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 (Korresp. Autor*in), Michail N. Taran, George R. Rossman

Veröffentlichungen: Beitrag in FachzeitschriftArtikelPeer Reviewed


Aluminosilicate garnet is an excellent phase to research solid-solution behavior in silicates. Natural almandine-pyrope, {equation presented} and almandine-spessartine, {equation presented} 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.

Seiten (von - bis)1149-1160
FachzeitschriftAmerican Mineralogist
PublikationsstatusVeröffentlicht - 1 Juni 2023
Extern publiziertJa

ÖFOS 2012

  • 105113 Kristallographie
  • 105116 Mineralogie