Introduction
Rock microstructure can preserve a wealth of information about geological history, particularly if the last recrystallisation event was insufficient to obliterate the traces of earlier episodes. Most microstructural studies interpret characteristics such as grain shape, the distribution of grain sizes and preferred orientation of grains (c.f. the comprehensive reviews by Vernon (2004) and Higgins (2006)). Other parameters of interest are the shape of grain boundaries and interfaces (e.g. Kruhl, 2001; Kruhl and Peternell, 2002). A comparatively neglected microstructural parameter is the dihedral angle, that angle subtended between two grain boundaries or interfaces (Smith, 1948; 1964). Although dihedral angles in rocks have been studied for decades, due to their importance in determining the equilibrium distribution of a liquid phase (Smith, 1964; Wray, 1976; Bulau et al., 1979; von Bargen and Waff, 1986), it has only recently become apparent that in many rocks the population of dihedral angles is far from equilibrium (e.g. Sawyer, 1999; Kruhl and Peternell, 2002; Holness et al., 2005a), and that this departure from equilibrium provides a window into rock history. Dihedral angles are particularly useful in that they are relatively straightforward to quantify and tend to undergo the earliest changes during recrystallisation (Voll, 1960): they are thus highly sensitive to changes in physical conditions (particularly temperature). The disequilibrium angle population is a useful record of processes such as recrystallisation, solidification and reaction and is used as a qualitative indicator of the thermal history. Here I review the current level of understanding of both equilibrium and disequilibrium dihedral angle populations, outline how they may be interpreted, and provide pointers for further research. The focus is on melt-bearing and solidified rocks, because the rarity of preserved volatile-filled pores in metamorphic and igneous rocks reduces the usefulness of volatile-solid dihedral angle data.