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CET/UWA Microstructure Course

1) Grain shape foliations Grain shape foliations are the preferred alignment of elongate grains (the particle orientations of Panozzo 1983), or the preferred orientation of grain boundaries (the surface orientations of Panozzo 1983).

Grain Orientations   (Particle Orientations)
 

These can be measured by measuring the longest axis of each grain in a thin section.

Grain Boundary Orientations (Surface Orientations)

These can be measured by breaking the grain boundaries into short line segments and measuring the orientation of each segment. For many rocks the two techniques will give similar results, however this does not have to be the case.

2) Processes that affect the development of a grain shape foliation

 
• The picture below shows the competing processes involved in grain shape foliation (GSF) development. Not all of these processes lead to a strengthening of the GSF, see Deformation Mechanisms table for further details.

a) Mica foliations
 
• The two early models for mica foliation development were proposed by March and Jeffreys
 
• March proposed that foliations formed by the micas acting as passive strain markers, ie that they deformed along with the matrix. This  implies that the micas changed shape during deformation.
 
• Jeffreys proposed that the foliations formed by the micas acting as rigid objects that rotated as the matrix deformed. This implies that the micas kept their shape during deformation.
 
• Slaty Cleavage- Slaty cleavages are primary cleavages which are now believed to form by a combination of processes: bend of micas, rigid body rotation, recrystallisation of existing micas and growth of new micas.

• Crenulation Cleavage- Crenulation cleavages are secondary cleavages that develop when a primary cleavage is shortened sub-parallel to its existing foliation plane. Crenulation cleavage development involves micro-buckling of the original foliation associated with rigid body rotation of micas, diffusive mass transfer of quartz away from the limbs to the hinges of the micro-buckles
 
• Pressure Solution Cleavage- The formation of a cleavage as closely spaced stylolites

b) Foliations in non-platy minerals   Pannozzo1983
• Processes causing grain shape changes:

Intra-crystalline processes:
• Dislocation Glide- leads to internal deformation of individual grains, and hence of grain boundaries
• Twinning- leads to internal deformation of individual grains, and hence of grain boundaries
• Climb- can lead to internal deformation of individual grains, and hence of grain boundaries
• NH Creep- leads to internal deformation of individual grains, and hence of grain boundaries
• Kinking- leads to internal deformation of individual grains, and hence of grain boundaries
• Cracking- leads to generation of new grain boundaries, their orientation can be either crystallographically controlled or by orientation of stress field, or some combination of both.
• Rotation Recrystallisation- newly formed subgrains are typically fairly equant, so if these become sufficiently reoriented to become new grains, they will degrade the grain shape foliation
Grain boundary processes
• Grain Boundary migration. Typically reduces the strength of the grain shape foliation, either during deformation, when dislocation density contrasts drive GBM or after deformation where in addition the surface energy driving force may be important.
• Coble Creep- can lead to change of shape of individual grains, and hence of grain boundaries

2) Crystallographic preferred orientations

Crystallographic preferred orientations (CPO), are also known as textures, petrofabrics, fabrics & lattice preferred orientations. The term refers to the observation that in many deformed rocks there is a clustering of crystallographic orientations. The figures below are contoured equal-area stereographic projections of the c-axes of quartz grains showing the relationship of the fabrics wrt the foliation (S) and the lineation (L)
   

Any processes that systematically changes the orientation of a grain, or part of a grain, or changes the size of a grain can potentially lead to a modification to the pattern of orientations.

a) Lattice rotationsEtchecopar 1977, Lister et al 1978

Processes causing lattice reorientations:

Intra-crystalline processes:
• Dislocation Glide- the glide of dislocations on slip planes generally leads to the reorientation of the crystal lattice.
• Twinning- twinning inherently involves the reorientation of tabular volumes within a crystal, and as these tabular bodies can widen and coalesce with subsequent deformation, the whole grain can have its lattice orientation altered
• Kinking- again tabular volumes will be reoriented
• Rotation Recrystallisation- reorientation of sub-grains as a result of addition of dislocations of like sign to sub-grain boundaries
Inter-crystalline processes
• Grain Rotation- rotation of a whole grain will necessarily reorient its lattice

b) Grain Boundary Migration Kamb 1972, Jessell 1988
• Grain boundary migration in deforming rocks is driven by contrasts in dislocation densities. Since grains with different crystallographic orientations will deform using different slip systems, their internal microstructures will be different. In particular the dislocation densities will be at least in part crystallographically controlled, so that there is a crystallographic control on which grains will grow at the expense of their neighbours.
• At low temperatures grains which find it difficult to deform because of their orientation tend not to deform at all, and as a result they have lower dislocation densities, and tend to be preserved as augen grains.
• At higher temperatures, more grains will deform (since the CRSS of all slip systems goes down with increasing temperature), and in this case the grains which deform at the lowest applied stresses will have the lowest dislocation densities. As a result grains which find it hard to deform because of their orientation will be consumed by their neighbours.

 
c) Diffusional processes - See Bons abstract in BASEL microstructure conference volume for explanation of how this might work.

3) Grain Size vs stress relationship

It is a common experimental observation that there is an inverse relationship between the median grain size in a deforming polycrystal and the applied deviatoric stress. This occurs because there is a balance between those processes leading to an increase in grain size, such as grain boundary migration, and those processes leading to a reduction in grain size, such as rotation recrystallisation.

In reality using this relationship can be quite difficult, since metadynamic processes such as grain growth can affect the observed grain size, and even if it appears that no such modification has taken place, we have to be able to demonstrate that the observed grain size was formed during steady-state flow. See Twiss

Greg Hirth & Jan Tullis 1992  (JSG 14, 145-159)and see examples in Tullis et al 2001


1) Dislocation climb difficult, low grain boundary mobilities, high dislocation density contrasts;
leads to dislocation glide accommodated by recovery and grain boundary migration.

2) Rate of dislocation climb increases, rotation recrystallisation dominates.

3) Mobility of grain boundaries increases, grain boundary migration and rotation recrystallisation both active.

Graph of Creep Regimes in experimentally deformed "as-is"quartzite