guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Cleavage and fracture
Figure 3.2-3: Deformation and recrystallisation phenomena
Figure 3-16. Plastic deformation and recrystallization
A,B: Deformed and partially recrystallized plagioclase crystals. C: Deformed and partially recrystallized
orthoclase. D,E: Deformed cordierite showing a chessboard subgrain structure, partially recrystallized to a
polygonal granular aggregate. F: Subgrain domains in sillimanite. G: Subgrain pattern in glaucophane. H: Bent
calcite showing undulatory extinction and subgrain formation. I: Calcite with lamellar deformation twins,
partially recrystallized statically to form a fine-grained polygonal aggregate.
Raith, Raase & Reinhardt – February 2012
Plastically deformed crystals tend to recrystallize at high temperatures, whereby crystal
defects are eliminated. Recrystallized crystals are typically smaller-grained than the original
deformed grains. The individual recrystallized grains show even extinction under crossed
polarizers and have high-angle grain boundaries with the adjacent new grains. Recrystallized
grains may have lobate to straight grain boundaries (Fig. 3-15 D,F,H; Fig. 3-16 B,C,I). Twin
lamellae or inclusions contained in the large deformed crystal are commonly not preserved in
the smaller recrystallized grain aggregate (Fig. 3-15 F; Fig. 3-16 E,I).
Recrystallization of deformed crystals may occur while deformation is active, or under static
conditions. Typical for dynamic recrystallization is a side-by-side occurrence of larger grains
showing undulatory extinction or subgrain structures and domains of finer-grained recrystallized
material (Fig. 3-15 G-I). At high temperatures a relatively large grain-size spectrum and
strongly curved or embayed grain boundaries are commonly observed.
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