FTIR spectroscopic study of natural andalusite showing electronic Fe–Ti charge-transfer processes: zoning and thermal evolution of OH-vibration bands

Abstract

We investigated a natural Brazilian Fe–Ti-containing andalusite and its thermal behavior by polarized infrared and optical spectroscopy. Polarized infrared spectra of the Brazilian andalusite and their evolution at thermal annealing in air clearly evidence that there are several types of OH-groups in the structure. Optical spectra and their evolution with temperature indicate that the incorporated iron (about 0.43 wt% calculated as FeO) is in the ferrous and ferric state. Incorporation of ferrous iron in the Al-sites of andalusite is discussed as a possible incorporation mechanism for hydrogen. The weakening and disappearance of the Fe2+/Ti4+ IVCT band in the andalusite spectra under annealing in air is caused by oxidization of Fe2+ to Fe3+ in Fe2+/Ti4+ IVCT pairs. The process of oxidation is accompanied by a rearrangement of the hydroxyl groups and dehydration of the sample, especially vivid at the final stage of the thermal annealing at 1,200 °C. During thermal annealing, structural hydroxyls of different types apparently transform into each other: the most distinct are the hydroxyls causing the doublet at 3,516 and 3,527 cm−1 (i.e., H bonded to O1) which seem to transform into the hydroxyls causing the line at 3,461 cm−1 (i.e., H bonded to O2). The infrared spectra scanned across differently colored zones of the crystal clearly show that some amount of hydroxyls is related to Fe2+/Ti4+ IVCT pairs which are the cause of the red-to-black coloration of the sample in E||c-polarized illumination: it is evident that in a part of the hydroxyl groups, OH-vector changes orientation aligning directly along crystallographic a-axis due to some kind of interaction with Fe2+/Ti4+ IVCT pairs.