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. E -twinning is a common plastic deformation mechanism in calcitedeformed at low temperature. Strain rate, temperature and confining pressurehave negligible effects on twinning activation which is mainly dependent ondifferential stress. The critical resolved shear stress (CRSS) required fortwinning activation is dependent on grain size and strain hardening. ThisCRSS value may obey the Hall–Petch relation, but due to sparse experimentaldata its actual evolution with grain size and strain still remains a matterof debate. In order to provide additional constraints on twinning activation anddevelopment, new mechanical tests were carried out at room temperature onunconfined single crystals of calcite, with different sizes andcrystallographic orientations. Uniaxial deformation was performed at acontrolled displacement rate, while the sample surface was monitored usingoptical microscopy and a high-resolution CCD (charge-coupled device) camera. The retrieved macroscopicstress–strain behavior of the crystals was correlated with the surfaceobservations of the deformation process. Results show (1) the onset of crystal plasticity with the activation of thefirst isolated mechanical twins during the strain hardening stage, and(2) the densification and thickening of twin lamellae during the steady-stateflow stress stage. Such thickening of twin lamellae at room temperatureemphasizes that calcite twin morphology is not controlled solely bytemperature. The different values for the CRSS obtained for the activation ofisolated twins and for the onset of twin densification and thickeningraises questions regarding the appropriate value to be considered when using calcite twin datafor stress inversion purposes.

Uniaxial compression of calcite single crystals at room temperature: insights into twinning activation and development