Mössbauer and infrared spectroscopic studies of Belgian chloritoids

Abstract

Three chloritoid samples from the Stavelot massif (Belgium) and one sample from the Serpont massif (Belgium) have been characterized by chemical analyses and differential X-ray diffraction. A classification of chloritoid is proposed. Mossbauer spectra at temperatures between 78 and 360 K and in external magnetic fields were obtained for a triclinic and for a monoclinic specimen. The spectra show a superposition of a weak Fe3+ doublet (less than 10%) and an intense Fe2+ doublet. It is found that a decomposition of the ferrous absorption into two distinct quadrupole doublets leads to smaller deviations between experimental and calculated line shapes, especially at low temperatures. This suggests that Fe2+ is present in both cis and trans O2(OH)4 octahedral positions in the trioctahedral layer. However, the structural data derived from the temperature dependence of isomer shifts and quadrupole splittings, are found to be inconsistent with known crystallographic data. It is therefore concluded that Fe2+ is present in only one type of lattice site and that the numerically imposed decomposition into two ferrous doublets is merely an artifact due to thickness saturation effects and to the distributive character of the hyperfine parameters. The negative sign of the electric field gradient further confirms the assignment of the Fe2+ doublet to a cis octahedral configuration. Finally, only minor differences between the Mossbauer results for triclinic and monoclinic chloritoid are observed. The infrared absorption spectra of the four samples are almost identical except in the region around 600 cm−1 at which the monoclinic phase exhibits two absorption bands instead of one band for the triclinic samples. All absorption bands can be well assigned to the different vibrations. Inter-layer hydrogen bonding is evidenced by the occurrence of two vO-H absorption bands. Furthermore, the specific nature of the infrared spectra enables a fast identification of chloritoid samples.