Brittle and Crystal‐Plastic Defect Dynamics of Calcite Single Crystals

Abstract The response of Earth materials to stress has a first‐order control on solid‐Earth dynamics ranging from earthquakes to volcanic eruptions to landslides. Thus, understanding the rheological behavior of minerals, preferentially at a micro‐scale to allow process‐based upscaling, has broad applications across the Earth sciences. Cracks, dislocations, and point defects are microscale agents of deformation that govern the macro‐scale response of rocks under stress. Complexity in deformation behavior arises from the interplay among these defect types. Here, we deformed calcite single crystals under uniaxial loading to study defect dynamics using an array of piezo‐sensors synchronized with fast‐camera recordings. We demonstrate that brittle cracking co‐occurs with crystal‐plastic twinning deformation. Furthermore, each mechanism produces distinct impulsive ultrasonic signals. Cracking produces energetic signals often with high‐frequency patterns. Meanwhile, mechanical twinning produces less energetic, shorter duration, low‐frequency signals correlated with propagating twin fronts. These signals can be fit with a second derivative of a Gaussian function for the first few cycles after their onset. Our results illustrate key microscale processes that drive permanent deformation in calcite, and thus contribute to building an experimentally based, process‐driven understanding of rheological behavior. We highlight the utility of using ultrasonic signals, with detailed time‐series analysis, to help infer in situ microscale deformation at a range of spatiotemporal scales.