H. Cynn et Am. Hofmeister, HIGH-PRESSURE IR-SPECTRA OF LATTICE MODES AND OH VIBRATIONS IN FE-BEARING WADSLEYITE, J GEO R-SOL, 99(B9), 1994, pp. 17717-17727
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.