Features in Thin Section - Crossed Polarizers
Steven Dutch, Natural and Applied Sciences, University
of Wisconsin - Green Bay
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The Cardinal Rule of Thin Sections
What you see in thin section is a random cross-section through a grain whose properties vary in three dimensions. Therefore, always view as many grains of a mineral as possible to get an idea of the total range of properties you observe. Also, bear in mind that you may not get a grain that displays some property to optimum effect.
Interference Color
Extinction
Top: As a grain is rotated, it becomes black every time its privileged directions line up with the polarizers, that is, every 90 degrees. Halfway in between extinction positions, the grain shows its brightest interference colors. Bottom: If the grain goes extinct when its cleavage or crystal faces are parallel to the polarizers, it is said to have straight extinction (left). Minerals that go extinct in oblique orientations have inclined extinction (right). |
Some minerals do not go black at extinction or do not show normal interference colors. Sometimes the privileged directions for different wavelengths have different orientations, so that when the grain is at extinction for red light it still transmits blue light. Sometimes the mineral is isotropic for some wavelenths but not others, so that it may never transmit, say, green light. The colors that result are called anomalous interference colors
Twinning
Undulose Extinction
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Anomalous Interference Colors
Exsolution textures
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Created December 30, 1999, Last Update December 30, 1999
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