The post-stishovite phase transition in hydrous alumina-bearing SiO^sub 2^ in the lower mantle of the earth

Silica is the most abundant oxide component in the Earth mantle by weight, and stishovite, the rutile-structured (.../mnm) high-pressure phase with silica in six coordination by oxygen, is one of the main constituents of the basaltic layer of subducting slabs. It may also be present as a free phase...

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Bibliographic Details
Published in:Proceedings of the National Academy of Sciences - PNAS 2007-08, Vol.104 (34), p.13588
Main Authors: Lakshtanov, Dmitry L, Sinogeikin, Stanislav V, Litasov, Konstantin D, Prakapenka, Vitali B
Format: Article
Language:English
Subjects:
Alumina
Chemical reactions
Earth
Hydrogen
Phase transitions
Silica
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Summary:Silica is the most abundant oxide component in the Earth mantle by weight, and stishovite, the rutile-structured (.../mnm) high-pressure phase with silica in six coordination by oxygen, is one of the main constituents of the basaltic layer of subducting slabs. It may also be present as a free phase in the lower mantle and at the core-mantle boundary. Pure stishovite undergoes a displacive phase transition to the ... structure (Pnnm) at ...55 GPa. Theory suggests that this transition is associated with softening of the shear modulus that could provide a significant seismic signature, but none has ever been observed in the Earth. However, stishovite in natural rocks is expected to contain up to 5 wt % ... and possibly water. Here we report the acoustic velocities, densities, and Raman frequencies of aluminum- and hydrogen-bearing stishovite with a composition close to that expected in the Earth mantle at pressures up to 43.8(3) GPa [where (3) indicates an uncertainty of 0.3 GPa]. The post-stishovite phase transition occurs at 24.3(5) GPa (at 298 K), far lower than for pure silica at 50-60 GPa. Our results suggest that the rutile-... transition in natural stishovite (with 5 wt % ...) should occur at ...30 GPa or ...1,000-km depth at mantle temperatures. The major changes in elastic properties across this transition could make it visible in seismic profiles and may be responsible for seismic reflectors observed at 1,000- to 1,400-km depth. (ProQuest: ... denotes formulae/symbols omitted.)
ISSN:0027-8424
1091-6490