Zoisite Visible Spectra (400 - 1000 nm)

The blue variety, tanzanite, is colored by vanadium. It is highly pleochroic before it is heat-treated for the commercial gem market. Pink zoisite contains Mn³⁺. Green varieties contain Fe³⁺.

Images of the zoisite absorption spectrum

Tanzanite

GRR 1265 spectrum, from Tanzania, normalized to 3.0 mm thick. The color of this crystal varies considerably with the direction in which it is observed. It varies from greenish yellow to deep blue to red-violet. Data Files: E||a=gamma; E||b=alpha; E||c=beta
GRR 2292 spectrum - blue zone, from Arusha Region, Tanzania, normalized to 3.0 mm thick. This crystal consists of two zones, one yellow and the other blue. In polarized light, each of these zones shows distinct pleochroism. Data Files: E||a=gamma; E||b=alpha; E||c=beta.
GRR 2292 spectrum - yellow zone from Arusha Region, Tanzania, normalized to 3.0 mm thick. E||a=gamma; E||b=alpha; E||c=beta.
GRR 2891 spectrum, from the Merelani Hills, Tanzania, normalized to 5.0 mm thick. This sample was collected on the surface and was exposed to the sun for an unknown period of time. The material is bluish and lacks the brown color of most tanzanite when it is mined underground. Yet, it still shows spectacular pleochroism in linerally polarized light. Data Files: E||a=gamma; E||b=alpha; E||c=beta
Literature data from Faye and Nickel (1971) showing the spectrum of a tan colored zoisite (Tanzanite) before and after heating to 600°C
Literature data from Koziarska et al (1994)

Heated Tanzanite

GRR 3045 spectrum before heating, 5.747 mm in the common, naturally occuring brown color .
GRR 3045 spectrum after heating, 5.747 mm when it develops other colors as shown in the three polarizations.
GRR 3045 spectrum difference showing the loss that occurs when the sample is heated to 500 C.

Pink Zoisite

CIT 16781 spectrum, from Merelani Hills, Lelatema Mountains, Arusha Region, Tanzania, normalized to 3.0 mm thick. The color of this material is strongly pleochroic.
GRR 3495 spectrum from the Tanzanite mine. This sample is pale pink in color.
Synthetic manganese(3+)-rich zoisite, variety "thulite", digitized from Langer et al. (2002) normalized to 1.0 mm thick. This synthetic sample demonstrates that Mn³⁺ bands appear in many tanzanite samples. Data Files: E||a=gamma; E||b=alpha; E||c=beta

Green Zoisite

CIT-16940 spectrum, from Alchuri, Shigar Valley, Pakistan, normalized to 5.0 mm thick. This material also shows pleochroic colors, but not as spectacular as those of Tanzanite. Data Files: E||a=gamma; E||b=alpha; E||c=beta

Pale Yellow Zoisite

CIT 17283 spectrum, from the Merelani Hills, Tanzania, normalized to 10.0 mm thick. This crystal looks yellow with a hint of green to the eye but also shows significant pleochroism. Data Files: E||a=gamma; E||b=alpha; E||c=beta

Irradiated Zoisite

GRR 3073 spectrum from Alchluri, Pakistan, 4.562 mm thick
GRR 3073 spectrum after it was irradiated with gamma rays

Manganese-containing Zoisite - the variety thulite

Literature data from Langer (2002); 1 mm thick

Link to references about the spectra of zoisite

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last updated: 20-Dec-2014
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