Structural locations of the iron ions in cordierite: a spectroscopic study

Abstract

A series of 21 cordierites with 0.06<Fe²⁺ cations pfu <1.54 from different petrogenetic environments were studied by polarized electronic absorption single-crystal spectroscopy and ⁵⁷Fe Mössbauer spectroscopy (MS). The electronic spectra measured in the range 35,000–1,000 cm^–1 using microscope techniques were also obtained at different temperatures between 80 and 700 K on five different samples. The aim of this study was to answer the still-debated question of the location of iron in the cordierite structure. Both electronic absorption and ⁵⁷Fe Mössbauer spectra confirm the presence of Fe²⁺ on two different structural positions. The major fraction, 90–99% of the total Fe²⁺, occupies the octahedral site 8g in orthorhombic cordierite. Minor amounts of Fe²⁺ occur in a second, non-octahedral site. The octahedral and non-octahedral Fe²⁺ give rise to two MS doublets and to different absorption bands in the electronic absorption spectra, namely ν₁ and ν₂ at about 8,300 and 10,000 cm^–1 both of which are α-polarized for octahedral Fe²⁺, and β/γ-polarized ν₃ at about 10,500 cm^–1 for non-octahedral Fe²⁺. The integral intensities of the ν₁ and ν₂ bands increase linearly with increasing total iron contents. Their energies decrease slightly with increasing Fe²⁺. Increasing temperature causes a shift of ν₁ to lower energies, while the integral intensities of ν₁ and ν₂ increase. These observations permit an assignment of the ν₁ and ν₂ bands to transitions derived from the ⁵T_2g→⁵E_g transition of octahedral Fe²⁺. The integral intensity of the β- and γ-polarized ν₃ band correlates linearly with the concentration of non-octahedral Fe²⁺. Its high molar extinction coefficient, ca. 150 cm^–2 l mol^–1, and temperature independence are best explained by its assignment to dd transitions of Fe²⁺ in tetrahedral coordination. There occurs also a broad band at 18,000 cm^–1 polarized predominantly along b. Its properties are typical of a metal–metal charge transfer (CT) band involving Fe²⁺ on the octahedra and Fe³⁺ on the T₁1-tetrahedra, the two of which are edge-shared. All spectroscopic data, including changes in the electronic spectra caused by heating at 1,000° C in air, as well as crystal-chemical considerations, suggest that the ring-connecting T₁1-tetrahedra contain small amounts of Fe²⁺.