"The relationship between pleochroism and crystal structure offers a fertile field for investigation." (Bloss, 1994) Pleochroism Through the Dichroscope Trichroism and Dichroism in Gemstones 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. 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: Atoms experience non-uniform electrical attraction from dissimilar neighboring atoms to become polarized in the direction of of strongest attraction. The plane of maximum polarization is usually the plane of maximum atom population density, and the plane of minimum polarization is usually perpendicular to it. The polarization of an atom strongly influences the velocity of the transmitted lightwaves: the stronger the polarization, the higher the index of refraction, the slower the velocity. Not only do the two rays experience different velocities and plane polarization, but they also experience different absorption while traveling along those paths through the lattice, resulting in different colors (further study using a spectroscope fitted with a polarizing filter will show the nature of the color absorption of each ray in more detail). This last point can be expanded to include several concepts of causation of color affecting pleochroism, including ion coordination environment and associated distortions (epidote), intervalence charge transfer (iolite), and color centers (maxixi beryl). For more detailed explanation, see the references at the bottom of this page. It must be reiterated that 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): "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. The images below were taken with stones resting on a microscope stage fitted with a diffuser. A London dichroscope was used which incorporates two polarizing sheets mounted at right angles to each other for much the same effect as above, though a bit easier to photograph through. Not all of the angles are optimal for maximum contrast, but still the effects are very interesting. It should be noted that while the calcite dichroscope examines the two rays coming from the same exact area of a mineral, because of its construction, the London dichroscope neccessarily samples rays from two different areas.

Photo: E. Skalwold Dichroism in chrome tourmaline (dravite). Left side: extraordinary ray Right side: ordinary ray

Photo: E. Skalwold Pleochroism in andalusite. Left side: gamma ray Right side: alpha ray (beta not shown: green)

Photo: E. Skalwold Dichroism in benitoite. Left side: extraordinary ray Right side: ordinary ray (colorless; almost no absorbtion)

Photo: E. Skalwold Dichroism in purple scapolite. Left side: extraordinary ray Right side: ordinary ray

Photo: E. Skalwold Dichroism in sapphire. Left side: extraordinary ray Right side: ordinary ray

Photo: E. Skalwold Dichroism in ruby. Left side: ordinary ray Right side: extraordinary ray

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

Photo: E. Skalwold Dichroism in cat's eye pezzottaite. Left side: ordinary ray Right side: extraordinary ray Its pleochroism is distinctively different from red, pink or purplish beryls.

Photo: E. Skalwold Trichroism in zoisite, variety tanzanite. Blue and purple of the lateral axis directions are seen; bluish green of c-axis not shown, but present.

Photos: E. Skalwold Trichroism in unheated pink zoisite. When observing the uncut crystal, yellow is seen down the c-axis, dark pink along the longer lateral and pale pink along the shorter lateral axis ("face colors" in unpolarized transmitted light).

Of the transparent zoisite observed, all tanzanite showed purple (or purplish red) / blue / bluish green; green zoisite showed bluish green / yellowish orange / pale pink; yellow zoisite showed strong orangish yellow / greenish blue / pale pink; pink zoisite showed yellow / strong pink / pale pink. Note: heating zoisite can reduce pleochroism as in the case of tanzanite where the yellow, brown or green seen along the c-axis can turn blue upon treatment. This can make the third color harder or impossible to detect. In rare cases the color seen along the c-axis is a deep red. The two other colors parallel to the lateral axes are purple looking along the shorter a-axis and blue looking along the longer b-axis ( the famous Van Pelt portrait of this appears in G&G vol.28 no.2 and as the cover of the Mineralogical Record Vol. 24, no. 3). While living and working in Arusha, Tanzania, Dr. Allen Bassett reported on another interesting zoisite which exhibited pure red/yellowish orange (the optic-axis direction showed pure red in intense fiber optic transmitted light), greenish blue/yellowish orange along one lateral and greenish blue/red along the other. I asked his brother to inquire of Allen if there is a picture still around of this unique crystal, so stay tuned. It was reported in Gems & Gemology Vol. 27 no. 3 as "Unusual red zoisite" under the Gem News section.

References: Anderson, Basil W. (1990) Gem Testing. Rev. by E. A. Jobbins. 10th ed. London: Butterworth. 390 pages. Bloss, F.D. (1971) Crystallography and Crystal Chemistry: New York, Holt, Rinehart, and Winston, 545pages. Bloss, F.D. (1961) An Introduction to the Methods of Optical Crystallography: New York, Holt, Rinehart, and Winston, 294 pages. Dyar, M.D. and Gunter, M.E. (2007) Mineralogy and Optical Mineralogy. Mineralogical Society of America, Chantilly, Virginia, 708 pages. Gunter, M.E. (2003) Calcite, the mineral with the most forms. extraLapis English No. 4. East Hampton, CT. USA. pp 210-222. Kerr, P.F. (1977) Optical Mineralogy. McGraw Hill Book Company, 4th edition. New York, 492 pages. Nassau, Kurt. (2001) The Physics and Chemistry of Color: The Fifteen Causes of Color. 2nd edition Wiley Interscience, Chichester. 481 pages. Phillips, W.R. (1971) Mineral Optics: Principles and Techniques.W.H Freeman and Company, San Francisco, 249 pages. Sturman, Darko and Back, M.E. (2002) Doubling of images in gemstones. Journal of Gemmology, Vol. 28, 4, pp. 210-222. 40-45 Wood, Elizabeth. (1977) Crystals and Light: and Introduction to Optical Crystallography. 2nd ed. Dover Publications, Inc. 156 pages.

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