MISORIENTATION ANALYSIS AND THE FORMATION AND ORIENTATION OF SUBGRAINAND GRAIN-BOUNDARIES

In contrast to the behaviour of individual grains, both inter-and intr a-granular boundaries within rocks have received much less attention. However, many geological processes, particularly during deformation (e .g., yielding, dislocation creep, recrystallisation, superplasticity a nd various fracture mechanisms), and petrophysical properties depend t o some extent on the nature of boundaries present in a rock. In this c ontribution, we consider the role of intergranular and intragranular c rystal boundaries. A precise characterisation of such boundaries depen ds on defining the crystallographic and dimensional orientations of th e boundary and the misorientation between the adjacent regions (i.e. g rains, subgrains: etc.) separated by the boundary. Although several th eoretical descriptions of boundary configuration are available, practi cal precision is lacking and approximations are necessary. We describe two specific approximations for boundary formation and orientation ob tained using the SEM electron channelling technique. The first is a ge ometrical interpretation of electron channelling patterns (ECP) in ter ms of the like(, formation and orientation of the intervening boundary . The second considers the misorientation between adjacent regions acr oss a boundary. This involves a model which assumes a simple geometric al relationship between crystal slip systems responsible for the rotat ion and misorientation between adjacent regions, and the formation and orientation of the resulting boundary. These approximations are capab le of: (a) identifying trends in the dispersion of crystallographic di rections during deformation; (b) identifying active slip systems; (c) calculating the relative Schmid Factors for each crystal slip system ( and therefore the most likely system to be activated): (d) modelling s ynthetic misorientations and predicting the crystal slip systems and b oundary configurations to be expected; and (e) comparing real data wit h synthetic models. Our analyses are illustrated via natural examples of dynamic recrystallisation in quartzite and a theoretical simulation of the behaviour of an individual quartz grain during deformation.