Tourmalines Paraiba: the spectroscopic, crystal chemical and gemological aspects
| M. Taran M.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formationof NAS of Ukraine |
| E. Naumenko National Museum of Natural Historyof NAS of Ukraine |
| О. Andreyev Taras Shevchenko National University of Kyiv |
| Language: Ukrainian |
| Precious and Decorative Stones. No. 3 (89), 2017: 4-9 |
| Abstract Optical absorption spectra, chemical composition and colorimetric characteristics of three gem-quality Cu, Mn-bearing tourmalines from deposits of Brasilia, state Paraiba, widely known among mineralogists, gemologists and jewelers as Paraiba tourmalines, and three similar to them by color Fe-bearing tourmalines from Mawi deposit, Nuristan (Afghanistan), which sometimes appear at the market as Paraiba-type tourmalines, were investigated. As found, in spite of some resemblance by coloration, the nature of the latter in the samples studied is different. Thus, blue and purple colors of the Paraiba-tourmalines are predominantly caused by two chromophore ions, Cu2+ and Mn3+, although some effect to appearance of greenish hues are caused by an admixture of Fe3+. Bluish colors of the tourmalines from Afghanistan are caused by Fe2+ at relatively low concentrations of Fe3+. Copper contents are lower than the detectable level of microprobe. Taking in consideration a certain resemblance by color to the Paraiba tourmalines, such Fe2+-bearing tourmalines from Afghanistan, as well as similar iron-bearing tourmalines for other locations, are proposed to be called pseudo-Paraiba tourmalines. It is found that the means of falsification of the composition by impregnation of the polished surfaces of cut tourmalines with copper by rubbing them with a copper plate in order to represented the stones as Paraiba-type tourmalines, gives some effect of appearance of detectable signalі of copper in X-ray fluorescence spectra, the copper “content” maintains about two orders of magnitude lower than in the real Paraiba tourmalines. |
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