The amount of fixed Li, 0.09-0.67 per O20(OH)4, increased with increasing temperature, ..... IR spectra of MI-M8 samples in the 1300-400 cm -1 region. 466.
Clay Minerals (1996) 31,233-241
PREPARATION AND INFRARED SPECTROSCOPIC C H A R A C T E R I Z A T I O N OF R E D U C E D - C H A R G E M O N T M O R I L L O N I T E WITH V A R I O U S LI C O N T E N T S J. MADEJOV/k, J. BUJD,/~K, W. P. G A T E S * AND P. K O M A D E L Institute of Inorganic Chemistry, Slovak Academy of Sciences, 842 36 Bratislava, Slovakia, and *Savannah River Ecology Laboratory, University (?f'Georgia, Aiken, South Carolina 29802, USA (Received 19 May 1995; revised 4 October 1995)
A series of reduced-charge montmorillonites (RCMs) was prepared from Limontmorillonite from Jelgov2~Potok (Slovakia) by heating at various temperatures (105-210~ for 24 h. The amount of fixed Li, 0.09-0.67 per O20(OH)4, increased with increasing temperature, confirming preparation of a set of samples of variable layer charge from the same parent Limontmorillonite by varying only the preparation temperature. Infrared spectroscopy revealed that Li was trapped in the hexagonal cavities of the tetrahedral sheet at all temperatures. Partial deprotonation of the samples, reflected in the decrease of the intensities of the OH-bending bands, was observed after treatments above 120~ Analysis of the OH-stretching region showed Li in the previously vacant octahedra in the samples heated above 150~ Weak inflections near 660 and 720 cm -l confirmed development of local trioctahedral character of octahedral cations coordinated with OH groups in the sample heated at 210~ Gradual decrease of the layer charge due to Li fixation led to a shift of the S i - O stretching band to higher frequencies and to the appearance of new, pyrophyllite-like bands at 1120 and 419 cm -~. ABSTRACT:
Lithium fixation and invoked changes in swelling have been widely used by mineralogists to distinguish between different smectites (Hofmann & Klemen, 1950). The irreversible collapse of the layers of Li-montmorillonite after heating was proposed as the criterion to distinguish montmorillonite from beidellite (Greene-Kelly, 1955; Schultz, 1969). It is widely considered that, on heating, Li cations migrate from the interlayer space into the vacant octahedra (Hofmann & Klemen, 1950; Greene-Kelly, 1955; Sposito et al., 1983; Srasra et al., 1994), into the hexagonal holes of the tetrahedral sheet (Tettenhorst, 1962; Luca & Cardile, 1988; Alvero et al., 1994) or into both (Russell & Farmer, 1964; Calvet & Prost, 1971). Recent studies have found that Li-H exchange occurs with heat treatment as well since dioctahedral smectites retained some non-exchangeable Li in excess of the reduction in CEC (Jaynes & Bigham, 1987). Lithium saturation and heating of these clays at 250~ for 8 - 1 2 h caused acidification in direct proportion to total Fe content, apparently as a result of deprotonation of structural
hydroxyl groups. Charge regeneration of thermally treated Li-montmorillonite by treatment with a strong base involved proton extraction from the sheets, each Li ยง entering the structure being balanced by a H + leaving it (Williams et aL, 1991). Many studies have used various temperatures (usually in the range 200-300~ reaction times and clays for the preparation of reduced-charge montmorillonites (RCMs), making direct comparisons somewhat difficult. Calvet & Prost (1971) could fix 3 1 - 9 2 % of total Li in Camp Berteaux montmorillonite heated for 24 h at 108-220~ and indicated by infrared (IR) spectroscopy that Li + ions moved within the structure towards the octahedral vacancies, presumably those which lie near the sites of isomorphic substitution. Structural modification took place with Li migration and creation of local trioctahedral configuration. Only a fraction of the non-exchangeable Li was identified in the structure, the remainder being in the hexagonal cavities. However, no relationship was reported between Li in the structure and the temperature of preparation and no spectra of samples prepared below 220~ were shown.
9 1996 The Mineralogical Society
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J. Madejovd et al.
The complex OH-stretching region of layer silicates can be reliably deconvoluted into structural components with good agreement with chemical analysis (Slonimskaya et al., 1986; Madejovfi et al., 1994). A detailed analysis of the IR spectra of RCMs using spectrometers with Fourier transform capabilities could greatly improve the ability to discern evidence of the location of Li. It was decided, therefore, to prepare and characterize in detail a series of RCMs from one parent clay and FFIR spectroscopy was used to provide better insight towards the process of reduced-charge formation and final Li location. Such preparation by varying only the temperature minimizes differences in possible admixtures, tetrahedral A1 for Si substitution and octahedral Mg and Fe for A1 substitutions, etc. The differences observed within such a set of samples can then be ascribed directly to differences in fixed Li content and/or to invoked changes of the layer charge. MATERIALS
AND
METHODS
The CaZ+-saturated bentonite Jelgov~ Potok (JP) from the clay deposit in the Kremnica mountains in central Slovakia (Sucha et al., 1992; ~amajov~ et al., 1992) was fractionated to
