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Licensed Unlicensed Requires Authentication Published by De Gruyter December 30, 2019

Incorporation of Mg in phase Egg, AlSiO3OH: Toward a new polymorph of phase H, MgSiH2O4, a carrier of water in the deep mantle

Luca Bindi, Aleksandra Bendeliani, Andrey Bobrov, Ekaterina Matrosova and Tetsuo Irifune
From the journal American Mineralogist


The crystal structure and chemical composition of a crystal of Mg-bearing phase Egg with a general formula M1x3+Mx2+SiO4H1x(M3+=Al,Cr;M2+=Mg,Fe),where x = 0.35, produced by subsolidus reaction at 24 GPa and 1400 °C of components of subducted oceanic slabs (peridotite, basalt, and sediment), was analyzed by electron microprobe and single-crystal X‑ray diffraction. Neglecting the enlarged unit cell and the consequent expansion of the coordination polyhedra (as expected for Mg substitution for Al), the compound was found to be topologically identical to phase Egg, AlSiO3OH, space group P21/n, with lattice parameters a = 7.2681(8), b = 4.3723(5), c = 7.1229(7) Å, b = 99.123(8)°, V = 223.49(4) Å3, and Z = 4. Bond-valence considerations lead to hypothesize the presence of hydroxyl groups only, thereby excluding the presence of the molecular water that would be present in the hypothetical end-member MgSiO3·H2O. We thus demonstrate that phase Egg, considered as one of the main players in the water cycle of the mantle, can incorporate large amounts of Mg in its structure and that there exists a solid solution with a new hypothetical MgSiH2O4 end-member, according to the substitution Al3+ ↔ Mg2+ + H+. The new hypothetical MgSiH2O4 end-member would be a polymorph of phase H, a leading candidate for delivering significant water into the deepest part of the lower mantle.


Paul Asimow, George Rossman, and Fabrizio Nestola are thanked for the efforts to obtain an IR/Raman spectrum of Mg-bearing phase Egg. The manuscript took advantage from the revision of three anonymous reviewers. Zhicheng Jing is gratefully acknowledged for the efficient handling of the manuscript.

  1. Funding The research was supported by “progetto di Ateneo 2016, University of Firenze” to L.B. and by the Russian Science Foundation, project no. 17-17-01169 to A.B., A.B., and E.S.

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Received: 2019-07-12
Accepted: 2019-09-17
Published Online: 2019-12-30
Published in Print: 2020-01-28

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