Infrared study of OH sites in tourmaline from the elbaite-schorl series

Abstract

Two different behaviors controlled by local lattice environment of crystallization are observed in the IR spectra of polycrystalline natural tourmalines from the elbaite-schorl series in the O-H stretching region (3800-3100 cm-1). The first case is characterized by the presence of three O-H stretching bands, and is observed in elbaite [Na(Li,Al)3Al6(BO3)3Si6O18(OH)4], schorl [NaFe2+3Al6(BO3)3Si6O18(OH)4] and Fe-elbaite. The second case is observed in Li-rich schorl and is marked by four O-H stretching bands. This behavior is due to the presence of Li-rich and Fe-rich domains in schorl crystallized in Li-bearing-pegmatites. Band assignments are discussed using the results of the factor group analysis for a C3v5 crystal structure and considering the interactions between the O-H and the atoms in the Y and Z sites in the crystal. The interpretation presented differs from previous conclusions.