Fe-bearing phase E coexisting with ringwoodite and wadsleyite has been synthesized at near-geotherm temperatures in hydrous KLB-1 peridotite compositions held at 18 and 19 GPa, and 1400 °C for 27 h. The long heating duration time of syntheses implies that phase E can be a stable component of the mantle under hydrous conditions. Single-crystal X-ray diffraction analyses show that the M1 octahedral site is 72.1–75.2 at% occupied, whereas the M2 and tetrahedral Si sites are 2.4–2.9 at% and 18.9–19.8 at% occupied, respectively. The M1 site occupancies show a positive correlation with Fe/Mg molar ratios, indicating that Fe mainly occupies the M1 site in the phase E structure. High-pressure Raman spectroscopy shows that the framework Raman frequencies of Fe-bearing phase E increase continuously with increasing pressures up to 19 GPa at room temperature, and there is no indication for a major change in the crystal structure. If transition-zone regions adjacent to subducting slabs are hydrated by fluids generated at the top of the lower mantle, Fe-bearing phase E is expected to occur at wadsleyite-ringwoodite phase transition boundary (about 520 km) as an important phase for incorporating water.

In ultrahigh-pressure (UHP) metamorphic rocks, rutile is an important accessory mineral. Its high-pressure polymorph TiO₂II can be a significant indicator of pressure in the diamond stability field. In the present study, in situ high-pressure Raman spectroscopic measurements of natural rutile in UHP eclogite from the main hole of the Chinese Continental Scientific Drilling Project (CCSD) have been conducted up to ~16 GPa. Rutile and recovered TiO₂II have also been analyzed via single-crystal X-ray diffraction and FTIR spectroscopy. The results indicate that (1) the phase transition from rutile to baddeleyite-type TiO₂ terminates at about 16 GPa under compression at ambient temperature; (2) the metastable TiO₂II in the exhumated UHP rocks formed during deep continental subduction can be characterized by a highly distorted octahedral site in the crystal structure. X-ray powder diffraction analyses (with Cu Kα radiation) at ambient conditions are sufficient for identifying the lamellae of TiO₂II within natural rutile based on the angles (2θ) of two strong peaks at 25.5° and 31.5°; (3) rutile and recovered TiO₂II in the continental slabs can contain certain amounts of water during deep subduction and exhumation. The estimated water contents of rutile in the present study range from 1590 to 1780 ppm of H₂O by weight. In the crystal structure of TiO₂II, hydrogen can be incorporated close to the long O-O edges (>2.5143 Å) of the TiO₆ octahedra. Further studies on the pressure–temperature stability of hydroxyls in rutile and TiO₂II may help to understand the transportation and release of water in subducted continental slabs.

Lepidolite is widespread in the upper crust and can be used as a lithium ore. In the present study, in-situ high-temperature Raman and infrared spectroscopic measurements of natural Rb/Cs-free 2M₂ lepidolite in pegmatite were conducted up to 700 °C for investigating the thermal response of lepidolite. In addition, single-crystal X-ray diffraction and electron microprobe analyses were also conducted to determine the polytype and composition of the lepidolite sample. The results show that (1) in the temperature range from 25 to 700 °C, the crystal structure of 2M₂ lepidolite is not expected to undergo remarkable transformations upon heating and cooling; (2) the stabilities of hydroxyls in high-charge environments are lower than those in low-charge environments at elevated temperatures up to 700 °C. As a result, hydrogen ions in 2M₂ lepidolite can transfer from the non-silicate oxygens in high-charge environments to those in low-charge environments during heating; (3) the transfers of the hydrogen ions between non-silicate oxygens in high-charge and low-charge environments may lead to the dehydration of lepidolite. Thus, lepidolites in subduction zones at temperatures below 700 °C can be sufficient to trigger partial melting and result in the release of lithium in subducted oceanic and continental slabs during subduction and exhumation.