Plastic Deformation of Quartz:
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Click image to enlarge |
18. Undulatory extinction in quartz -
Bands of undulatory extinction in quartz form a striking pattern in protomylonitic
granodiorite of the Borrego Springs mylonite zone, southern California. Deformation
was at middle greenschist facies. Quartz has formed elongate, ribbon-like grains,
with long grain boundaries trending lower left to middle right on image. Feldspar
(upper left of image) has deformed by brittle failure. The quartz lattice within
each ribbon-grain has undergone deformation by dislocation glide, leading to the
formation of undulatory extinction. Where there is little dislocation climb or other
recovery mechanism operative, the distribution of dislocations causes a smooth and
gradual change in extinction from one part of the grain to another (see also image
#19). Where there has been some recovery, however, the change in extinction angle
occurs more abruptly (for example at lower center of image; see also image
#20). Abrupt changes in extinction across elongate zones within a single grain
are termed 'deformation bands'. Unlike new grain boundaries, these extinction boundaries
are not visible in plane light. Click here to view Flash animation in a new window. Click here to view Flash animation in a new window. Click here to view Flash animation in a new window.
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19. Ribbon-grains in
quartz and biotite - Individual ribbon-like grains of quartz may have formed
from original igneous quartz grains by a process analogous to kinking. These ribbon-like
quartz grains occur in an adjacent sample to image #18, and show smooth undulatory
extinction along their length. There is some grain boundary migration recrystallization
of the quartz along ribbon-grain boundaries at the top and left of image. Biotite
(lower left) has formed ribbon-like grains by kink-band migration (see also images
#66 and #67).
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20.Subgrains in quartz- This example
of deformed quartz in protomylonitic tonalite from the Borrego Springs mylonite zone,
shows polygonal 'patches', the optical manifestation of subgrain formation. Deformation
was at middle greenschist facies. As the lattice plastically deforms, dislocations
sweep through it and produce undulatory extinction. If temperatures are sufficiently
high, or strain rate is low enough, the lattice undergoes strain 'recovery', or organization
of dislocations into ordered, lower energy structures that can be seen optically
as discrete zones across which the extinction angle changes by a few degrees. If
these zones are very elongate, the resulting structures are called deformation bands.
If they are patchy and more or less equant, as in this example, they are 'subgrains',
because they look almost like individual small grains except that their boundaries
are not visible in plane light. Click here to view Flash animation in a new window.
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