Natural mineral fracturing in rocks is evaluated in terms of anisotropic thermal and elastic properties of crystals; models are proposed for calculating anisotropic thermal stresses. Computed thermal stresses that result from differential thermal contractions, with reasonable temperature change, between different coexisting minerals or between the same minerals of different orientation are adequate to cause brittle rupture of the crystals or provide elastic strain energies for annealing recrystallization. Thermal strains are relieved by brittle rupture in the ferromagnesian minerals, whereas in the quartz-feldspar system, annealing recrystallization is the mechanism. Fracture stress is evaluated in terms of theoretical cohesive strength of minerals. It is proposed that the modulus of rigidity, less than half the value of Young's modulus, be substituted for it in the descriptive fracture stress equation.