Deformations in Minerals

Minerals submitted to tectonic stresses may deform plastically, i.e., continuously at the lattice scale. Various mechanisms of plastic deformation may be involved, depending on several parameters, e.g., temperature T, pressure P, applied stress σ, strain rate έ, chemical composition of the mineral, presence of fluids. Values of these parameters are different for different tectonic events; in crustal deformation, pressure and temperature may be 0.5 Gpa and 500 °C, whereas in the upper mantle, they are 10 Gpa and 1500°C. Deformation occurs at the time scale of geological events, i.e., with strain rates as low as 10−12 s−1. The first important results on the mechanisms of plastic deformation were obtained from transmission electron microscope studies of naturally deformed samples (Christie and Ardell 1976). Nevertheless, precise determination of these mechanisms needed to know exactly the deformation conditions and therefore implied the study of laboratory deformed samples (i.e., with strain rates ca. 10−4−10−6 s−1).