Petrographic microscopy lab - this is our old Nikon E600Pol microscope - a steady old workhorse that's being upstaged by our new Zeiss AxioLab 5 Pol scopes.  This scope is still great for group work, though, because we've hooked it up to a 65-inch screen so people can discuss in real time slides on the stage.  Kaylan Tripp is studying chalcedony samples from a historic Native American jasper quarry.  (1.8 Mb image)

The petrography lab has several kinds of optical microscopes:

Nikon E600Pol petrographic microscope with 24 megapixel Nikon DS-10 digital camera for studying thin section slides of rocks or other crystalline samples

Zeiss AxioLab 5 Pol petrographic microscopes (12) each mounted with 8 megapixel AxioCam 208 digital cameras

Leica fluid inclusion microscope and stage for studying tiny pockets of H2O trapped inside crystals

Zeiss Stemi 508 binocular microscope with 8 megapixel AxioCam 208 digital camera

Crystals interact with polarized light in a directional sense.  In this example, the mineral appears green (bottom left) when the long red chemical bonds in the crystal (upper left) align with the polarized light (blue arrows).  The crystal appears purple (bottom right) when rotated 90º so that the short green chemical bonds align with the light light polarization (upper right). (2.2 Mb original image)

Light is type of wave that oscillates back and forth, kind of like waves on the ocean where the water molecules move up and down while the wave itself moves laterally toward the shore.  Similarly, light waves oscillate perpendicular to the direction the light is shining, but the oscillation vibrates up/down, side-to-side, and everything in between.  We live in a world mostly of unpolarized light.

We can filter light so it vibrates in only one direction.  In this diagram, the light has been filtered to vibrate in an east-west (side-to-side) direction, indicated by dark blue arrowed lines.

The circle represents the view we see looking down a microscope.  The stick-and-ball drawings are the atoms and chemical bonds in a crystal.  Most crystals are "stretched out" in one direction compared to the other.  This crystal has long chemical bonds indicated in red, and short bonds shown in green.  When light passes through the sample, light interacts with the atoms and chemical bonds that are lined up with the vibration.

On the left, the crystal is oriented so the long red bonds are aligned with the blue vibrations of the light, so the optical view (what we see) is of a green crystal.  On the right, the crystal is rotated so the short green bonds align with the blue vibrations of light, so we see the crystal as being purple in color.  Pleochroism is when things are different colors along different directions.  It is only one of many properties we study with polarized light.

(One of the most annoying things I did in that drawing was to draw the crystal shape to be the same as the elongation of chemical bonds in the lattice.  I did that to make it easier for non-chemists to see, but it is the opposite of what actually happens as predicted by the Bravais law.  Sometimes we make oversimplifications to help make a point.  My apologies to the crystal chemists who see this page.)

These two images are of the same place on a slide of molybdenum skarn ore.  The left image was taken using light polarized in only one direction, so shows the true colors of the minerals.  The white band is a quartz vein.  The black stuff is molybdenite (MoS2) and brownish material is diopside pyroxene Ca(Mg,Fe)Si2O6. (1 Mb original)

When light passes through crystals, it can interact with more than one plane of atoms in ways that create artificial colors called interference colors that we can see when we use a second polarizing filter perpendicular to the first.  We call this "cross-polarized" light because the two polarizing filters are oriented cross-wise relative to one another.  Interference colors like this are diagnostic of the mineral and useful for identification. (4.1 Mb original)

If you look closely at the non-black crystals in this image, you should be able to see a subtle difference between greenish crystals and brownish crystals.  These are different grains of the same mineral, but oriented in different directions relative to the light polarization.  When minerals have different colors along different crystallographic directions, we call that pleochroism.  This pyroxene is pleochroic.  The black swath across the photo is the opaque mineral molybdenite.

(5.5 Mb original)

The same view of molybdenite and pyroxene seen with cross-polarized light.  The colors here are optical interference colors, not the true colors of the mineral.  Although they're not true colors, they are very useful in helping us identify the minerals in the sample.  (5.7 Mb original)