September 15, 2011
1. An optical property found in many minerals in which a crystal is able to absorb different wavelengths of transmitted light depending on the orientation of the crystal.
2. A useful characteristic for identifying minerals in thin section with an optical microscope.
3. A color-shifting mirage that adds extra pizazz and pop to certain gemstones.
Pleochroic crystals are the chameleons of the mineral world. Pleochroic means “many-colored” in ancient Greek, and pleochroic crystals certainly live up to their name, changing colors– often dramatically– when viewed from different directions. For example, the pleochroic mineral zoisite can appear clear or yellow or pink. The mineral tanzanite ranges from rich purple to rich blue, although gem-quality tanzanite is often heat treated to remove pleochroism and increase the brilliant blue hue. Some pleochroic minerals do not change color exactly but rather display different shades of the same mineral. For example, kunzite ranges from light to dark pink.
The color-shifting nature of pleochroic minerals is revealed through transmitted light, not reflective light. So, while many minerals are pleochroic, this pleochroism is only visible if the mineral is thin or clear enough for light to be transmitted. Thus, for many minerals pleochroism is only observable in translucent, gemstone-quality specimens or when a crystal is sliced thinly enough to allow the transmission of light through the crystal.
Geologists often examine pleochrosim in crystal or rock slices viewed under the microscope. The slices are generally 30-40 micrometers thick and are appropriately named “thin sections.” To increase pleochroism and other optical properties of minerals, these thin sections are usually viewed with a polarizing microscope, which orients (polarizes) the light in a very bright bulb placed below the thin section. These microscopes use both plain polarized light and cross-polarized light. I won’t go into too many details about optical mineralogy here. Thick reference guides and much practice under the microscope are needed to fully understand optical mineralogy, which many geology students find intimidating in these days of button-pressing, mechanical mineral identification. However, optical mineralogy still has many uses and is often a quick way to identify minerals. So, I highly recommend taking a course if you have the opportunity. All you need to know for now is that pleochroic minerals display different colors in thin section when rotated under plain polarized light. The pleochroism of a crystal– the colors displayed and the angles at which the colors change– is a very useful characteristic that geologists can use to identify minerals in thin section. Similar-looking minerals can often be distinguished by differences in pleochroism.
To give you a sense of what pleochroism looks like under the microscope, Shawn Wright of the geoblog Vi-Carius kindly sent me some wonderful sets of pictures displaying pleochroism in minerals viewed from different angles in plain polarized light using an optical microscope. In the set of pictures below, the mineral biotite changes color from dark brown to black when the thin section is rotated:
And in the set of pictures below, the mineral hornblende changes from brown to dark gray when the thin section is rotated:
In the hornblende pictures above, the large crystal of hornblende is actually a composite crystal of hornblende which formed at slightly different orientations. One section of the large horblende crystal is brown at the same time that another section has turned dark gray. This “composite pleochroism” is commonly observed in thin section and can sometimes provide useful information about crystal structure and growth.
Callan Bentley of Mountain Beltway has also put together a wonderful set of .gif files which illustrate pleochroism in biotite and riebeckite. He has slow and fast versions in his blog post. Here’s the two slow versions:
Not all minerals display pleochroism, however. Some minerals display none, some minerals display two different colors, and some minerals display three different colors. The type of pleochroism (or lack thereof) displayed by a mineral is determined by crystal structure of that mineral.
When light is transmitted through different axes of a crystal, there are three options: the same color is displayed (no pleochroism), two different colors are displayed (dichroism), or three different colors are displayed (trichroism). Because we live in a 3-dimensional world, crystals have three axes: x, y, and z. Or a, b, and c if you (and my mineralogy text) prefer. Minerals which do not display pleochroism have symmetrical crystal axes. That is, the crystal structure is identical along the a, b, and c axes. Minerals which have identical crystal structures along all three crystal axes are known as “isometric” or “cubic” minerals. Because isometric minerals have the same structure in all directions, changing the angle through which light is transmitted does not change the color of the mineral. Minerals with two identical crystal axes and one distinct crystal axis (trigonal/rhombahedral, tetragonal, or hexagonal crystals) can display two different colors. Minerals with three distinct crystal axes (triclinic, monoclinic, and orthorhombic crystals) can display three different colors.
Dichroic minerals can be a little bit tricky to identify in thin section. Because thin sections essentially represent a 2D slice of a 3D mineral, dichroic minerals may or may not display pleochroism in thin section. If the thin section displays two crystal axes which are distinct, then the mineral will display pleochroism. However, if the thin section displays two crystal axes which are symmetric, then the dichroic mineral will not display pleochroism. Similarly, trichroic minerals may display different pleochroism, depending on the orientation of crystals viewed in thin section. Thus, pleochroism is a useful tool not only for identifying minerals but also for identifying which crystal axes are visible in thin section.
Here’s another great set of pictures from Shawn Wright, this time showing tourmaline in thin section. Tourmaline is a trigonal mineral, which means that it is dichroic. When oriented so that two distinct crystal axes are displayed (the first set of pictures), pleochroism is visible, and the mineral color changes from gray to dark blue when the thin section is rotated. However, when oriented so that two symmetrical crystal axes are displayed (the second set of pictures), no pleochroism occurs when the thin section is rotated.
Pleochroism is found in many gemstones, and gemstones are often cut to either display or hide pleochroism. Here’s a great website with many images of pleochroism in gemstones.
“pleochroism, n.” The Oxford English Dictionary. 2nd ed. 1989. OED Online. Oxford University Press. 15 September 2011.