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.