Effect of Fe-Al substitution on the crystal structure of bridgmanite

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Marco E. Ciriotti
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Iscritto il: ven 25 giu, 2004 11:31
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Effect of Fe-Al substitution on the crystal structure of bridgmanite

Messaggio da Marco E. Ciriotti » sab 18 set, 2021 20:08

Referenza:
▪ Huang, R., Boffa Ballaran, T., McCammon, C.A., Miyajima, N., Frost, D.J. (2021): The Effect of Fe-Al Substitution on the Crystal Structure of MgSiO3 Bridgmanite. Journal of Geophysical Research: Solid Earth, 126, e2021JB021936.

Abstract:
The crystal chemistry of ten well-characterized bridgmanite single-crystals with Fe and Al contents ranging from 0 to 0.40 atoms per two-cation formula units were investigated by singlecrystal X-ray diffraction. Structural refinements indicate that Fe3+ and Al mainly occupy the Mg and Si sites, respectively, when present in similar proportions. Molar volumes of bridgmanite endmember components were refined using data from this and previous studies and found to decrease in the order Fe3+Fe3+O3 > MgFe3+O2.5 > Fe3+AlO3 > MgAlO2.5 > AlAlO3 > Fe2+SiO3 > MgSiO3. Fe3+AlO3 chargecoupled substitution leads to an anisotropic increase of B-O bond distances, resulting in more distorted octahedral B sites and in a more significant increase of the c-axis with respect to the a- and b-axes. Valence bond calculations indicate that the A site is more compressible than the B site for all bridgmanite samples studied, implying that octahedral tilting and distortion will dominate the bridgmanite compression mechanism. Guided by these crystal chemical observations, bulk moduli of bridgmanite endmember components were estimated using results of previous studies. The volume changes of equilibria controlling the speciation of bridgmanite components were then calculated at conditions relevant to the top of Earth's lower mantle. The proportion of oxygen vacancy components is predicted to decrease with pressure. While the stability of the bridgmanite Fe3+AlO3 component will drive charge disproportionation to produce iron metal at the top of the lower mantle, this appears to be much less favorable by 50 GPa. An increase in the proportion of the Fe3+Fe3+O3 bridgmanite component, however, may favor the formation of iron metal at higher pressures.

Plain Language Summary:
Fe-Al-bearing MgSiO3 bridgmanite is the most abundant mineral in the Earth's lower mantle. Its physical and chemical properties, which are closely related to its crystal structure, play a major role in controlling processes in the Earth's lower mantle. In this study, we synthesized several bridgmanite single crystals with various compositions, which allowed the substitution mechanisms and site occupancies of Fe and Al in the structure to be determined. The changes in volume and structure resulting from the different substitution mechanisms were identified and can be used to explain the compressibility of different bridgmanite endmembers. The effect of pressure on the composition of bridgmanite was then predicted. We found that pressure should favor charge-coupled substitution where 3+ cations enter both the Mg and Si cation sites. The alternative mechanism, where 3+ cations enter the Si site and oxygen vacancies are created, is not favored with pressure. Also Fe3+ and Al may become disordered onto both the Mg and Si sites as pressure increases. The formation of Fe metal through reduction and oxidation of Fe2+ may be suppressed at greater depths in the lower mantle, therefore, the deep lower mantle may be less reduced than the shallower region.
Marco E. Ciriotti

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