TRANSFORMATION OF HAUSMANNITE INTO BIRNESSITE IN ALKALINE MEDIA

-The transformation of hausmannite (Mn304) 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 Mn304 followed by reprecipitation of the 7-/~ phase. The ratedetermining step was the dissolution of Mn304. The reaction was accelerated by increasing the pH and/ or the temperature of the system. K-bimessite 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/~ and Co = 7.09/~ was found for this phase. Synthetic K-birnessite was stable in KOH at 70~ for many months. In neutral to slightly acidic media it converted rapidly to MnTO~3-5H20, and in more acid media, it dissolved and reprecipitated as 3,-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. I N T R O D U C T I O N The 7-~ phyllomanganates (or birnessites) are widely distributed in soils and sediments. Their reactivity makes them important regulators of M n availability in solution and also of trace metal uptake. These compounds consist of sheets of edge-sharing Mn(IV)O6 octahedra interlayered with sheets of water molecules. Systematic Mn(IV) vacancies in the sheets of Mn(IV)O6 octahedra lead to an overall negative charge on the sheets. This charge is compensated for by variable amounts of Mn 2+ and/or Mn 3+ ions in the interlayers above and below the vacancies in the framework sheets (Giovanoli et al., 1970). These birnessites usually contain other interlayer cations, most commonly Na +, but various transition metal ions can be accommodated as well. Such a structure permits the existence of a series of birnessites having a range of structures and degrees of ordering. Most Mn oxides can be produced by a variety of pathways, and interconversions between different oxides form an important feature of this class of compounds (Giovanoli, 1976). Although poorly crystallized birnessites can be obtained by precipitation from KMnO4 solution, the commonest preparative route involves oxidation of Mn(OH)2 to a 10-A phyllomanganate (or buserite) followed by dehydration to the 7-,~ phase (Giovanoli et al., 1976). This paper reports the formation of birnessite by an entirely new pathway involving dissolution of hausmannite (Mn304) in KOH. The transformation was followed by X-ray powder diffraction (XRD) and by transmission electron microscopy (TEM), and the birCopyright 9 1988, The Clay Minerals Society nessites produced were characterized. Two questions were examined; (1) are the birnessites prepared by a dissolution/reprecipitation reaction different from those obtained by topotactic transformation of Mn(OH)2; and (2) do the Naand K-birnessites have similar properties? Ultimately this study should provide information about the mechanism by which birnessite forms in soils. In natural waters oxidation of Mn 2+ leads to Mn304 and fl-MnOOH which further transform to 7-MnOOH (Stumm and Giovanoli, 1976). Hem and Lind (1983) used thermodynamic arguments to show that in such systems disproportionation reactions can convert Mn 3+ products to Mn(IV)O2. The pathways by which birnessites form in soils have not been clearly established; it seems probable, however, that as in natural waters, a dissolution/reprecipitation mechanism is involved. EXPERIMENTAL METHODS The transformation experiments were carried out by reacting 0.4 g of Mn304 (or Fe304 (magnetite) or MnFe204 (jacobsite)) with 100 ml of KOH or NaOH. The oxide was lightly ground in an agate mortar to break up the aggregates of crystals. The concentration of alkali ranged from 0.001 M to 10 M. Most experiments were carried out with 1 M KOH. The temperature of transformation was usually 70~ although some experiments were conducted at 25 ~ or 90~ Additional experiments involved transformations in twice-distilled water or in 0.1 M HNO3. The transformations took from weeks to months to complete. The product