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CET/UWA Microstructure Course

3) Evidence of crystalline plasticity in naturally deformed rocks.
 

These micrographs display a variety of deformation microstructures, for this practical only worry about those which reflect deformation by crystalline plasticity.

a) Quartzite Weak to moderate deformation of a quartzite with minor micas at low metamorphic grade. Original clastic quartz grains show numerous features of intra-crystalline plasticity, including deformation bands, and undulose extinction.

The figure below shows a quartz grain with the trace of the basal plane. Using a stippled pen, show what this grain would look like (ie its extinction characteristics) in crossed polarised light, assuming the polarisers are parallel to the cross. 



Now look at grain A in the top right hand corner, and make a sketch of its basal plane (assuming the dark bands are sub-perpendicular to the basal plane, as they usually are in low temperature deformed quartz).

b) Marble A typical low grade deformed marble, showing extensive twinning. 





The quartz grains embedded in the middle of the calcite appear to be relatively undeformed, why?

Why are the twin planes in the calcite curved?

c) Mt Kosciusko Granite 



The quartz and feldspar in this granite appear to have deformed by quite distinct deformation mechanisms, why?

Place a cross on the generic deformation mechanism map below for both the quartz and the feldspar. On the same diagram show the calcite from the previous picture as an arrow showing its history.




The quartz grains have undergone at least a 10x reduction in grain size, how might this affect the mechanical response of the quartz?