Deformation of biotite and muscovite: Optical microstructure

Abstract

An optical comparison illustrates the difference in behaviour of the two mica minerals biotite and muscovite; their response to deformation, and to chemical processes such as grain dissolution. Non-passive mechanical rotation, segmentation of deformed grains by a recovery-recrystallization type process and syntectonic growth of the phyllosilicates all contribute to the development of a strong tectonic foliation within a deformed pegmatite from the Italian Alps. There are significant mechanical differences between the two micas. Biotite readily deforms by kinking whereas muscovite forms sinusoidal folds and seldom kinks. If kink-like structures ( deformation zones) are present in muscovite they are generally accompanied by fracturing. Fracturing and displacements are obvious in most deformed muscovites both parallel to axial surfaces and between (001) cleavages. Fracturing is occasionally recognised in biotite. However, it is often obscured by extensive dissolution and new grain nucleation. Evidence for dissolution processes in biotite is more abundant than in muscovite.