Spectroscopic studies on natural chloritoids

Abstract

Room temperature and low temperature Mössbauer and optical absorption spectroscopic data on six natural chloritoids characterized by means of electron microprobe and X-ray powder diffraction techniques are presented.

Two narrow quadrupole doublets with widths of 0.25–0.29 mm/s assigned to Fe²⁺ in a relatively large octahedral site and Fe³⁺ in a smaller octahedral site, are observed in the Mössbauer spectra.

Polarized optical absorption spectra reveal three main absorption bands. A broad absorption band at 16,300 cm⁻¹, which is strongly polarized in E‖X and E‖Y and shows a linear increase in integral absorption with increasing [Fe²⁺] [Fe³⁺] concentration product, is assigned to a Fe²⁺+Fe³⁺→Fe³⁺+Fe²⁺ charge transfer transition. This band displays also a temperature dependence different from that of single ion d−d transitions. Two absorption bands at 10,900 cm⁻¹ and 8,000 cm⁻¹ are, on the basis of compositional dependence and energy, assigned to Fe²⁺ in the large M(1B) octahedra of the brucite-type layer in chloritoid.

Combined spectroscopic evidence and structural and chemical considerations support a distribution scheme for ferrous and ferric iron which orders the Fe²⁺ ions in the M(1B) octahedra and the Fe³⁺ ions in the small M(1A) octahedral sites. Both types of octahedra are found in the brucite type layer of chloritoid.