Clays and Clay Minerals, Vol. 34, No. 5,511-520, 1986, ION EXCHANGE, THERMAL TRANSFORMATIONS, AND OXIDIZING PROPERTIES OF BIRNESSITE D. C. GOLDEN, J. B. DIXON, AND C. C. CHEN Department of Soil & Crop Sciences, Texas A&M University College Station, Texas 77843 Abstract--Synthetic sodium bimessite, having a cation-exchange capacity (CEC) of 240 meq/100 g (cmol/ kg) was transformed into Li, K, Mg, Ca, Sr, Ni, and Mn 2+ cationic forms by ion exchange in an aqueous medium. Competitive adsorption studies of Ni and Ba vs. Mg showed a strong preference for Ni and Ba by birnessite. The product of Mgz§ was buserite, which showed a basal spacin~ of 9.6 ,~ (22~ relative humidity (RH) = 54%), which on drying at 105~ under vacuum collapsed to 7 A. Of the cation- saturated birnessites with 7-A basal spacing, only Li-, Na-, Mg-, and Ca-birnessites showed cation ex- change. Heating birnessite saturated with cations other than K produced a disordered phase between 200* and 400~ which transformed to well-crystallized phases at 600~ K-exchanged birnessite did not transform to a disordered phase; rather a topotactic transformation to cryptomelane was observed. Generally the larger cations, K, Ba, and Sr, gave rise to hoUandite-type structures. Mn- and Ni-birnessite transformed to bixbyite-type products, and Mg-bimessite (buserite) transformed to a hausmannite-type product. Li- birnessite transformed to cryptomelane and at higher temperature converted to hausmannite. The hol- landite-type products retained the morphology of the parent birnessite. The mineralogy of final products were controlled by the saturating cation. Products obtained by heating natural birnessite were similar to those obtained by heating birnessite saturated with transition elements. Key Words--Bimessite, Bixbyite, Buserite, Cation exchange, Cryptomelane, Hausmannite, Hollandite, Oxidation, Thermal treatment. INTRODUCTION Birnessite is one of the most common manganese oxides in the clay-size fraction (<2 #m) of soils. It has been identified in soil concretions in Australia (Taylor et al., 1964) and the Soviet Union (Chukhrov and Gorshkov, 1981). Manganese minerals of this type are an important source of nutrient Mn. In addition, the relatively soluble nature of these minerals under wa- terlogged or extremely acid conditions may contribute to Mn toxicity in many plants. The cation-exchange preference of birnessite for heavy metals makes it im- portant from a pollution-abatement point of view and in controlling the availability of certain trace elements, some of which are essential to plants and animals (e.g., Co and Cu). Adams et al. (1969) showed that the avail- ability of cobalt to plants is controlled to a large extent by the amount of manganese in the soil. The chemistry, structure, and other properties of nat- ural bimessite and its cation-exchange derivatives, however, are not well understood. Its structure is thought to be similar to that of chalcophanite (Burns and Burns, 1977), although a precise determination has not been made. The structure probably consists of sheets of water molecules between sheets of edge-sharing MnO 6 octahedra, the arrangement being repeated about every 7.2/~ along the c-axis. One of six octahedral sites in the MnO~ octahedra is unoccupied, and Mn 2+ and Mn 3+ lie above and below each octahedral vacancy. These low-valence Mn ions are coordinated to oxygens in Copyright 9 1986 The Clay Minerals Society both the octahedral MnO 6 sheet and the water slaeet (Figure 1). Synthetic birnessite was reported to be non- stoichiometric (Giovanoli et al., 1973), the formula for the sodium form being Na4Mn14026.9H20 and the sodium-free form, MnvO~3-5H20 (Giovanoli et al., 1970). To increase our understanding of the behavior of birnessite in the soil system, synthetic birnessite was characterized by X-ray powder diffraction, chemical, and infrared spectroscopic techniques. The cation-ex- change properties and the thermal stabilities of various exchanged products were also investigated and com- pared with those of the natural material. oooooooooooooooooooooo Oxygen T " "o ................. Mn O OOOOOOOOOOOCOOOOOOO o 9 9 Mn(ll) or Mn(lll) 7.0 A eeoc eooeeo ooeooe oeo. H20 • 9 000000000000000000000 eoeeee eel lee tee 9 ee 9 0000000000000000000000 9 9149149 OOII, OQ 9 9149149 o o00; oo oooo000ooo0ooo0 ee Jo o l e l 9 1 4 9. 9 ee 9 9 ee O000GO000000000000000 Figure 1. Birnessite structure modeled after chalcophanite (after Burns and Bums, 1977) as seen in a section perpendic- ular to the b-axis. 511