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 3-15. Plastic deformation and recrystallization of quartz
A,B: Subgrain domains that are elongate parallel to c. D: Chess board pattern. D,E: Partially
recrystallized grains showing subgrain domains. F: Static recrystallization of the marginal domain of a
turbid quartz crystal. G,H: Platy quartz in a recrystallized matrix of quartz + feldspar (felsic granulite).
I: Strongly flattened quartz with serrated grain boundaries.
(Photomicrographs - E: Rainer Kleinschrodt, Universität Köln; F,I: Jörn Kruhl, TU München)
Raith, Raase & Reinhardt – February 2012
Deformation by mechanical twinning is restricted to certain minerals (e.g., plagioclase,
calcite). As opposed to growth twins, deformation twins typically show a wedge-shaped
outline (Fig. 3-16 A,B,I).
As the deformation process proceeds, dislocation migration and accumulation of dislocations
along preferred zones (subgrain boundaries) within a mineral grain can lead to the formation
of aggregates of dislocation-poor crystal domains (subgrains; Fig. 3-15 A-E, Fig. 3-16
D,F,G,H). The crystal lattice between adjacent subgrains is tilted by a few degrees only (lowangle
grain boundaries). Between crossed polarizers a distinctly discontinuous change in the
direction of extinction can be observed. (domainal undulatory extinction). Subgrain formation
is particularly common in quartz, but also observed in other minerals. (Fig. 3-16, D,F,G,H). In
quartz, subgrain domains may be elongate in the crystallographic c direction (Fig. 3-15
A,B,D,E). Cross-cutting subgrain patterns do also occur (chess-board pattern; Fig. 3-15 C).
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