Near - Infrared and Mossbauer Study of Cation Site Occupancies in Tourmalines

Mbssbauer and near-infrared spectral measurements have been made on several tourmaline minerals representing both the elbaite-schorl and dravite-schorl series. The specimens were characterized by cell dimension measurements, and by partial chemical analyses by electron microprobe. M'ssbauer data for the iron-bearing specimens further substantiated the presence of ferrous and ferric cations in both the larger 9(b) and smaller 18(c) octahedral sites in the tourmaline structure. Quantitative estimates of site occupancies of the ferrous ions were based on relative areas under component doublets in the M5ssbauer spectra, fitted with Lorentizian curves. M'ssbauer spectroscopy also revealed that the oxidation process of ferrous iron in tourmalines is favored in the 18(c) site. Optical/near-infrared spectroscopy indicated that the Fe2+ -+ Fe3+ intervalence transition band occurs at -15000 cm1 in tourmalines, and not at -14000 cm-1 as previously suggested. Component Gaussian curves fitted to tourmaline spectral envelopes conformed with earlier assignments of the 9000 cm-1 band to a crystal-field transition within Fe2+ ions in the tourmaline 9(b) sites. Curve fitting also led to the resolution of an additional peak at 14000 cm-1 , which was attributed to a crystal-field transition within Fe2+ ions in the tourmaline 18(c) sites. Correlations between the M6ssbauer and the electronic spectral data confirmed the assignment of the 14000 cm-1 peak to