UV/Vis Single-Crystal Spectroscopic Investigation of Almandine-Pyrope and Almandine-Spessartine Solid Solutions: Part I. Spin-Forbidden Electronic Transition Energies, Crystal Chemistry

Charles A. Geiger1, Michail N. Taran2, George R. Rossman3

 1 Department of Chemistry and Physics of Materials University of Salzburg
Jakob Haringer Str 2a
A-5020 Salzburg, Austria

M.P. Semenenko Institute of Geochemistry and Mineralogy and Ore Formation
National Academy of Sciences of Ukraine
Palladin Ave. 34
03142 Kyiv-142, Ukraine
3Division of Geological and Planetary Sciences  
California Institute of Technology
Pasadena, CA  91125-2500, USA


Aluminosilicate garnet is an excellent phase to research solid-solution behavior in silicates. Natural almandine-pyrope, {Fe2+3x,Mg3-3x}[Al2](Si3)12 and almandine-spessartine, {Fe2+3x,Mn2+3-3x}[Al2](Si3)O12, crystals were measured by UV/Vis/NIR (~29000 to 10000 cm-1) optical absorption spectroscopy using a microscope. The spectra and changes in energy of a number of Fe2+ and Mn2+ spin-forbidden electronic transitions of different wavenumber were analyzed as a function of garnet composition across both binaries. The spectra of Alm-Prp garnets are complex and show a number of Fe2+ and Fe3+ transitions manifested as overlapping absorption bands whose intensities depend on composition. There are differences in energy behavior for the various electronic transitions, whereby lower wavenumber Fe2+ transitions decrease slightly in energy with increasing pyrope component and those with 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 higher wavenumber ones. Four narrow and relatively intense Mn2+ spin-forbidden bands between 23000 and 25000 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. Finally, 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 27100 and 27500 cm-1 depending on the garnet composition. Fe3+-O2- bonding is analyzed using Racah parameters. Band-structure calculations are needed to understand the precise nature of electronic transitions in garnet and to interpret better UV/Vis spectra, especially for Alm-Sps garnets.