Structural systematics of hydrous ringwoodite and water in Earth’s interior

Abstract

Seven separate samples of hydrous ringwoodite with compositions ranging from Fo100 to Fo89 and hydrogen contents from 0.2 to 1.1 wt% were synthesized in the 5000 ton multi-anvil press at the Bayerisches Geoinstitut. Synthesis conditions ranged from 18 to 22 GPa and 1400 to 1500 rC. The crystals were characterized by single-crystal X-ray diffraction, electron microprobe, IR and Mossbauer spectroscopy, and by analytical and high-resolution transmission electron microscopy. The crystals are optically isotropic, and the Fe-bearing samples are deep blue in color. Mossbauer spectroscopy and ELNE spectroscopy applied to the Fe-bearing samples indicates about 10% of the iron is in the ferric state. High-resolution TEM examination of one of the Fe-bearing samples indicates that the crystals are homogeneous and free of significant inclusions or exsolution features. Infrared spectra show a broad absorption band extending from about 2500 to 3600 cm –1 with maxima ranging from 3105 for the pure magnesian samples to 3150 cm –1 for the Fo89 samples. The crystal structures of the seven ringwoodite samples were refined by X-ray single-crystal diffraction. Refinement of cation site occupancies indicates full occupancy of the tetrahedral site for all samples, whereas the occupancy of the octahedral site appears to decrease systematically with H content. The principal hydration mechanism involves octahedral cation vacancies. The IR spectra are consistent with protonation of the short O-O approach on the tetrahedral edge, which would imply partial Mg-Si disorder.