guide to thin section microscopy - Mineralogical Society of America

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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. 45
Guide to Thin Section Microscopy Cleavage and fracture Static recrystallization typically produces relatively even-grained aggregates of undeformed grains with straight grain boundaries (recovery texture). Minerals with no pronounced inherent shape anisotropy (e.g., quartz, calcite, feldspar) tend to form polygonal aggregates of equant grains (Fig. 3-17). Evidence for post-deformative recrystallization of minerals with a pronounced shape anisotropy (e.g., micas, amphiboles) is perhaps best observed in rocks where such minerals occur in an originally planar arrangement that had been subjected to later folding. Recovery of the deformed grains in microfold hinges leads to the formation of socalled polygonal arcs (Fig. 3-18). Figure 3-17: Static recrystallization and annealing A-C: Granoblastic-polygonal grain aggregates generated by high-temperature (granulite-facies) static recrystallization or annealing. Garnet (A), spinel (B), orthopyroxene (C), plagioclase (D), quartz (E), calcite (F). Raith, Raase & Reinhardt – February 2012 Figure 3-18: Static recrystallization of minerals with a strong shape anisotropy A-C: Static recrystallization in microfold hinges ("polygonal arcs"). Mica (A), chlorite (B), amphibole (C). 46
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