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 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 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.
© 1994 Mineralogical Society of America. Manuscript received November 24, 1993. Manuscript accepted May 13, 1994. We thank N. N. Pertsev for the section of core no. 485 and R.L. Hervig for discussion of ion microprobe analysis of olivine. We appreciate the comments of D.L. Kohlstedt and an anonymous reviewer on an earlier version of the manuscript. D.S. and G.R.R. acknowledge support from NSF grant EAR-9218980 (to G.R.R.), and the TEM was supported by NSF grant EAR-8903630 (to D.R.V.). The molecular orbital calculations were performed by D.S. at a Gaussian 92 workshop at the San Diego Supercomputing Center.