Lecture 3- Grain shape & crystallographic fabric
VIEPS/Mainz Microstructure Course
| Lecture 1
| Lab 1 a
| Glossary Table
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
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
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:
Grain boundary 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 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.
Processes causing lattice reorientations:
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
Grain Rotation- rotation of a whole grain will necessarily reorient its lattice
b) Grain Boundary Migration Kamb
c) Diffusional processes - See Bons abstract in BASEL microstructure conference volume for explanation of how this might work.
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
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.
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
4)Dislocation Creep Regimes in Quartz Aggregates
Greg Hirth & Jan Tullis 1992 (JSG 14, 145-159)
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
3) Mobility of grain boundaries increases, grain boundary
migration and rotation recrystallisation both active.
Graph of Creep Regimes in experimentally deformed "as-is"quartzite