Page 64 - Microtectonics

52 3 · Deformation Mechanisms

Fig. 3.39. Polygonal fabric of scapolite grains formed by static recrystallisation. Mt. Isa, Australia. Width of view 4 mm. CPL

Fig. 3.40. Effects of the anisotropy of minerals for grain boundary
energy on grain boundary orientation. a If grain boundaries all have
similar internal free energy, grains will be equidimensional and
boundaries are not preferentially associated with specific crystal-
lographic planes (indicated by lines in the crystal). b In the case of
hornblende, some grain boundaries ({110} planes) have relatively
lower internal free energy, and may be dominant in the aggregate.
c In the case of micas, grain boundaries parallel to (001) are fa-
voured over all others and idiomorphic grains are commonly abun-
dant. d Typical shape of a quartz-mica aggregate where the low-
energy (001) planes of micas dominate. e Illustration of the dihe-
dral angle between minerals A and B, where the boundary between
like minerals has a lower (top) or higher (bottom) energy than that
between unlike minerals