Dan Sykes, George R. Rossman
Division of Geological and Planetary Sciences, 170-25, California Institute of Technology,
Pasadena, California 91125, U.S.A.
Daved R. Veblen
Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland 21218, U.S.A.
Edward S. Grew
Department of Geological Sciences, University of Maine, Orono, Maine 04469, U.S.A.
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. H2O contents calculated from the defect stoichiometry and defect density are approximately one-tenth of the total H2O 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 BO4 groups within the olivine structure. Our data represent the first direct evidence of the coupled substitution B(F,OH)Si-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.
|Olivine GRR1677, Tayozhnoye Fe deposit
Plotted for 1.0 mm thick