Microstructures associated with GBM recrystallization
The process of GBM recrystallization is dominated by the fast migration of grain boundaries, which can sweep entire grains producing highly irregular grain boundaries, characterized by strongly sutured and interfingered lobes and protrusions. The resulting microstructure is said to be interlobate up to amoeboid (i.e. grain boundaries so irregular to resemble amoebas). In general, grain boundary migration occurs at high temperatures and it is hence associated with relatively coarse grains (above 100 μm, generally around 500 μm). GBM-recrystallization is so efficient that strained grains are rapidly removed and replaced by nearly strain-free grains. Subgrain boundaries may nucleate during GBM-recrystallization and rapidly evolve to become independent grains from their parent grain with their own lobate grain boundaries. For these reasons, it is not straightforward to tell apart new grains from old grains in a grain aggregate produced by GBM-recrystallization, except when markers, like trails of inclusions that could outline the shape of the original grains, are preserved.

Island grains. In grain aggregates produced by GBM-recrystallization, it is common to find relatively small and isolated grains surrounded by larger grains and that are in optical continuity (extinguish at the same time) with respect to nearby grains. These ‘island grains’ are an effect of the strongly lobate/amoeboid shape of the grain boundaries and are actually often linked to larger grains outside of the 2-dimensional thin section plane. Indeed, this microstructure was also named by Urai et al. (1986) ‘dissection microstructure’, as island grains are not actual smaller grains included in larger grains, but an artifact produced by sectioning grains with highly irregular grain boundaries.

CPL + λ

CPL

Pinning microstructures. These structures form when GBM-recrystallization occurs in the presence of secondary phases, like platy micas, included in quartz. In this case, secondary phases may ‘pin’ the migrating grain boundaries producing arcuate lobes around them, known as ‘pinning microstructures’. Jessell (1987) identified many types of these ‘pinning microstructures’ (figure below). For example, in the case of oriented inclusions of platy micas in quartz, the micas may act as dams, obstructing the migration of grain boundaries. However, if a ‘hole’ in the ‘mica dam’ is present, grain boundaries can migrate across it, producing small pinned boundaries, referred to as ‘window microstructure’. Another common effect related to mica inclusions is that migrating grain boundaries may be dragged laterally along an inclusion (‘dragging microstructure’).

Left-over grains. The migration of grain boundaries may almost completely erase a parent grain  leaving behind only small left-over grains in optical continuity. These differ from island grains as they are actual small grains that are not connected anymore. However, considering that grain boundaries are often lobate in aggregates deformed by GBM-recrystallization, it is rather hard to tell whether these grains are actually linked outside of the thin section plane or not.