Espectroscopia de refletância aplicada à caracterização de turmalinas gemológicas provenientes da Província Pegmatítica da Borborema, Nordeste do Brasil

This article aims to evaluate the effectiveness of the reflectance spectroscopy technique applied to Gemology, focusing on the characterization of tourmalines related to pegmatites located in the Borborema Pegmatite Province, Northeast Brazil. Elbaite tourmalines (n=9) were analyzed and classified in four groups based on their colours: (i) green and blue, (ii) turquoise (Paraíba); (iii) red and purple; (iv) colourless. Reflectance spectroscopy data were obtained using FieldSpec™ (ASD) spectroradiometer, as well as, geochemical data, by electron microprobe analysis. Reflectance spectra were investigated visually, and absorption features were interpreted, and related to chromophore elements (Fe, Cu, Mn). The application of the continuum removal technique became possible the correlation between depth of absorption features in the VNIR range, and contents of chromophore elements. Specifically, samples marked by high CuO and FeO contents showed more intense depth of absorption features centered at 920 and 730 nm, respectively. Additionally, diagnostic absorption features of elbaites were identified in the SWIR range, and related to molecular vibrational processes (e.g., OH, H2O, metal-OH). The results of this study show the applicability of the reflectance spectroscopy technique for characterizing tourmalines quickly, resulting in an initial spectral library. This spectral library will facilitate the characterization of other gemological tourmaline in the study region, including the possibility of estimating contents of determined elements.