Atomic arrangements around the O3 site in Al- and Cr-rich oxy-tourmalines. A combined EMP, SREF, FTIR and Raman study

A study of natural oxy-tourmalines belonging to the system oxy-dravite–chromo-alumino-povondraite– oxy-chromium-dravite from the Sludyanka crystalline complex (Southern Baikal region, Russia) was carried out to explore the characteristic vibrational bands in the principal (OH)-stretching frequency and their relations to the O3 anion site of the tourmaline structure. Relevant information was obtained using electron microprobe analysis (EMPA), structural refinement (SREF), infrared (IR) and Raman single-crystal spectroscopy. The studied oxy-tourmalines are characterized by the substitution Al ↔ Cr, which is accompanied by redistribution of Mg over the Y and Z sites. The occurrence of strong correlations between relative peak area intensities for two IR bands at 3,565 and 3,519 cm−1 and cation site populations derived from SREF and EMP data allowed assignment of the band at 3,565 cm−1 to the cluster [YMg ZAlZ(Al,Mg)]–O3 and the band at 3,519 cm−1 to the cluster[YCr Z(Cr,Al) Z(Cr,Al,Mg))]–O3. It appears that the combination of polarized IR and Raman spectra collected with the electric vector E⊥c and E//c may provide a useful characterization of the local (OH) environments around the O3 site of the tourmaline structure.