Spectroscopic characterization of Mn-rich tourmalines
Electronic and vibrational spectra of two different tourmalines: green and pink coloured minerals from Minas Gerais, Brazil have been investigated by UV-visible, NIR, IR and Raman spectroscopy. The behaviour of transition metal ions in their electronic spectra is presented. Both minerals show a strong broad band at 300 nm (33300 cm-1) due to Mn(II) ion. In pink tourmaline three sharp bands at 365, 345, and 295 nm (27400, 28990 and 33900 cm-1) are assigned to 6A1g (S) 4T2g (D), 6A1g (S) 4Eg (D) and 6A1g (S) T1g (F) transitions of Mn(II) ion. The pink colour arises from Mn(II) ion while the band at 520 nm comes from Mn(III) ion which acts as an aid to the transmission window from 700-to-900 nm in pink tourmaline. NIR spectroscopy provides evidence for Fe(II) through the observation of two bands at 10240 and 8000 cm-1. The observation of four OH stretching bands in the hydroxyl stretching region 3800 to 3200 cm-1 near 3650, 3600 and 3555 and 3460 cm-1 shows mixed occupancy of octahedral sites in the minerals studied. The band positions in the spectra of green and pink tourmalines differ slightly due to variations in composition. Experimentally, a number of cation-oxygen vibrational modes in the Raman spectra could be observed in the low wavenumber region 1200-to-200cm-1.
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