• journal_title：American Mineralogist
• Contributor：Tiziana Boffa Ballaran ; Daniel J. Frost ; Nobuyoshi Miyajima ; Florian Heidelbach
• Publisher：Mineralogical Society of America
• Date：2010-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2010.3462
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：95
• issue：7
• firstpage：1113
• section：Letters

The dense hydrous-magnesium silicate phase D, which has the ideal formula MgSi₂H₂O₆, may be an important link in a chain of hydrous phases that carry H₂O in the ultramafic portions of subducting lithosphere, into the Earth’s lower mantle. We have synthesized a new Al-rich form of phase D, containing up to 50 wt% Al₂O₃, using a multi-anvil device at ~1300 °C and 25 GPa. The phase, with the formula Mg_0.2Fe_0.15Al_1.8H_1.8SiO₆, was initially produced in a bulk composition designed to synthesize Al- and Fe-rich magnesium silicate perovskite with a composition similar to that produced in experiments on mid-ocean ridge basalt bulk compositions at lower mantle conditions. Further experiments using a starting mixture based on the composition of this Al-rich phase resulted in the synthesis of 60–70 μm long single crystals at similar conditions. The recovered crystals were slightly richer in H₂O (Mg_0.2Fe_0.12Al_1.5Si_0.92H_3.1O₆) and their unit-cell parameters were similar to those of MgSi₂H₂O₆ phase D. A refinement of the crystal structure was carried out in the P3̅1m space group and revealed a more disordered cation distribution than magnesium silicate phase D. All cation-oxygen distances are similar, suggesting a high degree of Si/Al disorder. Although the stability field of this new variant of phase D is yet to be determined, this phase may be an important host for H₂O within portions of subducted oceanic crust in the lower mantle.