Abstract: The transformation of hausmannite (Mn₃O₄) into a K-bearing, 7-A phyllomanganate (K-birnessite) in KOH was followed using X-ray powder diffraction and transmission electron microscopy. The transformation involved dissolution of Mn₃O₄ followed by reprecipitation of the 7-Å phase. The rate-determining step was the dissolution of Mn₃O₄. The reaction was accelerated by increasing the pH and/ or the temperature of the system.

K-birnessite precipitated initially as thin, irregular plates and films that gradually recrystallized to thicker, more structured plates and laths. A pseudohexagonal unit cell with a₀ = 2.87 Å and c₀ = 7.09 Å was found for this phase. Synthetic K-birnessite was stable in KOH at 70°C for many months. In neutral to slightly acidic media it converted rapidly to Mn₇O₁₃·5H₂O, and in more acid media, it dissolved and reprecipitated as γ-MnO₂. The replacement of K⁺ by Na⁺ was not achieved. Jacobsite and magnetite also underwent a dissolution/reprecipitation transformation in KOH.