ENHANCED H AND F INCORPORATION IN BORIAN OLIVINE

B-containing olivine from the Tayozhnoye Fe deposit (Siberia, Russia) has unusually high H and F contents of 0.012 H and 0.026-0.043 F per f ormula unit of four O atoms. There are a minimum of eight infrared O-H absorption bands, none of which correlates with the known O-H band fr equencies of hydrous magnesium silicates. The O-H band at 3672 cm-1 ha s not been observed in previously published olivine spectra. Transmiss ion electron microscopy indicates substantial replacement of olivine b y clinohumite and disordered mixtures of olivine and humite-group mine rals near the rims of the olivine grains, whereas the interiors of the olivine grains contain regions of unaltered olivine structure. H-2O c ontents calculated from the defect stoichiometry and defect density ar e approximately one-tenth of the total H-2O content estimated from the infrared data. The O-H bands, therefore, are believed to reflect the presence of OH within the olivine crystal structure. Infrared-active s tretching vibrations at 758, 1164, and 1257 cm-1 are consistent with t he presence of [4]BO4 groups within the olivine structure. Our data re present the first direct evidence of the coupled substitution B(F,OH)S i-1O-1 in orthosilicate minerals. As a potential carrier of B, F, and H in the subducted oceanic lithosphere, olivine may play a key role in recycling these elements into the mantle.