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Physical Chemistry

Missing Xenon Still At Large

Computational study shows xenon oxides would be unstable in Earth’s mantle

by Elizabeth K. Wilson
November 12, 2012 | APPEARED IN VOLUME 90, ISSUE 46

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Credit: Artem O. Organov
Crystal structures (left to right) of XeO at 100 GPa, XeO2 at 120 GPa, and XeO3 at 200 GPa.
09046-scicon-xenoncxd.jpg
Credit: Artem O. Organov
Crystal structures (left to right) of XeO at 100 GPa, XeO2 at 120 GPa, and XeO3 at 200 GPa.

A computational study of the stability of xenon oxides has implications for the “missing xenon problem,” which refers to the fact that Earth’s atmosphere is depleted of xenon relative to other noble gases (Nat. Chem., DOI: 10.1038/nchem.1497). Studies have shown that xenon can form oxides and silicates under high temperatures and pressures. Scientists also have hypothesized that the missing atmospheric xenon might be sequestered as oxides or silicates deep inside Earth’s mantle. A study of xenon oxides by an international team led by Artem R. Oganov of SUNY Stony Brook predicts that XeO, XeO2, and XeO3 will become thermodynamically stable at pressures above 83 gigapascals (about 820,000 atm), such as those in the mantle. However, the compounds are strong oxidants and couldn’t exist in Earth’s mantle, which is a strongly reducing environment because of metallic iron, Oganov explains. Additionally, the study shows that xenon silicates, such as XeSiO4, will spontaneously decompose at the high pressures in the mantle. The researchers do give some hope for the mantle sequestration hypothesis: They suggest that xenon could be trapped in tiny air pockets formed by lattice defects and grain boundaries.

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