Page 65 - Microtectonics

3.10 · Grain Boundary Area Reduction (GBAR) 53

Fig. 3.41. Statically recrystallised quartz in a fabric with alternating quartz and feldspar layers. Feldspar is recrystallised and very fine-
grained (e.g. in layers left of the centre and at right). Grain size of quartz depends on the width of the quartz layer; in thin layers, quartz
grains are limited in their growth, leading to a clear dependence of statically recrystallised grain size on layer width. It is possible that quartz
grain size was similar in all layers at the end of the deformation that formed the layering and before static recrystallisation started. De-
formed quartz vein. Yilgarn Craton, Australia. Width of view 4 mm. CPL

Besides the anisotropy of individual minerals which ferent minerals (Vernon 1976). In general, there is a ten-
influences interfacial angles in monomineralic aggre- dency for high energy boundaries to decrease, and for
gates, the nature of different minerals in contact is low energy boundaries to increase in length. Consequently,
also of importance. In polymineralic aggregates where the interfacial angle between the boundaries separating
weakly and strongly anisotropic minerals are in contact, unlike minerals (also known as the dihedral angle; Hunter
the grain boundaries tend to be defined by the more 1987; Fig. 3.40e) deviates from 120°.
strongly anisotropic phase. For example, mica or tour- The process of grain growth tends to lower the inter-
maline grains included in quartz can be idiomorphic nal free energy of a grain aggregate even after a foam
(Figs. 3.35, 3.40d, 4.9). The anisotropy of minerals is also structure has been established, although grain growth
evident in the shape of included grains in rocks that becomes slower with increasing grain size (Olgaard and
underwent GBAR; grains of sillimanite in quartz, for Evans 1988; Kruhl 2001). The grain size that is finally
example, usually show a strong predominance of fa- reached after GBAR depends on temperature, but also
voured crystallographic directions for their boundaries. on the presence of other solid or liquid phases in grains
Notice, however, that this does not apply for inclusions and grain boundaries, variation in mineral chemistry and
in low to medium-grade rocks where inclusion bounda- crystallographic preferred orientation (Evans et al. 2001).
ries have been relatively immobile after the growth of Of these factors, the possibility of grains to grow with-
the host grain (Sect. 7.3). out obstruction by grains of other minerals seems most
In aggregates with phases of low anisotropy, another important (Masuda et al. 1991; Evans et al. 2001); conse-
effect may be visible. The boundaries between grains of quently GBAR in layered rocks results in relatively coarse
the same mineral can have another (commonly higher) grains in wide monomineralic layers, and small grains
grain boundary energy than those between grains of dif- in thin or polymineralic layers (Fig. 3.41, ×Video 3.41).