Trichroism and Dichroism in Gems and Minerals  Photos: Jeff Scovil, 2010 (animation: E.Skalwold) Zoisite, var. Tanzanite With 90 degree turns of the polarizing filter on his camera, Jeff Scovil demonstrates the dramatic dichroism seen along this mineral's short lateral axis Its third pleochroic color would be visible looking down the c axis from above (one can see a mixture of colors at the terminations in the angle seen in the image here). Note the reflection on the table for transmission direction of the filter. | |
 Photo: E. Skalwold "Iceland Spar" A cleavage rhomb of optical quality calcite (origin: Iceland). Shown above are three calcite dichroscope models which make use of Iceland spar's high birefringence to separate the mingled colors traveling through a gemstone. Left to right: RosGem Optics, a brass "antique" similar to that made by GIA and a wonderful OPL version from the very same people who bring you the indispensable Teaching Model hand spectroscope. What's a "rhomb?" Calcite has three directions of perfect cleavage which reflect its three-fold symmetry. When a crystal is broken, it tends to break on these directions forming a rhombohedron or "rhomb." Each cleavage face is inclined at the same angle to the c-axis; none are perpendicular or parallel to the c-axis (in calcite this is also the optic axis direction). This is a very important orientation when considering the doubling of images and is in part the premise on which the dichroscope works. For more on double refraction in calcite, see "Iceland Spar." | |
 Photo: E. Skalwold Objects viewed through the calcite appear double. Each image represents one of the rays of light which has been split off and polarized by passing through the doubly refractive calcite. | |
 Photo: E. Skalwold Construction of a calcite dichroscope. | |
| A dichroscope's deceptively simple design consists of a tube with a small aperture at one end which allows light transmitted through a transparent colored mineral to enter the instrument. Within the dichroscope there is a smaller version of the above calcite rhomb mounted with a glass prism at each end to direct the split rays directly towards the viewing window. By looking through the lens at the opposite end of the tube one will see an image of the two ray colors side by side. Because the rays are plane polarized at right angles to each other, by marking the vibration direction on the rim of the eyepiece, the observer can glean important directional information while observing the often times amazing colors of pleochroic minerals. This can be accomplished by observing polarized light generated by reflection, remembering that the light is polarized parallel to the reflecting plane as seen on the surface of Dr. Nassau's book in the image below. | |
 Photo: E. Skalwold Using the dichroscope to observe polarized light generated by reflection. | |
| It should be noted that no pleochroism occurs looking along the optic axis of the mineral (one in uniaxial, two in biaxial) and none will be visible if the vibration directions of the rays are at 45 degrees to those of the calcite. Therefore, in order to find the maximum pleochroic effect, it is very important to check a specimen in all directions while also rotating the dichroscope (see examples). | |
  Photos: E. Skalwold Trichroism in cordierite, variety iolite. Top Image - Left side: beta ray; Right side: gamma ray Bottom Image - Left side: beta ray; Right side: alpha ray For more information in the context of a historical theory, see: Iolite | |
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 Photo: E. Skalwold Apatite, Slyudyanka, Baikal Region, Russia. Top row: using non-polarized light source; Left: using a crystal as a polarizing filter. Right: looking down the c-axis. Bottom row: the polarizing filter in place (vertical transmission). (After Brian Jackson) | |
| This strongly pleochroic apatite is a uniaxial mineral which crystalizes in hexagonal system. Brian Jackson, Head Curator of Mineralogy at the National Museum of Scotland, first told me about these extraordinary specimens when we met at the British Gemmological Association's Centenary Conference in 2008. He has since presented the findings of his research into their properties at conferences and in publications. In regards to crossing two such crystals (as seen at left in the image above), he writes: "Unpolarized light passes from behind the crystal at the back. A second crystal is placed in front and orientated at right angles to the first. The crystal at the back polarizes the light by absorption and thus the crystal at the front only allows a single polarized ray through and that is orientated in the direction of the blue ray." | |
 Photo: E. Skalwold Top: non-polarized light source; bottom: polarizing filter in place The filter is oriented for vertical transmission. Dark blue = extrordinary ray; yellow = ordinary ray (After Brian Jackson) | |
| A sphere is an excellent form in which to explore the nature of light traveling through a mineral. Looking along the optic axis of a uniaxial quartz sphere - or even better, one of calcite - no doubling of images is observed. This is also true when viewing along the "equator" 90 degrees to the optic axis. Though this is the direction of maximum birefringence, the two rays are travelling in exactly the same path, one behind the other; maximum doubling of images occurs at 45 degrees to the optic axis (Sturman, 2002). A side note about that quartz sphere: with its much lower birefringence, in order to have the same separation of images shown by calcite, quartz must be 15 times thicker. In the case of pleochroism, maximum effect is seen at 90 degrees to an optic axis and while at the same time the vibration directions of these rays are exactly parallel with those of the calcite. | |
  Photo: E. Skalwold Quartz and Axinite spheres. | |
At this point you may be wondering why there are even two or three colors at all. First, keep in mind that that the pleochroism one sees is intimately related to optical orientation and specific causation(s) of color (Dyer and Gunter, 2007). As illustrated above and at "Iceland Spar," light entering a doubly refractive (anisotropic) mineral is split into two plane polarized rays by the crystal lattice of the mineral. From W. Revell Phillips:
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| The causation of color and phenomena are often the result of several factors acting in concert (see Color Change) Consider the problem given to me almost 2 years ago by Professor Bassett when devoloping the Viking Navigation teaching module (see iolite and iolite cubes): "a crystal must be anisotropic in order to show pleochroism and a crystal must be anisotropic to show birefringence. But a crystal needn't be birefringent in order to show pleochroism. It could have a birefringence of zero and still show strong pleochroism." This gave meaning to the quote by Dr. Bloss at the top of this page! For example, composition alters the optical orientations of andalusite while its crystal symmetry remains orthorhombic; at one point in its series Mn-andalusite is optically isotropic, but still exhibits strong yellow/green pleochroism when rotated in polarized light (beautifully demonstrated in a little movie by Dyar and Gunter,2007). This is just an example of how complex pleochroism can be and why simple axioms can not be taken too literally. Most important of all is to just enjoy exploring minerals with the dichroscope and discover the beautiful colors which are illuminated with this wonderful instrument. | |
 Photo: E. Skalwold Axinite sphere showing pleochroism in transmitted non-polarized light. The middle view is actually much more purple than shown in this light. Cut by Dr. Allen Bassett (20mm, origin Baja) | |
 Photo: E. Skalwold A slice perpendicular to the c axis of chiastolite with an "open" core. Just rocking back and forth over a non-polarized light source dramatically exhibits the eye-visible pleochroism which andalusite is famous for. Some of the inclusions radiating out from the core show the same color change and add color to the rim which is almost colorless otherwise. Approx. 2.5x2.5x0.5 cm; origin unknown. | |
| See more pleochroic gemstones. References and suggested reading:
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