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 Mssbauer 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 Mssbauer 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 Ȼ and ȼ at about 8,300 and 10,000 cm-1 both of which are α-polarized for octahedral Fe2+, and β/γ-polarized Ƚ at about 10,500 cm-1 for non-octahedral Fe2+. The integral intensities of the Ȼ and ȼ bands increase linearly with increasing total iron contents. Their energies decrease slightly with increasing Fe2+. Increasing temperature causes a shift of Ȼ to lower energies, while the integral intensities of Ȼ and ȼ increase. These observations permit an assignment of the Ȼ and ȼ bands to transitions derived from the 5T2g→5Eg transition of octahedral Fe2+. The integral intensity of the β- and γ-polarized Ƚ 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+.