A STUDY OF BERNALITE, FE(OH)(3), USING MOSSBAUER-SPECTROSCOPY, OPTICAL SPECTROSCOPY AND TRANSMISSION ELECTRON-MICROSCOPY

To study the crystal chemistry of bernalite, Fe(OH)(3), and the nature of the octahedral Fe3+ environment, Mossbauer spectra were recorded f rom 80 to 350 K, optical spectra were recorded at room temperature and a sample was studied using transmission electron microscopy. The Moss bauer spectrum of bernalite consists of a single six-line magnetic spe ctrum at 80 K. A broadened six-line magnetic spectrum with significant ly less intensity is observed at higher temperatures, and is attribute d to a small fraction of bernalite occurring as small particles. The v ariation of hyperfine magnetic field data for bulk bernalite with temp erature is well described by the Weiss molecular field model with para meters of H-0 = 55.7 +/- 0.3 T and T-N = 427 +/- 5 K. The centre shift data were fitted to the Debye model with parameters delta(0) = 0.482 +/- 0.005 mm/s (relative to alpha-Fe) and Theta(M) = 492 +/- 30 K. The quadrupole shift is near zero at 300 K, and does not vary significant ly with temperature. Absorption spectra in the visible and near infrar ed range show three crystal field bands of Fe3+ at 11300, 16000 and 23 200 cm(-1), giving a crystal field splitting of 14 570 cm(-1) and Raca h parameters of B = 629 cm(-1) and C = 3381 cm(-1). Infrared reflectio n spectra show two distinct OH-stretching frequencies, which could cor respond to two structurally different types of OH groups. A band was a lso observed at 2250 cm(-1), suggesting the presence of molecular CO2 in the large cation site. Analytical transmission electron microscopy indicates that Si occurs within the bernalite structure as well as alo ng domain boundaries. Electron diffraction and imaging show that berna lite is polysynthetically twinned along {100} planes with twin domains ranging from 3 to 20 nm in thickness. Results are discussed with resp ect to the nature of the octahedral Fe3+ site, and compared with value s for other iron oxides and hydroxides.