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 (1753 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 (3277 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 (2161 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. A few silicates show small negative relief, but strong negative relief is limited mostly to non-silicates like fluorite.

Becke Line

If a grain is not perfectly in focus, it will often appear to be bordered by a bright line called the Becke Line. The Becke Line is useful for determining which of two neighboring grains has the highest refractive index.

beckepls.gif (4220 bytes) A grain that has greater refractive index than its surroundings will refract and relect light inward like a crude lens.
  • If the focal plane of the microscope is centered within the thin section (purple line) the grain boundary is in sharp focus (left).
  • If the focal plane is too high, rays that would normally appear at the grain boundary now appear inside it and a bright border appears inside the grain (center).
  • If the focal plane is too low, rays that would normally appear at the grain boundary now appear outside it and a bright border appears outside the grain (right).
beckeneg.gif (4233 bytes) A grain that has lower refractive index than its surroundings will refract and relect light outward like a crude diverging lens.
  • If the focal plane of the microscope is centered within the thin section (purple line) the grain boundary is in sharp focus (left).
  • If the focal plane is too high, rays that would normally appear at the grain boundary now appear outside it and a bright border appears outside the grain (center).
  • If the focal plane is too low, rays that would normally appear at the grain boundary now appear inside it and a bright border appears inside the grain (right).

To sum up: As you move away from the thin section (raising the objective or lowering the stage), the Becke Line appears to move into the material with greater refractive index. Although it's not a wholly accurate analogy, picture the grain focusing rays like a lens and picture what happens as you move along the cone of converging rays.

beckep-m.gif (1695 bytes) Minerals may be surrounded by grains of both higher and lower refractive index, so the Becke Line may move both in and out around the margins of a grain.

Carbonates have a maximum refractive index much higher than most rock forming minerals, and a minimum index much lower. It shows high relief all around, but some is positive and some is negative. A lone carbonate grain in a uniform mounting medium will still show a Becke Line that moves in along some parts of the grain boundary and out along others.

Cleavage

cleavg.gif (2795 bytes) Cleavage is much easier to see in thin section than in hand specimen. Cleavage along 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

xlshape.gif (1965 bytes)
  • Grains that show no recognizable crystal form are said to be anhedral (A).
  • Grains that show imperfect but recognizable crystal form are said to be subhedral (B).
  • Grains that show sharp and clear crystal form are said to be euhedral (C).

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Created December 30, 1999, Last Update 02 Oct 2007

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