57Fe Mössbauer spectroscopic analysis of chlorite

Abstract

Five chlorite samples have been studied by 57Fe Mossbauer spectroscopy at different temperatures. The spectra are consistently described by a superposition of one ferric and three ferrous quadrupole doublets. All hyperfine data exhibit only minor variations from sample to sample, meaning that the structural and electronic properties of chlorite are rather insensitive to compositional changes and local cation disorder. Two of the ferrous doublets are ascribed to cis and trans co-ordinations in the T-O-T layers. The third ferrous doublet is interpreted as being due to ferrous ions in the hydroxide sheets. Its hyperfine parameters are close to those of the former doublets and the observed deviations from the parameter values found for pure brucite are discussed. Approximately 20 to 25 percent of the total iron is situated in the hydroxide sheets. All five chlorites contain about 10 percent Fe3+ which is most probably located on octahedral sites. The temperature dependence of the ferrous quadrupole splittings for two chlorites has been interpreted on the basis of the thermal populations of the two lowest electronic states of the Fe2+ ions. From this, the deformations of the different octahedral co-ordinations could be estimated.