guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Extinction
4.2.4 Extinction positions in birefringent crystal sections
Vibration direction and extinction character
In optically anisotropic crystals, crystal symmetry, and in low-symmetry solid solutions also
composition, controls the form and spatial orientation of the indicatrix (Figs. 4-6,7). The
regular spatial relationships imply that the orientation of vibration directions with respect to
morphological elements (crystal faces, crystal edges, cleavage planes, twin planes) is fixed in
birefringent crystal sections. The resulting extinction behaviour allows a distinction between
minerals of higher symmetry (hexagonal, trigonal, tetragonal and orthorhombic) and those of
lower symmetry (monoclinic, triclinic) (Fig. 4-38). Three general types of extinction can be
distinguished:
Straight extinction: the vibration directions lie parallel to the morphological reference
directions (ε = 0˚).
Symmetrical extinction: the vibration directions bisect the angles between two equivalent
morphological reference directions (ε 1 = ε 2 ).
Inclined extinction: the vibration directions form any angle (ε ≠ 0˚, ≠ 90˚) with morphological
reference directions.
Raith, Raase & Reinhardt – February 2012
Figure 4-38. Crystal symmetry and extinction behaviour.
The orientation of the crystals shown here corresponds to the extinction position. The cross in each
figure indicates the two polarizer directions resp. the crosshairs.
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