Hydrothermal Synthesis of Braunite in the Systems MnOOH-SiO₂, Mn₃O₄-SiO₂, and MnO₂-SiO₂

Abstract

Braunite (MnMn₆SiO₁₂) was synthesized at low temperatures with pressures from 1 to 2500 bar in the systems manganite (γ-MnOOH)-SiO₂, hausmannite (Mn₃O₄)-SiO₂, and pyrolusite (β-MnO₂)-SiO₂. Braunite-pyrolusite assemblage was produced in the first system and braunite-pyroxmangite (MnSiO₃) assemblage in the second system. In the third system, only braunite was formed as solid phase. These reaction equations are as follows. 8MₙOOH+SiO₂=MnMn₆SiO₁₂+MnO₂+4H₂O 3Mn₃O₄+3SiO₂= MnMn₆SiO₁₂+2MnSiO₃ 7MnO₂+SiO₂= MnMn₆SiO₁₂+2O₂ These reaction temperatures, under 1000 bar H₂O total pressure, are 272℃, 459℃, and 521℃ (Po₂=l bar), respectively. P-T diagrams were constructed to show the stability field of braunite. A good correspondence was found between the braunite assemblage synthesized in our laboratory and those observed in bedded manganese ore deposits.