Features in Thin Section - Plane Polarized Light

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.

Color

color.gif (2634 bytes) Color in thin section tends to be more consistent than in hand specimen. Most major rock forming minerals are colorless (A). Some have distinctive colors (B). Some minerals like hematite (C) which appear opaque in hand specimen are transparent on thin edges in thin section. The most common truly opaque minerals (D) are metallic oxides (magnetite, ilmenite) and sulfides (pyrite).

Pleochroism

pleochr.gif (4698 bytes) Colored minerals often show different colors in thin section depending on how the grain is oriented relative to the polarizer directions.
  • Top: Most minerals change from lighter to darker as the stage is rotated.
  • Middle: Some minerals change color entirely as the grain is rotated.
  • Bottom: In a few cases the color change is so extreme that the mineral is, in effect, a natural polarizer. Thin slices of tourmaline were often used as polarizing filters before good synthetic filters became available.

Relief

relief.gif (3044 bytes) Relief is the contrast between a mineral and its surroundings due to difference in refractive index. The four grains shown here show increasing relief clockwise from left.

Relief is positive when the grain has higher refractive index than its surroundings, negative if lower. Negative relief compared to quartz, feldspar and normal slide mounting media is relatively rare.

Becke Line

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beckep-m.gif (3358 bytes)  

Cleavage

cleavg.gif (3760 bytes) Cleavage is much easier to see in thin section than in hand specimen. Cleavage alongth the length of the grain is exhibited by many minerals (A). Pyroxenes viewed end on (B) usually show the characteristic 87-degree cleavage, while cross-sections of amphibole show the characteristic 56-degree cleavage (C).

What you see will depend on the orientation of the grain. A true cross-section of an amphibole will show 56-degree cleavages but an oblique section will show other angles and a longitudinal section will show longitudinal cleavage as in (A).

Crystal Form

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Created November 19, 1999, Last Update November 22, 1999

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