Back 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 with the calcite dichroscope, 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 |
 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. |
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