The influence of Al2O3 on the structural properties of MgSiO3 akimotoite

Abstract

Akimotoite, a MgSiO3 polymorph, present in the lower transition zone within ultramafic portions of subducting slabs and potentially also in the ambient mantle, will partition some amount of Al, raising the question of how this will affect its crystal structure and properties. In this study, a series of samples along the MgSiO3 akimotoite -Al2O3 corundum solid solution have been investigated by means of single-crystal X-ray diffraction in order to examine their crystal chemistry. Results show a strong non-linear behavior of the aand c-axes as a function of Al content, which arises from fundamentally different accommodation mechanisms in the akimotoite and corundum structures. Furthermore, two Al2O3-bearing akimotoite samples were investigated at high pressure in order to determine the different compression mechanisms associated with Al substitution. Al2O3-bearing akimotoite becomes more compressible at least up to a content of 20 mol% Al2O3, due likely to an increase in compressibility as the Al cation is incorporated into the SiO6 octahedron. This observation is in strong contrast to the stiffer corundum end-member having a KT = 250 GPa larger than that of the akimotoite end-member (KT = 205(1) GPa). These findings have implications for mineral physics models of elastic properties, which have in the past assumed linear mixing behavior between the MgSiO3 akimotoite and Al2O3 corundum end-members in order to calculate sound wave velocities for Al-bearing akimotoite at high pressure and temperature.

Phase Stability of Al‐Bearing Dense Hydrous Magnesium Silicates at Topmost Lower Mantle Conditions: Implication for Water Transport in the Mantle

“…Both theoretical and experimental studies indicate that the incorporation of Al in the structure of akimotoite produces a decrease of its unit‐cell volume (Panero et al., 2006; Siersch, 2019). The unit‐cell volume of akimotoite in our experiment is also smaller compared to the unit‐cell volume of akimotoite with 20 mol % Al 2 O 3 (Siersch, 2019) (Figure 3a). Our interpretation is that the Al content in our akimotoite is higher than 20 mol%: if during the decomposition of Al‐bearing shy‐B all the Al released from shy‐B dissolves in the structure of akimotoite, the Al 2 O 3 content in akimotoite would be 23.8 mol% with a (Mg 0.763 Al 0.238 ) (Al 0.238 Si 0.763 )O 3 composition.…”

“…The unit‐cell volume of akimotoite, formed during decomposition of Al‐bearing shy‐B, is 1.6% smaller than that of Al‐free akimotoite (Wang et al., 2004). Both theoretical and experimental studies indicate that the incorporation of Al in the structure of akimotoite produces a decrease of its unit‐cell volume (Panero et al., 2006; Siersch, 2019). The unit‐cell volume of akimotoite in our experiment is also smaller compared to the unit‐cell volume of akimotoite with 20 mol % Al 2 O 3 (Siersch, 2019) (Figure 3a).…”

“…Similar to our results, Ohira et al (2014) showed that Al-bearing bridgmanite could react with water forming the solid solution of δ-AlOOH-MgSiO 2 (OH) 2 and preventing Al incorporation in bridgmanite between 68 GPa and 2010 K. Our study, however, indicates that the Al-depleted bridgmanite can be formed already between 52 and 54.8 GPa. In addition, a recent study also indicates strong Al partitioning in both Ph-D and Phase H resulting in Al-depleted (Wang et al, 2004); green line: 5 mol.% Al-bearing akimotoite (Siersch, 2019); black line: 20 mol.% Al-bearing akimotoite (Siersch, 2019). (b) Compression curves of bridgmanite with different Al contents.…”

“…Comparison of the unit-cell volumes of akimotoite and bridgmanite with different Al-content at high pressure and 300 K. (a) Compression curves of akimotoite with different Al contents; akimotoite in our experiment is produced by the decomposition of Al-bearing shy-B. Blue line: Al-free akimotoite(Wang et al, 2004); green line: 5 mol.% Al-bearing akimotoite(Siersch, 2019); black line: 20 mol.% Al-bearing akimotoite(Siersch, 2019). (b) Compression curves of bridgmanite with different Al contents.…”

Elastic properties of Fe-bearing Akimotoite at mantle conditions: Implications for composition and temperature in lower mantle transition zone

“…Previous studies suggested that the velocities and density of the pyrolite model can match the one-dimension (1D) seismological models such as PREM [2] and AK135 [3] for the upper mantle [4] and the lower mantle [ 5 , 6 ]. However, the shear wave velocity and density of the pyrolite model are lower than PREM or AK135 up to 4% and 2% in the lower mantle transition zone (MTZ) [7] , [8] , [9] . Based on the complicated slab dynamics in the lower MTZ, different hypotheses including the enrichment of harzburgite-rich materials were introduced to reconcile this discrepancy [ 8 , 9 ].…”