What are thin sections?  Thin sections are made from small slabs of a rock sample glued to a glass slide (~1 inch by 2 inches), and then ground to a specified thickness of 0.03mm (30 microns).  At this thickness most minerals become more or less transparent and can then be studied by a microscope using transmitted light. Thin sections are time consuming and costly to prepare.

Thin sections are viewed using a petrographic microscope under two different lighting conditions- plain polarized light and crossed polarizers. Plane polarized light is light that is constrained to a single plane. The light wave is a simple sine wave that has the vibration direction lying in the plane of polarization. When viewing under plane polarized light, a single polarizer (lower polar) is used. Inserting the upper polarizer is referred to as crossed polarizers (or, crossed nicols), the name given because the two polarizing lenses are set at right angles to each other.

For a good look at what someone sees with a polarizing microscope visit this site for their virtual tour http://met.open.ac.uk/vms/vms.htm

Rock-forming materials that are isotropic include glass and minerals such as Fe-Ti oxides (magnetite and ilmenite). Volcanic glass will appear as some shade of color (tan to brown) when viewed under plane polarized light, but will be black when viewed under crossed polarizers. Fe-Ti oxides are black under either condition.

Anisotropic minerals show different colors when viewed under crossed polarizers. The colors seen are called interference colors, which can be used to help identify the minerals. Quartz appears as white when viewed under plane polarized light, and will be white or some shade of gray when viewed under crossed polarizers. Unfortunately, plagioclase is also white when viewed under plane polarized light, and also has similar interference colors. Clinopyroxene will be tan to green when viewed under plane polarized light, and will be a more vibrant color such as blue, pink, or green when viewed under crossed polarizers.

For more information on a polarizing microscope and descriptions of shape and cleavage, interference colours, relief, extinction angle, color and pleochrosim, twinning, opacity, and vibration directions visit this site.  They have excellent descriptions of what we are looking for in a thin sections using the polarizing microscope: http://www.bris.ac.uk/Depts/Geol/opmin/mins.html#microscope

The following photos are thin sections of minerals and rock textures you may encounter in the study of Cassia Mountains ash-flow tuffs. Thin sections of minerals and volcanic microtextures are from the ISU Geology Department, as well as from several Internet sources, with many from the Geology Department of the University of N. Carolina.

Minerals

Augite - Note the pigeonite twin lamellae (thin parallel linear) in this grain. Pigeonite is a Ca-poor clinopyroxene. The view is under crossed polarizers. (The sample is from Georgia.)

Another example of clinopyroxene. This one is in basalt. The view is in plane polarized light.

Quartz- this slide shows quartz in a range of crystal orientations, all showing typical grayish interference colors. View is under crossed polarizers.

Volcanic microtextures

Rhyolite- This crystal-rich rhyolite contains phenocrysts of quartz, K-feldspar (sanidine), plagioclase, and biotite in a fine-grained groundmass. Note the interesting shape of the (partially resorbed? skeletal?) quartz grain in the center of the photograph. Biotite (brown) is in the upper left corner, plagioclase is in the lower left. View is under crossed polarizers.

Scoria is another name for a highly vesicular (almost "frothy") basalt. The black, oval features in this photomicrograph are vesicles. Note the acicular, white plagioclase laths throughout and the white olivine crystal at the lower right. View is under crossed polarizers. Sample is from Hawaii.

Fiamme are pumice and glass that have collapsed and compacted. Fiamme often display flame structures (think of flames in a fire). The elongated white mass in the center is fiamme. It shows flame structures on the left and along the bottom. Note the flattened glass shards draping over minerals.

Spherulites are radiating masses of fibrous crystals in a glassy matrix. These spherulites are probably composed of alkali feldspars and some polymorph of SiO2, and in this cross-polarized shot, appear as round objects with dark crosses. Note the large crystal, which forms the nucleus of one of the spherulites at center-left. View is under crossed polarizers. Sample is from Wyoming.

In this tuff, the irregularly shaped glass shards are still relatively undeformed. Also note the phenocrysts of quartz (clear) and biotite (dark red) in this slide. View is under plane polarized light. Sample is from Nevada.

Poorly welded rhyolitic tuff- in this sample, the glass shards are starting to get deformed. Note the phenocrysts of quartz (clear) and biotite (dark red) in this rock. View is under plane polarized light. Sample is from Nevada.

Welded rhyolitic tuff- in this sample, the glass shards are fused together in a swirly mass, and the large pumice fragment at center right is flattened. In contrast, quartz phenocrysts are relatively undeformed. View is under plane polarized light. Sample is from Nevada.

This photo is of a welded tuff with numerous glass shards and broken plagioclase grains. The view is under plane polarized light.

This photo is showing extinct glass matrix (elongated black shape), lithic clast (lower left), and broken plagioclase grains. The view is under crossed polarizers.

Obsidian vitrophyre - A vitrophyre is another name for a phenocryst-bearing obsidian. The phenocrysts in the photomicrograph are mostly plagioclase. The groundmass is volcanic glass. The view is under crossed polarizers. (Sample is from Montana.)

Volcanic glass that has completely devitrified will often have a felty texture, as shown in this photograph. The minerals that you can see are plagioclase (white), clinopyroxene (shades of green), and Fe-Ti oxides (black). View is under plane polarized light.

Volcanic glass showing perlitic texture (concentric curved fractures). A bleb of glass with clinopyroxene and plagioclase is located below and to the right of the large plagioclase phenocryst.

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This slide shows a texture called granophyric. Granophyric texture is an intergrowth of quartz and alkali feldspar. In this case, the granophyric texture radiates out from large plagioclase grains (lower left-gray, lower right-gray/black). View is under crossed polarizers.

http://www.brocku.ca/earthsciences/people/gfinn/minerals/database.htm

This site has links to a bunch of sites all concerning rocks in thin sections.
http://www.uni-wuerzburg.de/mineralogie/know3.html#thin

This site has pictures of volcanic thin sections.
http://www.geolab.unc.edu/Petunia/IgMetAtlas/volcanic-micro/volcanicmicro.html