Mineral-specific meso-mechanical degradation and microcrack evolution in granite induced by cyclic heating and LN2-cooling

LN 2 -assisted cyclic fracturing has been demonstrated to be an effective method for enhancing fracture networks in deep hot dry rock (HDR) geothermal reservoirs. This study investigates the influence of mineral phases on the mesoscopic mechanical response and microcrack evolution in granite subjected to cyclic heating and LN 2 -cooling. Nanoindentation, scanning probe microscopy (SPM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), and computed tomography (CT) were employed to evaluate mechanical degradation and microcrack development. A novel two-parameter statistical screening method was developed based on the relative content of mineral phases. The results indicate that cyclic heating and LN 2 -cooling significantly degraded the mechanical properties of granite, with progressive reductions in the slope of load-displacement curves across all mineral phases as the number of cycles increased. After 20 thermal cycles, meso-mechanical parameters exhibited exponential decreases: Young's modulus decreased by 39.86% to 88.53%, hardness by 26.65% to 78.53%, and fracture toughness by 19.79% to 69.08%. Deformation parameters continued to rise, with contact depth, peak depth, and creep depth increasing by 23.65% to 145.04%, 24.09% to 150.10%, and 39.92% to 329.70%. Mechanical properties are critical in characterizing mineral deformation: contact depth is primarily governed by Young's modulus, whereas peak and creep depths show strong correlations with hardness. Furthermore, structural differences between minerals result in distinct microcrack evolution patterns: quartz predominantly propagates along grain boundaries, feldspar fractures preferentially along cleavage planes, and mica exhibits layered delamination. Microcrack width, density, and quantity exhibited exponential growth with continued cycling, leading to progressive meso-mechanical deterioration.