Maohokite: Newly Discovered High-Pressure Mantle Mineral

Researchers for a long time believed that the lower mantle was made up of bridgmanite (Mg, Fe)SiO₃ and magnesiowüstite (Mg, Fe)O, in which Fe²⁺ resides. This view was revised when experiments proved that Fe²⁺ simply cannot exist at the temperature and pressure of the lower mantle. What really is present is Fe³⁺. The two phases (Mg, Fe)SiO₃ and (Mg, Fe)O both shed Fe²⁺ and, in turn, MgO and MgSiO₃ remain. However, which mineral hosts Fe³⁺ continued to be a secret.

Now researchers have a promising answer: Maohokite, a recently discovered high-pressure mineral. It may be what makes up the Earth’s lower mantle together with magnesiowüstite MgO and bridgmanite MgSiO₃. Details of this new mineral have been published in Meteoritics & Planetary Science.

Maohokite was discovered by CHEN Ming’s team from the Guangzhou Institute of Geochemistry of the Chinese Academy of Sciences and SHU Jinfu from the Center for High-Pressure Science and Technology Advanced Research. The mineral was christened after Hokwang Mao, in recognition of his significant contribution to high-pressure research.

The mineral and its name have been made official by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association under the designator IMA 2017-047.

Natural minerals can be split into two categories: low-pressure minerals and high-pressure minerals, according to their formation pressures. The temperature and pressure required for the formation of high-pressure minerals can only be provided by the mantle’s environment or the hypervelocity collision between celestial bodies.

Maohokite is the second scenario. It was discovered in shock-metamorphosed rocks from the Xiuyan impact crater in China.

This high-pressure mineral came to be after the decomposition of ferromagnesian carbonate via a self-oxidation-reduction reaction at a temperature >900 °C and impact pressure >25 GPa (a pressure range prevailing at depths more than 670 km below Earth’s surface). In this reaction, Fe²⁺ oxidizes into Fe³⁺ and then later integrates with Mg²⁺ to form maohokite, thus making it a likely important constituent of the lower mantle.

Maohokite, with a composition of MgFe₂O₄, possesses an orthorhombic CaFe₂O₄-type structure. The current mineralogical model of the Earth’s mantle exhibits that the ferromagnesian lower mantle is mostly made up of magnesiowüstite (Mg, Fe)O and bridgmanite (Mg, Fe)SiO₃. Thus, the fact that maohokite comprises of Mg and Fe, two main components of the lower mantle, only makes the case stronger that maokohite is a significant mineral in the lower mantle.