guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Double refraction
4.2.3.2 Observation with analyzer inserted (crossed-polarizers mode)
Extinction behaviour: The rotation of a birefringent crystal section between crossed
polarizers involves a periodic change between a bright image and a dark image. A full
rotation of the stage involves four extinction positions separated by 90˚ and four bright
images in between (Fig. 4-23). The four orientations of maximum brightness are also referred
to as diagonal positions.
Figure 4-23. Extinction positions and diagonal positions of a quartz grain during a 360˚
rotation of the stage.
Raith, Raase & Reinhardt – February 2012
In the extinction position the E-W vibrating waves leaving the polarizer are exactly parallel
to one of the two possible vibration directions of the crystal (Fig. 4-23). Hence, the waves are
not split up and pass the mineral without any change in vibration direction as E-W vibrating
waves which propagate with the velocity specific to that direction in the crystal. Taking the
optically uniaxial quartz as an example, either the E- or the O-waves with the refractive
indices n e ' resp. n o are parallel to the polarizer. In the general case of an anisotropic mineral,
the waves are those that relate to the refractive indices n z ' and n x '. After leaving the crystal,
the E-W vibrating waves are blocked by the N-S-oriented analyzer, and the crystal appears
black.
If the crystal is rotated out of the extinction position, the plane of polarisation of the light
entering the crystal is no longer parallel to any of the principal vibration directions in the
crystal (n o , n e ', n z ', n x '). The E-W vibrating waves leaving the polarizer are therefore split up
in the crystal into two orthogonally vibrating waves with refractive indices n e ' and n o resp. n z '
and n x ' in the general case of an anisotropic mineral (Fig. 4-23,24). As light enters the crystal,
the relative amplitudes a 1 and a 2 of the two newly generated waves depend entirely on the
orientation of n x ' and n z ' (or n o and n e ') with respect to the polarizer (Fig. 4-24, bottom row).
At small angles, one of the two waves very much dominates in terms of light intensity. In
the 45° diagonal position the a 1 and a 2 amplitudes are identical. However, without the analyzer
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