Microstructure of slate from Lady Loretta, Queensland, Australia

The preferred orientation of mica in dolomitic slaty siltstone units from Lady Loretta, Queensland, Australia, and its relation to grain size and shape show that the larger, thicker grains have (001) subparallel to bedding, and the thinnest grains have (001) parallel to cleavage. The overall mica orientation depends primarily on bedding-cleavage angle; it bears little relation to bulk composition within the restricted range observed. The shapes of quartz dolomite-rich blebs in two of the specimens have been measured to determine the orientation and shape of their finite strain ellipsoid. The λ₁λ₂ plane of this ellipsoid is at a low angle to the mica (001) preferred orientation defining the slaty cleavage. This is interpreted to be due either to a difference in strain between the blebs and their matrix, or to the operation of a mica-orienting mechanism that is not related rigorously to the finite strain. Predominantly mechanical orienting mechanisms are considered to be incompatible with the bimodal orientation and grain shape of mica. It is suggested that there was a well-defined preferred orientation in bedding, imposed during diagenesis, and that the cleavage mica grew with a strong preferred orientation during the deformation. A mechanism for this growth control similar to that proposed by Etheridge and others (1974) is favored.