Braunite\nSynthesized under Simulated Diagenetic Conditions:\nImplications for the Ancient Manganese Cycle

Many manganese minerals\nare effective paleo-redox proxies due to\nmanganese’s high redox potential and ability to exist in a\nwide range of valence states. Manganese is largely soluble unless\noxidized by oxygen or O<sub>2</sub>-related species, so manganese\nenrichments are typically inferred to indicate the interaction between\nMn(II) and oxygen. Braunite [Mn(II)Mn(III)<sub>6</sub>O<sub>8</sub>(SiO<sub>4</sub>)] is a pervasive component of ancient sedimentary\nmanganese deposits, which have been associated with fluxes in global\noxygen. Braunite’s sedimentary textures suggest a diagenetic\norigin; however, the precursor precipitates and postdepositional processes\nthat lead to braunite’s mineralization are poorly constrained.\nWe hypothesized that the requisite conditions for braunite mineralization\ninvolve the deposition of a manganese oxide precursor mineral that\nsubsequently undergoes diagenetic reactions with sedimentary aqueous\nsilica and Mn(II). We tested this idea by synthesizing various Mn(III,IV)\noxide minerals, along with Mn(II) minerals like rhodochrosite, and\naging these synthetic precursors at late-stage diagenetic temperatures\nin a siliceous solution while including and omitting aqueous Mn(II).\nAt the end of the aging period, we identified that braunite had only\nformed from Mn(III,IV) oxide precursors. Furthermore, the Mn(III)\noxide feitknechtite (Mn(III)OOH) required Mn(II) in solution to transform\ninto braunite, while feitknechtite solutions lacking Mn(II) stabilized\ninto an MnOOH polymorph. Mn(IV) oxides were less stable and could\nhydrothermally transform into braunite without aqueous Mn(II) under\nanoxic conditions. The evolution of precursor manganese oxides into\nbraunite upholds this mineral’s capability to be used as an\nindicator for paleoenvironmental manganese oxidation, and thus most\nlikely implies a past redox boundary where concentrated Mn(II) and\nO<sub>2</sub> interacted.