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UV/Vis single-crystal spectroscopic investigation of almandine-pyrope and almandinespessartine solid solutions: Part I. Spin-forbidden Fe2+,3+ and Mn2+ electronic-transition energies, crystal chemistry, and bonding behavior

  • Charles A. Geiger ORCID logo , Michail N. Taran and George R. Rossman
Published/Copyright: May 31, 2023
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American Mineralogist
From the journal American Mineralogist Volume 108 Issue 6

Abstract

Aluminosilicate garnet is an excellent phase to research solid-solution behavior in silicates. Natural almandine-pyrope, F e 3 3 2 + M g 3 − 3 x A l 2 S i 3 O 12 , and almandine-spessartine, F e 3 x 2 + , M n 3 − 3 x 2 + A l 2 S i 3 O 12 , crystals were measured by UV/Vis/NIR (~29 000 to 10 000 cm–1) optical absorption spectroscopy using a microscope. The spectra and changes in energy of several Fe2+ and Mn2+ spin-forbidden electronic transitions of diferent wavenumber were analyzed as a function of garnet composition across both binaries. The spectra of Alm-Pyp garnets are complex and show several Fe2+ and Fe3+ transitions manifested as overlapping absorption bands whose intensities depend on composition. There are diferences in energy behavior for the various electronic transitions, whereby lower wavenumber Fe2+ transitions decrease slightly in energy with increasing pyrope component and those of higher wavenumber increase. The spectra of Alm-Sps solid solutions show both Fe2+ and Mn2+ spin-forbidden bands depending upon the garnet composition. The variations in energy of the different wavenumber Fe2+ transitions are unlike those observed in Alm-Pyp garnets. The three lowest wavenumber electronic transitions appear to vary the most in energy across the Alm-Sps join compared to those at higher wavenumber. Four narrow and relatively intense Mn2+ spin-forbidden bands between 23 000 and 25 000 cm–1 can be observed in many Sps-Alm garnets. Their transition energies may increase or decrease across the join, but scatter in the data prohibits an unequivocal determination. A consistent crystal-chemical model and Fe2+-O bond behavior, based on published diffraction and spectroscopic results, can be constructed for the Alm-Pyp binary but not for the Alm-Sps system. The spectra of the former garnets often show the presence of high-wavenumber spin-forbidden bands that can be assigned to electronic transitions of Fe3+ occurring at the octahedral site. The most prominent band lies between 27 100 and 27 500 cm–1 depending on the garnet composition. Fe3+-O2– bonding is analyzed using Racah parameters. State-of-the-art electronic structure calculations are needed to understand the precise physical nature of the electronic transitions in garnet and to interpret better UV/Vis/NIR spectra.

Keywords: UV/Vis/NIR spectroscopy; garnet; solid solutions; electronic spin-forbidden transitions; crystal chemistry

Funding statement: This research was supported by a grant to C.A.G. from the Austrian Science Fund (FWF: P 30977-NBL). He also thanks the “Land Salzburg” for financial support through the initiative “Wissenschafts- und Innovationsstrategie Salzburg 2025.”

Acknowledgments

Several different individuals and sources provided garnet samples used in this study. In the cases where they are known, they are listed in Tables 1a and 1b. We thank the various institutions and individuals for their generosity. We also thank O.A. Vyshnevskyi (Kyiv) for making microprobe EDS analyses on the samples studied spectroscopically in Kyiv and M. Grodzicki (Salzburg) for helpful discussions on crystal/ligand field theory. U. Hålenius and an anonymous referee are thanked for their useful comments that improved the manuscript.

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Received: 2022-02-22
Accepted: 2022-07-27
Published Online: 2023-05-31
Published in Print: 2023-06-27

© 2023 by Mineralogical Society of America

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https://doi.org/10.2138/am-2022-8499
Keywords for this article
UV/Vis/NIR spectroscopy; garnet; solid solutions; electronic spin-forbidden transitions; crystal chemistry

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  2. Incorporation of chlorine in nuclear waste glasses using high-pressure vitrification: Solubility, speciation, and local environment of chlorine
  3. Experimental constraints on miscibility gap between apatite and britholite and REE partitioning in an alkaline melt
  4. Thermal expansion of minerals in the tourmaline supergroup
  5. Viscosity of Earth’s inner core constrained by Fe–Ni interdiffusion in Fe–Si alloy in an internal-resistive-heated diamond anvil cell
  6. The distribution of carbonate in apatite: The environment model
  7. Low-temperature crystallography and vibrational properties of rozenite (FeSO4·4H2O), a candidate mineral component of the polyhydrated sulfate deposits on Mars
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  15. A novel method for experiments in a one-atmosphere box furnace
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