Conclusions
The impetus for studies of dihedral angles came from metallurgists interested in predicting the behaviour of composite materials and alloys. Their work was noticed by Earth scientists who realized that textural equilibrium might control melt migration in the mantle. Much of the early petrographic work was therefore aimed at understanding textural equilibrium, and determining equilibrium values of the dihedral angle. However, a cursory look at most crustal igneous and metamorphic rocks demonstrates that textural equilibrium is actually rather rare; dihedral angles are generally out of equilibrium. The potential for this disequilibrium can only be realized by studies determining the true distribution of these angles, and in practical terms this means using a universal stage. At present we are in the earliest stages of putting disequilibrium dihedral angles to good use, and almost all published work is purely qualitative, based on comparison of dihedral angle populations from a large number of well-located samples. Full exploitation of the observations can only be achieved if we make the next step forward into quantitative studies. We need to develop not only an understanding of the processes leading to the initial dihedral angle distribution but also an understanding of the processes and rates of its subsequent modification in either the super- or the sub-solidus. Future work should be aimed at solving these problems.