Evidence for the charge disproportionation of iron in extraterrestrial bridgmanite

Abstract

Bridgmanite, MgSiO 3 with perovskite structure, is considered the most abundant mineral on Earth. On the lower mantle, it contains Fe and Al that strongly influence its behavior. Experimentalists have debated whether iron may exist in a mixed valence state, coexistence of Fe 2+ and Fe 3+ in bridgmanite, through charge disproportionation. Here, we report the discovery of Fe-rich aluminous bridgmanite coexisting with metallic iron in a shock vein of the Suizhou meteorite. This is the first direct evidence in nature of the Fe disproportionation reaction, which so far has only been observed in some high-pressure experiments. Furthermore, our discovery supports the idea that the disproportionation reaction would have played a key role in redox processes and the evolution of Earth.

Six ‘Must-Have’ Minerals for Life’s Emergence: Olivine, Pyrrhotite, Bridgmanite, Serpentine, Fougerite and Mackinawite

“…Yet its presence was only initially surmised from experimental work [ 93 ]. This is because bridgmanite is a very rare mineral, generally found only in meteorites where it is generated by transitory high shock waves pressures of 18 to 25 GPa [ 95 , 103 , 104 , 105 ].…”

“…However, Tutolo et al [ 131 ] show that in the silica-rich Hadean ocean, minimal hydrogen would be generated at nearly two orders of magnitude lower than we had formerly assumed. Nevertheless, we do know that large quantities of hydrogen would have degassed from sources in the Earth’s mantle and from the mantle/core boundary [ 31 , 32 , 103 , 132 , 133 , 134 ]. Also, Tutolo et al [ 131 ] did demonstrate that the pH contrast at the Hadean submarine alkaline—as an ambient proton motive force—to be up to two orders of magnitude greater than we had first surmised [ 4 ].…”

Dehydration of δ-AlOOH in Earth’s Deep Lower Mantle

“…Our experiments also suggest that δ-AlOOH in warmer slabs could be dehydrated at shallower depths ( Figure 3A,B) and released H 2 O could react with local metallic iron. The lower mantle is thought to contain 1 wt.% of metallic iron produced by the charge disproportionation of Fe in bridgmanite [42][43][44], which has been regarded as an important redox buffering agent in the lower mantle [45]. Based on our observations, we proposed two cases of dehydration of δ-AlOOH in the lower mantle.…”

Systematic investigations of high‐pressure polymorphs in shocked ordinary chondrites

“…2014) and hiroseite (Bindi et al. 2020), respectively. The two minerals were not described because they could not be identified by the Raman spectrum alone.…”

“…In this paper, both phases were described as solely majorite because the difference in their crystal structures is too small to be distinguished by their Raman spectra. MgSiO 3 and FeSiO 3 with a perovskite structure are bridgmanite (Tschauner et al 2014) and hiroseite (Bindi et al 2020), respectively. The two minerals were not described because they could not be identified by the Raman spectrum alone.…”