Mineral interphases control the thermal weakening and strengthening of granite

Abstract

While existing studies on hot rocks across various scales have provided valuable insights, they have yet to offer mechanistic explanations at the mineral level for the controversy in temperature-dependent changes in the granite mechanical properties. Most prior work, moreover, relies on samples that are heated and then cooled before testing, a step that relaxes thermal stresses and may introduce cooling-induced microcracks. Unlike these cooled-then-tested measurements, this study examines mineral-to-mineral interactions in granites recovered from two geothermal sites of contrasting mineralogy, at elevated temperatures using instrumented nanoindentation. The findings indicate that temperature causes predominantly inter- rather than intra-mineral changes in granite. Interphases of minerals with different strengths and coefficients of thermal expansion, like quartz-biotite and albite-biotite, degrade as temperature increases, whereas interphases between minerals with similar mechanical and thermal properties, like quartz-albite, are strengthened by temperature. Granite properties at elevated temperatures therefore result from a competition between thermal degradation in mismatching mineral interphases and thermal strengthening of interphases of comparable minerals. This competition leads to an overall thermal degradation in biotite-rich granite, but thermal strengthening in quartz- and albite-rich granite, and thus, the dominant interphase type set by the mineral assemblage governs whether a given granite weakens or strengthens. These interphase-controlled mechanisms reconcile the conflicting weakening and strengthening behaviors reported for granite and inform the prediction of crystalline-rock performance in enhanced geothermal systems.