guide to thin section microscopy - Mineralogical Society of America

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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 47
Guide to Thin Section Microscopy Twinning Raith, Raase & Reinhardt – February 2012 Figure 3-20. Twinning in minerals (I) A: Staurolite, penetration twin on (320). B: Kyanite, simple twin on (100). C: Titanite, simple twin on (100). D: Simple and lamellar twins in chondrodite grains. E-G: Simple and lamellar twins on (100) in augite. H: Pigeonite twin on (100) inverted to orthopyroxene with augite exsolution lamellae on (001). I: Amphibole, simple twin on (100). J: Wollastonite, twins on (100). K: Epidote, lamellar twins on (100). L: Margarite, simple twins on (001). 48
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