Plastic Deformation of Quartz:
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Click image to enlarge |
21. Rotation recrystallization of quartz - Foliation in this sample of deformed quartz from the Borrego Springs mylonite zone,
southern California, is aligned from lower left to upper right. Deformation was at
middle greenschist facies. Most of the image shows a single grain of quartz with
undulatory extinction at the left side, grading into a few patchy subgrains in upper
center of image. To the right, the subgrain patches become increasingly misoriented
with respect to the surrounding lattice, and eventually (after a lattice misfit of
about 15° or more), the boundary between a subgrain and its surroundings becomes
a new grain boundary, visible as a sharp line in both plane light and with crossed
nicols. Several 'rotation-recrystallized' new grains occur at lower center and right
of image. The microstructure is called 'core-and-mantle' structure - an undeformed
core surrounded by a mantle of fine, recrystallized new grains. A distinguishing
factor between rotation-recrystallized and grain boundary-recrystallized new grains
is size: rotation-recrystallized grains are the same size as adjacent subgrains,
whereas grain boundary migration-recrystallized grains are usually much smaller than
any subgrains present (see image # 48).
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22. Rotation recrystallization
of quartz - Foliation in this deformed vein quartz from the Rockfish Valley Fault
Zone, central Virginia Blue Ridge province, is aligned from lower left to upper right.
Deformation was at lower greenschist facies. A large, single, original quartz grain
with slight undulatory extinction is visible at upper left. The lower right part
of the image contains numerous equant subgrains and recrystallized new grains of
the same size as the subgrains. A relict of the quartz 'host' grain for these rotation
recrystallized new grains is visible at lower left.
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23. Rotation recrystallization in quartz
- Same view as image # 22 but with a gypsum plate inserted to illustrate
the close lattice orientation of the recrystallized new grains.
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24. Rotation recrystallization
in quartz - In this sample of deformed tonalite from the Borrego Springs mylonite
zone, southern California, original igneous quartz is deformed (across center of
image from lower left to upper right) between two rigid feldspar grains at top left
and bottom right. Deformation was at middle greenschist facies. At lower left, the
quartz has formed elongate deformation bands that become narrower and better defined
toward the center of image as the lattice misfit increases. Subgrains and rotation
recrystallized new grains of quartz occupy the upper right part of the image.
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25. Rotation recrystallization in quartz
- Same view as image # 24 but with a gypsum plate inserted to illustrate
the close lattice orientation of the deformation bands and of the recrystallized
new grains.
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