부산대학교 도서관

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 , $\left\{\mathrm{Fe}_{3 \mathrm{3}}^{2+} \mathrm{Mg}_{3-3 \mathrm{x}}\right\}\left[\mathrm{Al}_{2}\right]\left(\mathrm{Si}_{3}\right) \mathrm{O}_{12},$and almandine-spessartine, F e 3 x 2 + , M n 3 − 3 x 2 + A l 2 S i 3 O 12 , $\left\{\mathrm{Fe}_{3 \mathrm{x}}^{2+}, \mathrm{Mn}_{3-3 \mathrm{x}}^{2+}\right\}\left[\mathrm{Al}_{2}\right]\left(\mathrm{Si}_{3}\right) \mathrm{O}_{12},$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.