Octahedral-site Fe2+ quadrupole splitting distributions from Mössbauer spectroscopy along the (OH, F)-annite join

Abstract

We have performed a detailed Mössbauer study of synthetic annites on the (OH, F)-join. Recently developed data treatment and spectral analysis methods were used to extract true intrinsic Fe2+ quadrupole splitting distributions (QSDs) that represent the most information that can be resolved from the spectra. The overall room temperature (RT) QSDs can be consistently interpreted in terms of four QSD contributions (or populations) centered at: QSHH∼2.55 mm/s for Fe2+O4(OH)2 octahedra (cis and trans not resolved), QSHF∼2.35 mm/s for Fe2+∼O4(OH)F octahedra (cis and trans not resolved), QScFF∼2.15 mm/s for CiS-Fe2+O4F2 octahedra, and QStFF∼1.5 mm/s for trans-Fe2+O4F2 octahedra. Each such contribution has a width (σΔ∼0.2 mm/s) caused by distortions of the octahedra. Minor contributions due to Fe2+O5(OH) and Fe2+O5F octahedra probably also contribute to the overall Fe2+ QSDs. The ferric iron spectral components were also characterized. Here, two distinct types of octahedral Fe3+ contributions are seen and interpreted as being due mainly to Fe3+O5OH and Fe3+O5F octahedra, respectively. Tetrahedral Fe3+ is seen only in the OH-annite end-member and the total Fe3+ content drops significantly on addition of F.