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

Abstract

Aluminosilicate garnet is an excellent phase to research solid-solution behavior in silicates. Natural almandine-pyrope, {Fe²⁺₃ₓ,Mg₃₋₃ₓ}[Al₂](Si₃)O₁₂, and almandine-spessartine, {Fe²⁺₃ₓ,Mn²⁺₃₋₃ₓ}[Al₂](Si₃)O₁₂ 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 different 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 differences 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.

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