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The internal energy associated with dislocations is calculated in quartz as a function of dislocation density. These calculawhich would drive the diffusive mass transfer of SiO 2 out of the fault zone.

The chemical role of water and other aqueous environments in crack propagation was examined in a series of crack propagation tests in natural and synthetic quartz. The potential roles of surface free energy reduction and zeta potential (surface electrostatic potential) in chemical weakening were specifically examined by running the crack propagation tests in the presence‐of six surface active aqueous environments which varied sub‐stantially in these parameters with respect to quartz. Calorimetric and electrophoresis tests were also undertaken in order to determine the degree of reduction of the surface energy of quartz produced by each chemical environment and the zeta potential between each environment and quartz. It was found that there was a moderate correlation between reduction of the crack propagation stress of quartz and the degree by which the surface energy was reduced in the presence of a particular environment. No such correlation was found with respect to zeta potential. It was also observed that some of the chemical environments appeared to produce highly branched cracks in quartz. This branching effect is probably related to the velocity of crack propagation.

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The role of surface active aqueous environments in chemomechanical weakening of geologic materials is examined in light of the results of hydraulic fracture tests in sandstone, calorimetric studies, and crack propagation tests in synthetic quartz. In hydraulic fracture tests employing Crab Orchard Sandstone it was found that the effective hydraulic fracture pressure was reduced, over t•at attained with distilled water, when 5 x 10--M aqueous solutions of dodecyl trimethyl ammonium bromide (DTAB) were used as the hydraulic fracture medium. The degree of branching of the fractures was also increased in the presence of the DTAB solution. Previously reported crack propagation stress values in quartz exposed to distilled water and various DTAB solutions displayed the same trend. When examined in this study, the cracks propagated in the presence of DTAB solutions also displayed a greater degree of branching than those propagated in the presence of distilled water or the ambient atmosphere. These results and results from calorimetric measurements of the heats of adsorption and desorption from quartz of distilled water and DTAB are synthesized in a model relating the weakening and morphological effects to a reduction in the surface free energy of quartz due to adsorption of species from the chemical environment onto the surfaces of the quartz and sandstone.