Electronic transitions of iron in almandine-composition glass to 91 GPa

Valence and spin states of Fe were investigated in a glass of almandine (Fe3Al2Si3O12) composition to 91 GPa by X ray emission spectroscopy and energy and time domain synchrotron Mössbauer spectroscopy in the diamond anvil cell. Changes in optical properties total spin moment and Mössbauer parameters all occur predominantly between 1 bar and 30 GPa. Over this pressure range the glass changes from translucent brown to opaque and black. The total spin moment of the glass derived from X ray emission spectroscopy decreases by 20. The complementary Mössbauer spectroscopy approaches reveal consistent changes in sites corresponding to 80–90 Fe2+ and 10–20 Fe3+. The high spin Fe2+ doublet exhibits a continuous decrease in isomer shift and increase in line width and asymmetry. A high spin Fe3+ doublet with quadrupole splitting of 1.2 mm/s is replaced by a doublet with quadrupole splitting of 1.9 mm/s a value higher than all previous measurements of high spin Fe3+ and consistent with low spin Fe3+. These observations suggest that Fe3+ in the glass undergoes a continual transition from a high spin to a low spin state between 1 bar and 30 GPa. Almandine glass is not expected to undergo any abrupt transitions in electronic state at deep mantle pressures.