variation in chemical composition (Ross & Hendricks, 1945; Brindley, 1980), consisting of. 2 : 1 silicate layers in .... Lake John, Colorado.
Clay Minerals (1988) 23, 147-159
E V I D E N C E F O R T H E M U L T I P H A S E N A T U R E OF BENTONITES FROM MOSSBAUERAND EPR SPECTROSCOPY B. A . G O O D M A N ,
P. H. N A D E A U
AND J. C H A D W I C K *
The Macaulay Land Use Research Institute, Craigiebuekler, Aberdeen AB9 2QJ, Scotland, and *Oliver Lodge Laboratory, Department of Physics, University of Liverpool, Liverpool L69 3RX, UK (Received 20 January 1988)
ABSTRACT : M/Sssbauer spectra of several smectites demonstrate the existence of at least three phases with distinct Fe populations: (i) a component with very low Fe content (< 1~), which shows slowly-relaxing paramagnetic hyperfine structure at both 4.2 K and 77 K; (ii) a component with intermediate Fe content (~ 1-10H) which is seen as doublets in the spectra at 4.2 K, 77 K and ambient temperature; (iii) an Fe-rich phase (> 30~ Fe), which shows magnetic ordering at 4.2 K and 77 K. These data are consistent with components (i) and (ii) corresponding to Fe incorporated in aluminosilicate structures from distinct phases, whereas (iii) is characteristic of an iron oxide phase, probably goethite in most cases. These conclusions are supported by EPR measurements which show magnetically-dilute Fe in more than one type of structural environment plus an additional component with magnetically-interacting ions.
The term smectite refers to an important group of microcrystalline clay minerals with a large variation in chemical composition (Ross & Hendricks, 1945; Brindley, 1980), consisting of 2 : 1 silicate layers in which planes of cations with octahedral coordination are sandwiched between sheets of tetrahedrally-coordinated cations. A variety of cations can occur in both the tetrahedral and octahedral sheets, but primarily Al and Si occur in tetrahedral coordination, and Mg and A1 in octahedral coordination. Iron, in both the Fe(III) and Fe(II) forms, occurs primarily, but not exclusively, in octahedral positions. In chemical studies of smectites, it is usually assumed that a given sample consists of a single phase, and a structural formula is calculated accordingly. However, evidence has been reported that a sample of beidellite, an aluminous variety of smectite, consists of physically separable components of different compositions (Nadeau et al., 1985). Despite the fact that most smectites cannot be similarly fractionated, their possible multi-phase nature cannot be ruled out and this was investigated using spectroscopic techniques in a study of the F e distribution in a group of eight bentonite clay samples. There are many examples in the literature of investigations of smectites using M6ssbauer spectroscopy, and in virtually every one, results obtained at 77 K or above have been analysed in terms of a number of doublets which have been assigned to Fe in the structural sites with octahedral coordination. In dioctahedral smectites, Fe(III) is usually the d o m i n a n t form and, for samples where its absorption envelope has been fitted to two doublet components, these have been assigned to the two different types of octahedral crystallographic site in the structure (e.g. Rozenson & Heller-Kallai, 1977; Bart et al., 1980). There have also been 9 The Macaulay Land Use Research Institute 1988
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reports of an interlayer Fe(III) component (Cardile & Johnston, 1985) which is exchangeable, and magnetic ordering at low temperatures (Cardile et al., 1986; Dickson & Cardile, 1986) as well as several instances of associated iron oxide species. Although smectites with a wide range of Fe contents exist, common minerals, such as bentonite montmorillonites, generally have only comparatively small amounts of Fe (0-10~), but most of the M6ssbauer spectroscopic work has been performed with samples having Fe contents in the top 50~oof this range. The Fe(III) occurs in the high spin 6S state, which in the absence of magnetic exchange interactions (the case for low Fe contents if Fe is randomly distributed), is split by the crystal field into 3 Kramer's doublets, having m~ = + 5/2, + 3/2 and _+1/2. The separations of the Kramer's doublets are of the order of 1 cm -1 and consequently are all approximately equally occupied except at very low temperatures. Most of the published results can be explained in terms of rapid exchange between these states, with the result that no magnetic hyperfine structure is seen in the M6ssbauer spectra. In magnetically-dilute materials, however, the rate of relaxation between states decreases with increasing separation of the paramagnetic ions in the structure. Hyperfine structure from slowly-relaxing paramagnetic Fe(III) has been reported in synthetic A1203 doped with small amounts of 57Fe (Wickman & Wertheim, 1966). Such hyperfine structure is also seen commonly in biological samples in, for example, proteins where the Fe is held specifically in magnetically-dilute environments (e.g. Oosterhuis & Spartalian, 1976) and in complexes, particularly in frozen solution, where M6ssbauer spectroscopy can be used to identify mononuclear species (Goodman & Cheshire, 1987). In naturally-occurring mineral samples, reports of this hyperfine structure from slowly-relaxing paramagnetic ions are rare, although Fysh et al. (1983) have demonstrated its occurrence at 4.2 K in a kaolinite sample. Similar structure might, therefore, be expected to occur in other aluminosilicates with low Fe content and the search for such structure in bentonites was the original objective of this work. One potential problem in the identification of hyperfine structure from slowly-relaxing paramagnets is distinguishing it from hyperfine structure from magnetically-ordered phases. In natural mineral specimens, particularly those with low Fe contents, there is the possibility that a significant fraction of the Fe is in the form of an oxide or oxyhydroxide, which has a total concentration too low to be detected by conventional methods of mineral analysis. However, the magnitude of the internal field is influenced by the exchange interaction and is generally smaller than for coordination to the same atoms in the absence of exchange. Thus the naturally-occurring iron oxyhydroxides, goethite, akaganeite and lepidocrocite have hyperfine fields in the range 45.8-50.6 T at 4.2 K (Murad & Johnston, 1987) and the largest field for an oxide is 54.17 T for hematite in its antiferromagnetic state below its Morin transition. A somewhat smaller saturation hyperfine field of 53.5 T is observed for hematite in its weakly ferrimagnetic state. If distinction between magnetically-dilute and ordered forms cannot be made by the magnitude of the hyperfine splitting, the application of a small magnetic field may be useful. This has the effect of slowing down relaxation processes in paramagnetic materials but leaving unaffected any components with antiferromagnetic properties. An alternative way of studying materials in the presence of a magnetic field is by the use of electron paramagnetic resonance (EPR) spectroscopy. Magnetically-interacting ions are readily distinguished from those not involved in exchange interactions and the spectra also show variations with the magnitude and symmetry of the crystal fields at the sites of the magnetically-dilute ions. EPR measurements have, therefore, been performed on the bentonites in order to gain further information on the environments of the Fe(III).
Mbssbauer and EPR study of bentonites MATERIALS
149
AND METHODS
Smectites from bentonites in Cretaceous marine pelitic rocks from the Western Interior of North America (Table 1) were Na-saturated using 3 washes with 1 M NaC1 solution (once overnight), dialysis being employed for the removal of excess ions, and then size fractionated. The mineralogy of the clay fractions was established by X-ray diffraction and I R spectroscopy, and the total iron compositions determined by X-ray fluorescence (Nadeau & Bain, 1986). TABLE1. Details of samples studied. Sample
Locality
Size fraction
Total Fe*
WMB LJB RIB MCB GCB C3B VLB RPB
Upton, Wyoming Lake John, Colorado Rangeley,Colorado Mt. Carmel, Utah Glen Canyon, Utah Canon City, Colorado Vernal, Utah Rio Puerco, New Mexico
