Shape, size, and orientation of muscovite crystals in a schist of variable metamorphic grade

Abstract

Measurements of muscovite crystals from a pelitic schist unit show that with an increase in metamorphic grade from the lower to the upper amphibolite facies the crystals on average become larger, more equidimensional in shape (the ratio of length to thickness decreases) and the number per unit volume of rock decreases. In the high grade rocks all sizes of crystals, including small inclusions of muscovite within porphyroblasts of plagioclase feldspar, tend to be relatively equidimensional. The change in shape is thus not simply related to the increase in average size, and the two variables must be considered as independent. Both variables reduce the surface area of the crystals relative to the volume as the metamorphic grade increases.Contrary to previous estimates of the preferred orientations of micas, small muscovite crystals from lower grade zones are more scattered in orientation than larger crystals from higher grade zones. The scatter is mainly due to the presence in the low grade rocks of short, relatively equidimensional crystals lying at high angles to the foliation.It is suggested that the textural changes reflect processes of continuous recrystallization and growth during prograde metamorphism and the development of the schistosity. Crystals may have reached an equilibrium shape, which is temperature dependent. These changes may be driven by surface energy forces, so that the total interfacial free energy of the rock tends to be minimized for any metamorphic grade.