Precious and Decorative Stones

2016: No. 3 (85) CONTENTS

Gem tourmalines of uvite compositions from Brumado (Brazil): the optical spectroscopy study

M. Taran
M.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of NAS of Ukraine

Ye. Naumenko
National Museum of Natural History of NAS of Ukraine

Language: Ukrainian

Precious and Decorative Stones. No. 3 (85), 2016: 24–28

Abstract
Three color varieties of tourmaline of predominantly uvite compositions from Brumado deposit (Bahia, Brasil), light- green, brownish-yellow and dark-red, were investigated by the methods of optical spectroscopy and microprobe analysis. In the samples studied from chromophore ions only vanadium and iron are present in the amounts high enough to generate the coloration. Thus, the light-green color is caused by V³⁺, which occupies the octahedral Y-sites of the tourmaline structure. The appearing of the brownish-yellow color is caused by a combined influence of V³⁺, Fe²⁺ and Fe³⁺. The two latter in addition to the bands of V³⁺ cause the appearance of the absorption bands of the exchange-coupled [Y]Fe²⁺ — [Y]Fe³⁺ pairs, whereas the intensification of the short-wavelength absorption edge is very likely due to electronic charge-transfer transitions of О^2- ➙ Fe³⁺ type. The dark-red color is caused by high content of iron admixture mainly in form of the exchange- coupled [Y]Fe³⁺ — [Y]Fe³⁺ pairs. From comparison of the obtained optical absorption spectra with literature data one can arrive to the conclusion that all chromophore ions, V 3+ , Fe²⁺ and Fe³⁺, enter only into Y-sites of the tourmaline structure in spite of the fact that a part of Mg²⁺ occupies the smaller and more regular octahedral Z-sites and, in accordance with abundant in natural silicates Mg, Fe³⁺ - distribution, they could be partly substituted, at least, by Fe²⁺, but this is not the case.

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