Polarized-Light Petrographic Microscopy

Petrography lab with Kaylan using microscope

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)

Petrographic microscope lab

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

Diagram illustrating how polarized light reacts differently with different crystal lattice planes

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)

Why use polarized light?

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.)

Polarized light microscope image of molybdenum ore

Granite porphyry from Qiushuwan molybdenum deposit viewed under cross-polarized light.  Photo taken by David Muller (4.5 Mb original image)

Qiu shu wan molybdenum skarn

Two really excellent research students worked with me studying the Qiushuwan molybdenum deposit (David Muller and Michael Perrotta - both were phenomenal!).  The Qiushuwan deposit formed when a molten magma injected into rock about a mile underground, forming a big magma-filled cave called a magma chamber.  As the magma cooled, it began to form crystals that would eventually become an igneous rock called granite.  Before it crystallized completely, however, the magma started bubbling hot, salty water which rose to the top of the chamber and chemically reacted with the surrounding rocks to deposit minerals rich in molybdenum (a metal that strengthens steel and makes it more resistant to corrosion).  This image is an example of what one of those granite rocks looks like with a polarized-light microscope. (4.5 Mb original)

Petrographic microscope slide of quartz vein cutting molybdenum ore

Plane-polarized light

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)

Petrographic microscope slide of quartz vein cutting molybdenum ore

Cross-polarized light

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)

Plane-polarized image of pyroxene with molybdenite

Plane-polarized light

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)

Cross-polarized image of pyroxene with molybdenite

Cross-polarized light

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)

Prairie Agate

Plane-polarized light

Agate is a very fine-grained intergrowth of microscopic quartz crystals.  The crystals typically grow in a radiating pattern from nucleation points resulting in bulbous blobby surface.  In this plane-polarized light image, you can see the true nearly colorless nature of the crystals, their radial crystal pattern, and see how different generations of quartz grew on top of earlier generations.

Cross-polarized light

Under cross-polarized light, we can see the interference colors of quartz (gray to yellow), and see the radial nature of growths, but it's harder to see how one layer grows on top of another.

Cross-polarized light with gypsum plate

When we insert an additional slide of gypsum in the light path, we get accentuated interference colors.  From this, we can tell the orientations of the individual crystallographic directions of each tiny needle-like crystal!

Each type of lighting tells us something different about the sample.  Diversity is good.