Understanding the storage forms of manganese during Earth's evolution is essential for elucidating the origin of large-scale manganese deposits associated with the Great Oxygenation Event. Here, in addition to known manganese oxides, we identify an unconventional Mn-rich compound, ${\mathrm{Mn}}_4\mathrm{O}$, which exhibits thermodynamic stability from ambient pressure up to at least 150 GPa—the highest pressure explored. Additionally, a high-pressure polymorph of ${\mathrm{Mn}}_3{\mathrm{O}}_4$ with monoclinic $Cc$ symmetry emerges as stable above 72 GPa. Ab initio molecular dynamics simulations confirm that both phases remain in the solid state across a wide pressure-temperature range relevant to deep-mantle conditions. We propose that ${\mathrm{Mn}}_4\mathrm{O}$ may form via redox reactions between manganese and oxidized species transported by subducted hydrated lithosphere, and subsequently be brought to the surface through mantle convection, where it oxidizes into conventional manganese ores in the presence of ${\mathrm{O}}_2$. Owing to their low sound velocities, these phases could contribute to the seismic anomalies observed in ultralow velocity zones near the core-mantle boundary. These findings suggest a previously unrecognized deep-mantle reservoir for manganese and offer insights into the deep-cycle behavior of this key transition metal.

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• High-pressure studies
• Phase transitions
• Crystal structures
• Planets & planetary systems
• First-principles calculations