Elastic strain and preferred orientation in monoclinic crystals

Microcline phenocrysts in a monzonite gneiss, and hornblende and actinolite crystals in a series of amphibole schists, form strong lineations of elongate crystals perpendicular to axes of small folds. The crystallographic c axes of the amphibole and microcline crystals form strong maxima parallel to the A fabric axis of the rocks; the b axes of microcline form a strong maxima parallel to the C fabric axes and the b axes of the amphiboles form maxima parallel to the B fabric axis. If the folds and foliation planes are products of inelastic shear strain, these orientations correspond to the maximum elastic strain energies per unit stress for these crystals and agree with the prediction of preferred orientation by MacDonald (1960) . A predicted effect of positive and negative shear strain was not observed in these crystals. Preferred mineral orientations and recrystallization may be interpreted by use of elastic strain theory to give additional insight into the problems of rock deformation.