guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Twinning
3.3. Twinning
Twins are generated through crystal-structure-controlled intergrowths of two or more
individual crystal segments with a defined symmetrical relationship. Twinning can also result
from deformation (as in calcite). The individual parts of a twinned mineral are intergrown
such that they either mirror each other's orientation (the mirror plane being the twin plane), or
they are rotated against each other by a specific angle (the rotation axis being the twin axis),
or both. The twin interface commonly corresponds to the twin plane.
For many mineral species twinning is an important property for identification. There are
different kinds of twinning such as contact twins, penetration twins, simple twins, multiple
twins, polysynthetic (or lamellar) twins (Fig. 3-19).
In thin section, twinning is commonly easily recognised under crossed polarizers if the
mineral is anisotropic. The individual parts of twinned crystals show different brightness and
interference colour, and on turning the microscope stage different extinction positions are
revealed (Fig. 3-20). There are exceptions, however, as not all types of twins can be recognised
under the microscope. If the indicatrix orientation of the individual parts of twinned
crystals is identical, they are indistinguishable under crossed polarizers (e.g., the most
abundant quartz twins have a twin axis parallel to c, which means the indicatrices are in
parallel alignment. Thus, the twins go into extinction simultaneously).
Raith, Raase & Reinhardt – February 2012
Figure 3-19. Types of twinning
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