Two aspects of the brittle deformation of Sioux quartzite have been studied; water-induced changes in the amount of energy consumed during fracture of the quartzite and the occurrence of microcracking during crack propagation. Wedge-loaded double cantilever beam test specimens were cracked in several environments (air, water, aqueous solutions of NaOH, HCl, NaCl, methanol) at crack velocities of 4.2¿10-2mm/s and 4.2 mm/s and temperatures of 17¿-60¿C. Scanning electron microscopy and the determination of enhanced permeability were used to detect fracture induced microcracks. The fracture experiments measured the fracturs energy of the quartzite, the specific energy requaired to produce a unit increment of crack extension, using the analysis of linear elastic fracture mechanics. The fracture energy of Sioux quartzite in water is 15% lower than that of quartzite cracked in air (ambient humidity). The dependence of the quartzite fracture energy on crack velocity (time) and on the concentration of water in the test environment is characteristic of the occurrence of stress corrosion as a mechanism of enhanced crack growth in the presence of water. Other theories of the chemomechanical weakening of rock reduction in surface energy and changes in dislocation mobility, do not explain the observed environmentally induced changes in the fracture energy of the quartzite. Although increased crack velocity is quartzite cracked in hot water is evidence of thermally enhanced water weakening, the fracture energy of quartzite cracked in hot air in hot water (40¿-60¿ C) is 12-13% higher than room temperature values. The increase in fracture energy is attributed to changes (reduction in residual stress, existence of thermal stresses) in the net stress field responsible for micro-cracking and crack propagation in the quartzite.