In order to gain further insight into changes in the properties and microstructures of granite due to temperature changes, two typical granites with different granularities, Westerly and Fujioka granite, were subjected to a onecycle heating and cooling test under various conditions: two maximum maintenance temperatures, 300 and 500°C, and two heating rates. Properties reflecting the internal microstructures, such as size, weight, effective porosity, and P-wave velocity, were measured before and after the thermal cycle and microstructures were visualized and observed using the fluorescent approach.
No significant differences in sample size and weight were identified. However, an increase in the effective porosity and gradual decrease in P-wave velocity were clearly identified after the test. This suggests that changes in microstructure occur within granites. Marked differences due to the heating rate were not identified in our examination. Microcracks along grain boundaries developed and widened in the tested samples, and this tendency was pronounced in more strongly heated samples. These microscopic observations agree well with the observed changes in properties. Furthermore, a difference in the crack growth pattern between the two granites was observed. In fine-grained Westerly granite, few new cracks formed in quartz grains and crack development was mainly along grain boundaries, while in coarse-grained Fujioka granite, many cracks developed in a network pattern in quartz grains in addition to the grain boundary cracks. It was inferred that one of the causes of the different crack growth patterns was the variation in grain size and shape. In both granites, nearly identical characteristic crack development patterns were observed in other major constituent minerals, feldspar and biotite. In feldspar, with increasing temperature, cracks grew straight and wide along the weak planes such as cleavages and twin planes. Cracks in biotite grains mainly developed along cleavages in the samples heated up to 500°C.