Braunite Synthesized under Simulated Diagenetic Conditions: Implications for the Ancient Manganese Cycle

Many manganese minerals are effective paleo-redox proxies due to manganese’s high redox potential and ability to exist in a wide range of valence states. Manganese is largely soluble unless oxidized by oxygen or O2-related species, so manganese enrichments are typically inferred to indicate the interaction between Mn(II) and oxygen. Braunite [Mn(II)Mn(III)6O8(SiO4)] is a pervasive component of ancient sedimentary manganese deposits, which have been associated with fluxes in global oxygen. Braunite’s sedimentary textures suggest a diagenetic origin; however, the precursor precipitates and postdepositional processes that lead to braunite’s mineralization are poorly constrained. We hypothesized that the requisite conditions for braunite mineralization involve the deposition of a manganese oxide precursor mineral that subsequently undergoes diagenetic reactions with sedimentary aqueous silica and Mn(II). We tested this idea by synthesizing various Mn(III,IV) oxide minerals, along with Mn(II) minerals like rhodochrosite, and aging these synthetic precursors at late-stage diagenetic temperatures in a siliceous solution while including and omitting aqueous Mn(II). At the end of the aging period, we identified that braunite had only formed from Mn(III,IV) oxide precursors. Furthermore, the Mn(III) oxide feitknechtite (Mn(III)OOH) required Mn(II) in solution to transform into braunite, while feitknechtite solutions lacking Mn(II) stabilized into an MnOOH polymorph. Mn(IV) oxides were less stable and could hydrothermally transform into braunite without aqueous Mn(II) under anoxic conditions. The evolution of precursor manganese oxides into braunite upholds this mineral’s capability to be used as an indicator for paleoenvironmental manganese oxidation, and thus most likely implies a past redox boundary where concentrated Mn(II) and O2 interacted.