Description: Ringwoodite [(Mg,Fe) 2 SiO 4 ] is the high-pressure polymorph of olivine stable in the upper mantle between ~525 to 660 km. Information on its temperature-dependent water content and Fe-oxidation state bears important implications on the hydrogen cycle and oxidation state of the Earth’s interior. We conducted several multi-anvil experiments to synthesize iron-bearing (0.11 ≤ x Fe ≤ 0.24) hydrous ringwoodite under oxidizing and reducing conditions. The experiments were performed at 1200 °C and pressures between 16.5 and 18.3 GPa. The incorporation of hydrogen and iron in ringwoodite was studied using Fourier transform infrared (FTIR), Mössbauer (MB), ultraviolet-visible (UV-VIS), and electron energy loss (EEL) spectroscopy. For MB spectroscopy, ringwoodite enriched in 57 Fe was synthesized. The IR spectra of ringwoodite show a broad OH band around 3150 cm –1 and two shoulders on the high-energy side: one intense at 3680 cm –1 and one weak at around 3420 cm –1 . The water content of the samples was determined using FTIR spectroscopy to have a maximum value of 1.9(3) wt% H 2 O. UV-VIS spectra display a broad band around 12 700 cm –1 and a shoulder at 9900 cm –1 representing the spin-allowed dd-transitions of VI Fe 2+ . The weaker band around 18 200 cm –1 is a distinct feature of Fe 2+ -Fe 3+ intervalence charge transfer indicating the presence of Fe 3+ in the samples. EEL spectra yield Fe 3+ fractions ranging from 6(3)% at reducing conditions to 12(3)% at oxidizing conditions. We performed heating experiments up to 600 °C in combination with in situ FTIR spectroscopy to evaluate the temperature-dependent behavior of ringwoodite, especially with respect to hydrogen incorporation. We observed a color change of ringwoodite from blue to green to brown. The heat-treated samples displayed hydrogen loss, an irreversible rearrangement of part of the hydrogen atoms (FTIR), as well as oxidation of Fe 2+ to Fe 3+ evidenced by the appearance of the spin-forbidden dd-transition band for Fe 3+ and the ligand-metal (O 2– -Fe 3+ ) transition band in the optical spectra. An increased Fe 3+ fraction was also revealed by EEL and MB spectroscopy (up to 16% Fe 3+ /Fe). Analyses of MB data revealed the possibility of tetrahedral Fe 3+ in the annealed ringwoodite. These results lead to a reinterpretation of the broad OH band, which is a combination of several bands, mainly [V Mg (OH) 2 ] x ), a weaker high-energy band at 3680 cm –1 ([V Si (OH) 4 ] x ) and a shoulder at 3420 cm –1 ([(Mg/Fe) Si (OH) 2 ] x ).

Description: Valence and spin states of Fe were investigated in a glass of almandine (Fe 3 Al 2 Si 3 O 12 ) 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% Fe 2+ and 10–20% Fe 3+ . The high-spin Fe 2+ doublet exhibits a continuous decrease in isomer shift and increase in line width and asymmetry. A high-spin Fe 3+ 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 Fe 3+ and consistent with low-spin Fe 3+ . These observations suggest that Fe 3+ 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.

Description: We present 27 Al and 29 Si magic angle spinning nuclear magnetic resonance (MAS-NMR) spectra of Al- and Fe-bearing, high-pressure pyroxene and perovskite samples, synthesized in a multi-anvil apparatus at 26 GPa and 1900 °C at targeted compositions of (Mg 1–x Fe x )(Si 1–x Al x )O 3 (x = 0.01, 0.025, and 0.05). 27 Al MAS-NMR spectra of the perovskite samples indicate that Al 3+ replaces both Si 4+ in the octahedral site and Mg 2+ in the larger 12-coordinated site. NMR signal loss caused by paramagnetic interactions is often a severe complication when performing NMR on materials containing Fe 2+,3+ ; however, careful measurement of signal loss and comparison to total Fe content in these samples sheds light on the nature of Al and Fe incorporation. NMR signal loss for the pyroxenes is linearly related to total Fe content as would be expected in the case of uncorrelated substitution of randomly distributed Al and Fe. However, 27 Al signal loss for the perovskite samples increases only slightly between samples with x = 0.01 and 0.025 indicating similar coordination of Al by Fe and non-random distribution. Complete signal loss at Fe/(Fe + Mg) = 0.05 suggests the upper limit of Fe 2+ and Fe 3+ concentration at which useful NMR data can be obtained for this system.