Enhanced H and F incorporation in borian olivine

Abstract

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 formula 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 frequencies of hydrous magnesium silicates. The O-H band at 3672 cm^(-1) has not been observed in previously published olivine spectra. Transmission electron microscopy indicates substantial replacement of olivine by clinohumite and disordered mixtures of olivine and humite-group minerals near the rims of the olivine grains, whereas the interiors of the olivine grains contain regions of unaltered olivine structure. H_2O contents calculated from the defect stoichiometry and defect density are 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 stretching vibrations at 758, 1164, and 1257 cm^(-1) are consistent with the presence of [4]BO_4 groups within the olivine structure. Our data represent the first direct evidence of the coupled substitution B(F,OH)Si_(-1),O_(-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.