Transformation of Hausmannite into Birnessite in Alkaline Media

R. M. Cornell1 and R. Giovanoli2
1 ETH Zürich, Laboratory for Inorganic Chemistry, ETH-Zentrum CH-8092 Zürich, Switzerland
2 University of Bern, Laboratory for Electronmicroscopy Freiestrasse 3, CH-3000 Bern 9, Switzerland

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.

Key Words: Alkaline solution • Birnessite • Dissolution • Hausmannite • Manganese • Synthesis

Clays and Clay Minerals; June 1988 v. 36; no. 3; p. 249-257; DOI: 10.1346/CCMN.1988.0360306
© 1988, The Clay Minerals Society
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