guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Cleavage and fracture
Deformation and recrystallization
Minerals in metamorphic rocks respond in different ways to tectonic stress, depending on prevailing
temperature conditions, deformation rates, level of differential stress and presence or
absence of fluids. Generally speaking, minerals at low temperatures tend to show brittle
behaviour, whereas they deform plastically at high temperature. The temperatures of the
transition between brittle and plastic behaviour are mineral-specific (about 300˚C for quartz,
400-500˚C for feldspars, at deformation rates that are typical for orogenic processes).
Figure 3-13 shows typical microstructures produced through brittle deformation at low
temperature. At the brittle-ductile transition, mineral grains may show features of both brittle
(fracturing, cataclasis) and plastic behaviour (undulose extinction, kinking, bending).
Figure 3-13. Brittle deformation behaviour of mineral grains
A: Domino-style fracturing of sillimanite crystal. B: Cataclastic deformation of quartz. C: Broken and kinked
glaucophane crystals in microfold. D: Cataclastic deformation of plagioclase. E,F: Microboudinage of biotite
and tourmaline. The open fractures are filled with quartz.
(Photomicrographs − B: Michael Stipp, IFM-Geomar Kiel; E,F: Bernardo Cesare, University of Padova)
Raith, Raase & Reinhardt – February 2012
In the polarized-light microscope, plastic crystal deformation is indicated where mineral
grains show bending or kinking of normally planar morphological elements such as cleavage,
rational crystal faces or twin planes (Fig. 3-14). The continuous or discontinuous change of
crystal lattice orientation within a single mineral grain corresponds to a gradual or abrupt
shift of the extinction position as the microscope stage is turned. Even where distinct morphological
elements are lacking (as in quartz), deformation can be recognised from the nonuniform
extinction positions of different domains within a grain (Fig. 3-14 J,L). These deformation
features are caused by stress-induced intracrystalline processes such as dislocation
glide and dislocation creep. Plastic deformation of crystal lattices may be continuous over a
mineral grain or affect discrete grain domains only. Deformation lamellae or translation
lamellae may form along defined glide planes (Fig. 3-14 K; 3-15 H).
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