Real-Time Measurement of Mechanical Behavior of Granite During Heating–Cooling Cycle: A Mineralogical Perspective

Abstract

Changes in mechanical properties of rocks at high temperatures are significant to various scientific and engineering problems, such as efficient exploration in enhanced geothermal systems (EGS) and rock mass stability in radioactive waste disposal. However, a comprehensive understanding of changes in rock mechanical behaviors at high temperatures and corresponding mechanisms, especially granites, remains unclear, which calls for the examination of micromechanical properties of the constituent minerals. This work investigates four representative minerals in granites, including quartz, plagioclase, amphibole, and biotite. The micromechanical properties of these minerals in the heating–cooling cycle were characterized in real time by using the nanoindentation technique. The morphology of samples was observed not only under real-time cyclic heating–cooling conditions, but also after thermal treatment. Subsequently, changes in structure of the minerals at elevated temperatures based on X-ray diffraction (XRD) were studied. Thermogravimetric analysis coupled with Fourier’s transform infrared spectroscopy (TG-FTIR) was performed to examine physio-chemical alterations of minerals at high temperatures. The experimental results revealed that the reduced modulus and hardness of quartz decreased during heating and subsequently increased during cooling. During the heating–cooling cycle, the hardness of plagioclase varied similarly to that of quartz, but its reduced modulus increased up to approximately 300,^{circ }hbox {C} followed by a decrease during heating. Unlike the net decrease of modulus of quartz after exposure to the heating–cooling cycle, feldspar showed a net increase of modulus. The micromechanical properties of biotite were found to rise significantly during heating, followed by a decrease during cooling. XRD results suggested the alteration of mineral structure, including spacing of lattice planes, crystallinity, and phase transitions, which controlled the above changes in mechanical behaviors of quartz, amphibole, and plagioclase. However, the aperture of open cracks along the cleavage in biotite determined its micromechanical properties during heating and cooling. Thermally-induced microcracks were observed during heating; whereas, closure of pre-existing cracks and initiation of microcracks were found during cooling. The real-time measured micromechanical properties of minerals could relate to the macromechanical behaviors of rocks under heating–cooling conditions based on thermally-induced microcracks.