Tourmaline with the Usambara Effect (Optical Spectroscopic Study)

UDC 549.612.1

https://doi.org/10.15407/mineraljournal.37.01.012

М.N. Taran (1), E.V. Naumenko (1), A.V. Andreev (2)

(1) M.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of NAS of Ukraine

34, Acad. Palladina Pr., Kyiv-142, Ukraine, 03680 E-mail: m_taran@hotmail.com

(2) Taras Shevchenko Kyiv National University

90, Vasylkivska Str., Kyiv, Ukraine, 03022 E-mail: andreev@univ.kiev.ua

Tourmaline with the Usambara Effect (Optical Spectroscopic Study)

Language: Ukrainian

Mineralogical journal 2015, 37 (4): 12-21

Abstract: The Usambara effect, i.e., a change of tourmaline color from dark-red to deep-green at thinning of a sample, is studied by optical absorption spectroscopy and colorimetric calculations on a sample of Tanzanian tourmaline of predominant dravite composition with 0.108 a. p. f. u. Cr. As shown, the Usambara effect, by its nature, is tightly related to the alexan drite effect, although in this case the color change is not caused by the change of spectral composition of the light of illumination, but by a specific ratio of light transmission in two "windows of transparency", green and red, and by nonlinear, exponential dependence of light transmittance on the sample thickness. The important circumstance is high enough chromium content to form deep, well shaped windows of transparency in green and red parts of the visible range of optical absorption spectrum. By example of green chromium-bearing tourmaline from Ural (0.357 and 0.196 a. p. f. u. Cr and Fe, respectively) it is shown how admixtures of other chromophore ions, namely, Fe2+ and Fe3+, can suppress the Usambara effect in tourmaline.

Keywords: tourmaline, optical spectroscopy, color, pleochroism.

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