature lattice parameters and elastic stiffnesses are used. The angular differences are less than 3° in most cases. They may reach 20° in the labradorites because the input data are not well known. All observed exsolution boundaries have been accounted for and some unknown ones calculated. The results of the calculations may be used to explain the cause of the exsolution orientations and to suggest areas requiring further research.
