Fe²⁺/Fe³⁺ intervalence charge transfer and enhanced d-d absorption in mixed-valence iron minerals at elevated temperatures

A broad experimental survey of the optical absorption spectra at elevated temperatures of various mixed-valence iron minerals has been conducted. The minerals considered here are cordierite, chloritoid, lazulite, dumortierite, jeremejevite, beryl, osumilite, biotite (mica), pargasite (amphibole), and aegirine (pyroxene). All samples transiently lose significant Fe/Fe IVCT feature intensity at elevated temperatures. In beryl, osumilite, biotite, pargasite, and aegirine, spin-allowed Fe²⁺d-d features also decrease in integral intensity at higher temperatures; in all but beryl, the intensity loss is significant. This trend is consistent with d-d band enhancement via Fe²⁺/Fe³⁺ exchange coupling, which has not previously been identified in the majority of these minerals. It is contrasted against the behavior of ordinary spin-allowed Fe²⁺d-d bands in non-IVCT minerals forsterite (olivine) and elbaite (tourmaline). The depletion of Fe/Fe IVCT and enhanced Fe²⁺d-d band intensity at elevated temperatures may both be important mechanisms by which iron-bearing mineral phases become more optically transparent under conditions at depth.

Samples used in this study were obtained from Rock Currier, Herb Obodda, Natural History Museum of Los Angeles County, Lee Groat, the Smithsonian Institution, Ataide de Oliveira, Jack Huneke, Jonathan Berg, Quan Bai, and Helen Tuttle. This research was supported by grant EAR-2148727 from the National Science Foundation and by the White Rose Foundation. William Palfey is acknowledged for assistance with sample preparation, Eran Funaro and Mike Baker for help modifying our equipment to optimize it for these studies, and Paul Asimow for scanning electron microscope/electron microprobe library data. Emmanuel Fritsch and Maxence Vigier are thanked for discussions of IVCT minerals, Glen Waychunas for discussions of experimental best practices, and Claire Marshall, Michail Taran, and Ulf Hålenius for providing kind and helpful feedback to improve this manuscript as well as future experiments.