Synthesis, Crystal Structure, and Raman Spectroscopy of Phase D with Varying Al and Fe Contents at High pressures

Abstract

Dense hydrous magnesium silicates (DHMSs) are key hosts of water in the Earth’s mantle. In this study, we examined the composition, crystal structure, and Raman spectra of phase D synthesized at 20 GPa and 1100 °C with six different starting compositions varying in Al and Fe contents. Our results show that, under these conditions, Fe incorporation in phase D is limited to <0.06 atoms per formula unit (pfu), whereas Al can be incorporated in much higher amounts. Single-crystal X-ray diffraction data, together with previous studies, indicate that the dominant substitution mechanism is Al3+ + H+ = Si4+ when Al < 0.4 pfu. At higher Al contents (>0.5 pfu), both Al3+ + H+ = Si4+ and 2Al3+ = Mg2+ + Si4+ substitutions exist. Using first-principles calculations and ambient-pressure Raman spectroscopy at room temperature, we assigned major Raman vibrational modes for Mg-endmember phase D. We find that 0.15 pfu Al has only a minor effect on lattice-vibration modes, whereas even small amounts of Fe (<0.06 pfu) generate distinct hydroxyl group-related vibrational features. High-pressure Raman measurements up to 30 GPa on Mg-endmember, Al-bearing, and Al-Fe-bearing phase D reveal their pressure-dependent Raman behavior. These results provide new constraints on the incorporation of Al and Fe in phase D and offer insights into water storage in subducted slabs, such as those stagnating in the mantle transition zone beneath the Western Pacific.