TRANSFORMATION OF HAUSMANNITE INTO BIRNESSITE IN ALKALINE MEDIA

The transformation of hausmannite (Mn//3O//4) 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 Mn//3O//4 followed by reprecipitation of the 7-A phase. The rate-determining step was the dissolution of Mn//3O//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. In neutral to slightly acidic media it converted rapidly to Mn//7O//1//3 multiplied by (times) 5H//2O, and in more acid media, it dissolved and reprecipitated as gamma -MnO//2. Jacobsite and magnetite also underwent a dissolution/reprecipitation transformation in KOH.