Acoustic emission (AE) from laboratory samples of coarse-grained granite hit by a dropped weight at temperatures from 20 to 500 °C is recorded within a frequency range of 80 kHz–4 MHz. The time series of AE signals bear information on the size of primary defects, since the AE frequency is proportional to the growth rate of microcracks and inversely proportional to their lengths. According to AE data obtained in this study, impact fracture of granite produces cracks at grain boundaries at temperatures below 400 °C and additional defects inside grains at 400 °C and higher. The reason is that overheated water–vapor inclusions increase pressure in the grain interior, which affects the mechanic behavior of granite.

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