The coupled problems of the tectonic significance and mechanism of development of slaty cleavage and schistosity have been studied in high-temperature and pressure deformation experiments. The primary mechanistic question concerns whether preferred orientation of mica in slates and schists develops by rotation of tabular grains or whether it develops by mechanisms that require crystallization or re-crystallization. Most proposed orienting mechanisms predict the same relationship between principal strain directions and preferred orientation. This frustrates attempts to determine the principal orienting mechanisms.

Simple theoretical models of rotation of passive planar markers and of isolated rigid ellipsoids in a deforming fluid predict similar relationships between the magnitudes of the strain and the preferred orientation. In room-temperature experiments, these predictions are followed when interactions between adjacent grains are minimal; when interactions are important, the rotation mechanism is less effective.

In high-temperature experiments in which phlogopite is syntectonically crystallized from the constituent oxides or fluorophlogopite or biotite is syntectonically recrystallized, the preferred orientations match the predictions of the rotation models. This suggests that crystallization or recrystallization aided the rotation process by reducing mutual grain interference. Textural observations of the recrystallized samples support this interpretation.

In all of the deformation experiments, maximum, intermediate, and minimum principal compressive strain directions coincide with maximum, intermediate, and minimum concentrations of poles to mica basal planes, respectively, even for general noncoaxially accumulating strain. These results coupled with studies of natural slates suggest that mica preferred orientations may be used to estimate both magnitudes and directions of finite strain.

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