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

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Published/Copyright: February 3, 2026
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American Mineralogist
From the journal American Mineralogist Volume 111 Issue 2

Abstract

The absorption of light by Fe/Ti and Fe/Fe intervalence charge transfer (IVCT) bands has previously been found in aluminum oxide and Al2SiO5 aluminosilicate minerals to decrease markedly at elevated temperatures. Given the abundance of iron at depth in the Earth, assessing the generality with which and extent to which IVCT mineral phases become more optically transparent at temperatures than they are under ambient conditions has potentially significant implications for the modeling of mantle geophysical processes such as radiative conductivity.

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.

Keywords: High-temperature spectroscopy; optical absorption spectroscopy; intervalence charge transfer; enhanced absorption; mixed-valence iron; cordierite; chloritoid; lazulite; dumortierite; jeremejevite; beryl; osumilite; biotite; pargasite; aegirine

Acknowledgments and Funding

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.

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Received: 2024-12-19
Accepted: 2025-06-24
Published Online: 2026-02-03
Published in Print: 2026-02-26

© 2026 Mineralogical Society of America

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https://doi.org/10.2138/am-2024-9724
Keywords for this article
High-temperature spectroscopy; optical absorption spectroscopy; intervalence charge transfer; enhanced absorption; mixed-valence iron; cordierite; chloritoid; lazulite; dumortierite; jeremejevite; beryl; osumilite; biotite; pargasite; aegirine

Articles in the same Issue

  1. Controls on metal zonation in porphyry-skarn systems: Evidence from the Tonglushan Cu polymetallic deposit, Eastern China
  2. Anomalously high REE diaspore formed in bauxite overlying paleokarstic surface
  3. Arsenohauchecornite, ognitite, parkerite, and related minerals from Onça Preta orebody, Carajás, Brazil: Fingerprinting PGE signatures in hydrothermal Ni ores
  4. Quartz-petalite intergrowths in the Yichun pegmatite: Formation from late-stage Li-rich melts and implications for Li mineralization in rare-metal granites
  5. Investigating formation and enrichment processes of the super-large North Sanshandao gold deposit (Jiaodong Peninsula, Eastern China): An in-depth mineralogical perspective
  6. Seismic signature of the upper continental crust: Implications from the thermoelastic properties of liebermannite and K-hollandite II
  7. Scandio-fluoro-eckermannite, NaNa2(Mg4Sc)(Si8O22)F2, a new Sc-dominant amphibole-supergroup mineral from the Bayan Obo deposit, China
  8. New minerals auropolybasite [Ag15AuSb2S11] and auropearceite [Ag15AuAs2S11]: Implications for the formation of bonanza ores in epithermal systems
  9. Incongruent melting of garnet during garnet-spinel transition and its implication for lithospheric exhumation
  10. The optics of a possible new interference figure in mineralogy
  11. Fe2+/Fe3+ intervalence charge transfer and enhanced d-d absorption in mixed-valence iron minerals at elevated temperatures
  12. The first meteoritic ammonium mineral: Discovery of boussingaultite in the Orgueil CI1 carbonaceous chondrite
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