The equilibrium orientation of an anisotropic mineral in a stress field is one in which the potential energy of the external forces plus the potential energy of deformation is minimized. For materials obeying a linear elastic stress-strain law, the minimum total potential energy is equivalent to a maximum Helmholtz free energy in an isothermal system or a maximum internal energy in an adiabatic system. The minimum potential-energy theorem is applicable to the orientation of minerals in rocks undergoing plastic deformation provided temperature and elastic strain are sufficient parameters to describe the thermodynamic state of the mineral undergoing isothermal plastic flow.