Transformation of Hausmannite into Birnessite in Alkaline Media

The transformation of hausmannite (Mn 3 O 4 ) into a K-bearing, 7-Å phyllomanganate (K-birnessite) in KOH was followed using X-ray powder diffraction and transmission electron microscopy. The transformation involved dissolution of Mn 3 O 4 followed by reprecipitation of the 7-Å phase. The rate-determining step was the dissolution of Mn 3 O 4 . 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 c 0 = 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 Mn 7 O 13 •5H 2 O, and in more acid media, it dissolved and reprecipitated as γ-MnO 2 . The replacement of K + by Na + was not achieved. Jacobsite and magnetite also underwent a dissolution/reprecipitation transformation in KOH.