The 3.65 Å phase in the system MgO-SiO2-H2O; synthesis, composition, and structure

The 3.65 Å phase, a member of the family of dense hydrous magnesium silicates (DHMS) in the system MgO-SiO2-H2O, was synthesized in a 77 h multi-anvil press experiment at conditions of 10 GPa and 425°C by using a gel of composition MgSiO3 plus water in excess as starting materials. From our multi-me...

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Bibliographic Details
Published inThe American mineralogist Vol. 96; no. 8-9; pp. 1207 - 1214
Main Authors Wunder, Bernd, Wirth, Richard, Koch-Müller, Monika
Format Journal Article
LanguageEnglish
Published Mineralogical Society of America 01.08.2011
De Gruyter
Subjects
3.65 Å phase
coordination
crystal structure
dense hydrous magnesium silicates
DHMS
electron probe data
experimental studies
high pressure
high-pressure synthesis
infrared spectra
magnesium oxides
Mineralogy
octahedral-coordinated Si
oxides
pressure
Raman spectra
Rietveld refinement
SEM data
silica
spectra
stability
synthesis
synthetic materials
TEM data
water
X-ray diffraction data
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Summary:The 3.65 Å phase, a member of the family of dense hydrous magnesium silicates (DHMS) in the system MgO-SiO2-H2O, was synthesized in a 77 h multi-anvil press experiment at conditions of 10 GPa and 425°C by using a gel of composition MgSiO3 plus water in excess as starting materials. From our multi-methodical study including SEM, TEM, EMP, IR, and Raman analysis, we determined the composition of the 3.65 Å phase to be MgSi(OH)6. Powder XRD combined with Rietveld refinement revealed the 3.65 Å phase to be isostructural with δ-Al(OH)3. The 3.65 Å phase can be described as a hydrous A-site vacant perovskite with probably long-range random distribution of Si and Mg at octahedral sites. Locally, some ordering of Mg and Si might exist, as indicated from the spectroscopic measurements. The 3.65 Å phase represents the second DHMS with exclusively sixfold-coordinated Si, the other being phase D. The 3.65 Å phase is stable at pressures above about 9.0 GPa and temperatures below 500°C. This limited P-T stability together with its high water content makes the 3.65 Å phase an unrealistic mantle component. If at all, it might only occur under hydrous conditions in the coldest parts of deeply and extremely fast subducted old oceanic slabs.
ISSN:0003-004X
1945-3027
DOI:10.2138/am.2011.3782