Preferred Mineral Distributions of Polymineralic Rocks Related to Non-Hydrostatic Stresses as Expressions of Mechanical Equilibria

Abstract

Recrystallization of a mineralogically heterogeneous rock under non-hydrostatic stress conditions should result in a preferred mineral distribution as well as preferred mineral orientations that are related to the stress direction. A stress-strain model is developed wherein calculations show that different mineral distributions provide different strain energies per unit stress for the rock. The maximum strain energy per unit stress is achieved if the minerals are arranged to give the modal composition of the rock in the direction parallel to the maximum compressive stress direction and monomineralic layers parallel to the direction of applied shear stress. These mineral distributions or arrays provide the maximum work per unit stress and the maximum decrease in the mechanical potential energy for the system and are regarded as the equilibrium arrays. However, these arrays do not necessarily provide the minimum chemical potentials of the constituents of the strained minerals. It is concluded that the equilibrium form for the system is the mechanical equilibrium mineral array, and the results of strain are described by Kamb's (1959, 1961) treatment of chemical potentials of strained systems.