HIGH-PRESSURE IR-SPECTRA OF LATTICE MODES AND OH VIBRATIONS IN FE-BEARING WADSLEYITE

High-pressure infrared (IR) spectra of most lattice modes and of hydro us impurities of Fe-bearing wadsleyite were measured from 0 to 23 GPa. At ambient conditions, 29 of the 35 expected IR bands were revealed t hrough a combination of reflection and absorption measurements. Absorp tion spectra of a magnesium end-member at ambient conditions were simi lar. The pressure dependence of eight OH vibrations were established t o 12 GPa. The degree of hydrogen bonding is inferred from the values o f the hydroxyl frequencies and their response to pressure: Five hydrox yls with low nu have negative partial derivative nu/partial derivative P and thus moderate to strong hydrogen bonding, whereas three OH grou ps with high nu have near-zero or positive values of partial derivativ e nu/partial derivative P and thus weak or no hydrogen bonding. This i nformation, the response of the OH bands to Fe/Mg exchange, and X ray data were used to assign the most intense OH bands to the structural s ites, O(1) and O(2). These assignments of hydroxyl vibrations are cons istent with previous theoretical models. Our study of high-pressure OH vibrations in nominally anhydrous wadsleyite indicates that hydrogen impurities in wadsleyite are stable during compression. Our data are c onsistent with the previously observed minor structural change at arou nd 9 GPa which is possibly due to a slight change in the compression m echanism.