Page 63 - Microtectonics

3.10 · Grain Boundary Area Reduction (GBAR) 51

Box 3.8 Continued
In practice, characteristic features of different recrystallisa-
tion mechanisms can be found together in one sample, since tem-
peratures may change during deformation.
An aggregate of small, dynamically recrystallised grains
around a crystal core with the same mineral composition is
known as a core-and-mantle structure, provided that evidence
(as mentioned above) exists that the structure developed by dy-
namic recrystallisation of the core mineral along its rim
(Sect. 3.13; White 1976; Figs. 3.29, 5.22–5.25). If the mantle is ex-
tremely fine-grained and the mechanism by which it formed is
uncertain, the term mortar structure has been used instead (Spry Fig. 3.38. Illustration of the process of grain boundary area reduction
1969). However, this term has a genetic implication as “mechani- (GBAR) through grain boundary adjustment and grain growth, result-
cally crushed rock” and its use is therefore not recommended. ing in a decrease in grain boundary energy. Irregular grain boundaries
formed during deformation and dynamic recrystallisation are straight-
A completely recrystallised fabric may be difficult to distin-
guish from a non-recrystallised equigranular fabric. However, ened to a polygonal shape, and some small grains are eliminated
in an aggregate of grains formed by complete dynamic recrys-
tallisation, the grains will show evidence of internal deforma- rock will lead to the approach of an ‘equilibrium-fabric’
tion, a lattice-preferred orientation (Sect. 4.4.2) and a relatively of polygonal crystals with contacts tending to make tri-
uniform grain size (Fig. 3.31). ple junctions with interfacial angles of approximately 120°
Most arguments given above are based on optical microscopy. in three dimensions (Figs. 3.39, 3.40a; ×Video 11.10b–d).
SEM observations also promise to become important to distin-
guish the effect of different mechanisms by precise and quantita- Obviously, this angle can be smaller in oblique cross-sec-
tive characterisation of grain and subgrain size and the frequency tions (Fig. 12.2). Since similar structures form in foam,
and distribution of boundary misorientation (Trimby et al. 1998). e.g. in a beer bottle, the fabric is often referred to as a
foam-structure (Fig. 3.39; ×Video 11.10b–d). Large grains
with many sides tend to increase in size while small grains
3.10 with few sides shrink and eventually disappear during 3.10
Grain Boundary Area Reduction (GBAR) GBAR (×Video 11.10b–e).
Many aggregates where GBAR has been active show
Lattice defects are not the only structures that contrib- slightly curved grain boundaries. Small grains may
ute towards the internal free energy of a volume of rock; have strongly outward curving boundaries (Fig. 3.39,
grain boundaries can be considered as planar defects ×Video 11.10b–e). On close inspection these may con-
with considerable associated internal free energy. A de- sist of many differently oriented straight segments par-
crease in the total surface area of grain boundaries in a allel to crystallographic planes (Kruhl 2001; Kruhl and
rock can reduce this internal free energy (Vernon 1976; Peternell 2002). This curvature may be due to migration
Poirier 1985; Humphreys and Hatherley 1995; Humphreys of the grain boundary in the direction of the centre of
1997; Kruhl 2001; Evans et al. 2001). Straight grain bounda- curvature during GBAR (Vernon 1976; Shelley 1993).
ries and large grains are therefore favoured and any poly- However, care must be taken when applying this princi-
crystalline material will strive towards a fabric with large, ple to deformed rocks since in SGR and GBM recrystal-
polygonal grains with straight boundaries to reduce the lisation, new grains can have curved boundaries that mi-
internal free energy (Figs. 3.38, 11.6b,c; ×Videos 3.38, grate away from the centre of curvature (Figs. 3.26, 3.34).
11.6a,b). We call this process of grain boundary mi- If there is a correlation between grain boundary en-
gration resulting in grain growth and straightening of ergy and the orientation of the crystal lattice, minerals
grain boundaries grain boundary area reduction (abbre- are said to be anisotropic with respect to grain boundary
viated GBAR). The reduction in internal free energy energy (Vernon 1976). Minerals like quartz, olivine, feld-
gained by GBAR is generally much less than that gained spars, cordierite, garnet, carbonates, anhydrite and sul-
by GBM or SGR recrystallisation. Therefore, although phides are weakly anisotropic; the effect is hardly visible
GBAR occurs during deformation its effect is more ob- in thin section but interfacial angles between grain
vious and may become dominant after deformation boundaries in an equilibrium fabric commonly deviate
ceased, especially at high temperature (Sect. 3.11; Bons from 120° (Fig. 3.40a; ×Video 11.10e). Minerals like horn-
and Urai 1992). blende and pyroxene are moderately anisotropic and many
The free energy represented by a grain boundary may grain boundaries are parallel to {110} planes (Fig. 3.40b).
depend on the orientation of the boundary with respect Micas, sillimanite and tourmaline are strongly anisotropic
to the crystal lattice (Vernon 1976; Kruhl 2001). If the de- and show a strong dominance of certain crystallographic
pendence of grain boundary energy on the crystal lattice planes as grain boundaries (Figs. 3.40c, 4.28c); in micas,
is weak for a certain mineral, GBAR in a monomineralic (001) is dominant.