ELECTRON SPIN RESONANCE STUDIES OF MONTMORILLONITES

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1 Clay Minerals (1985) 20, ELECTRON SPIN RESONANCE STUDIES OF MONTMORILLONITES C. CRACIUN AND AURELIA MEGHEA* Institutul de Cercethri pentru Pedologie ~i A grochimie and *Institutul Politeehnie Bueure~ti, B-dul Mhr?t~ti 61, Bueure~ti 32, Romania (Received 6 June 1983; revised 28 February 1985) A B S T RA C T: Sixteen montmorillonites (8 normal and 8 abnormal using DTA dehydroxylation criteria) from the Gurasada bentonite deposits, Romania, were investigatecl by ESR. The spectra showed two features: a g-4.3 signal attributed to isolated Fe 3+ ions and a broad resonance with g ~ 2.0 which was interpreted as arising from exchange interactions between clusters of Fe 3+ ions. Additional experiments suggested that the latter resonance could have a structural component. The line-shape of the two resonances varied between normal and abnormal montmorillonites, indicating that the distribution of the Fe 3+ ions between internal phase (montmorillonite lattice) and external phase (montmorillonite surface) was different for the two varieties. In the Gurasada region of Romania, two montmorillonite varieties have been reported (Cr~ciun, 1978) which show differences in dehydroxylation behaviour. One variety shows a single dehydroxylation reaction at ~700~ and the other shows a double dehydroxylation peak system with peaks at ~550 and 680~ Using the terminology of Mackenzie (1970), these would be classed as normal and abnormal, respectively. In spite of the fact that the 001 basal spacings are similar, the two varieties show distinct differences in intensity and width of the 001 reflection after Ca-saturation and glycolation, the normal variety showing higher 001 reflection intensities and lower peak widths at half-intensity (Crficiun & Ggttfi, 1981). The purpose of the present study was to obtain, using the electron spin resonance method, information on the nature of the differences between the normal and abnormal montmorillonites. EXPERIMENTAL Sixteen montmorillonites from the Gurasada deposits were investigated by ESR (8 normal and 8 abnormal). These were examined in the form of < 1 /~m fractions. The original material was dispersed in 4% NaOH and the fine-grained product was Ca-saturated using 1 y CaC1 z solutions, with subsequent removal of excess chloride by centrifugation and washing (Cr~tciun, 1984a). Fe3+-treated samples were also investigated by ESR. Since the introduction of Fe 3+ into exchange sites of montmorillonites is a delicate operation (Whittig & Page, 1961; Helsen & Goodman, 1983), the experimental procedure is detailed below. The Na-clay suspensions were treated with 1 N FeC13 solutions. After removal of excess chloride by centrifugation The Mineralogical Society

2 282 C. Cr{lciun and A. Meghea and washing, the samples were dried at room temperature. No attempt was made to control acidity and hydration during exchange and because of this it was difficult to avoid the formation of polymeric iron oxyhydroxides. Not more than three weeks elapsed between the treatment and ESR investigation of the samples. X-band ESR spectra were recorded using an ART-5-IFIN Bucharest spectrometer. The klystrom was operated at 9.06 GHz and the field range was 0-5 kg. Spectra of powdered samples were obtained at room temperature in quartz tubes using an Mn 2+ standard. RESULTS AND DISCUSSION ESR spectra of the samples revealed two features, a g = 4-3 signal and a broad resonance at g ~ 2.0. Some of the characteristics of both features are shown in Table 1, together with some transition metal contents of the samples. TABLE 1. ESR and chemical data for the montmorillonites. Low-field region ESR data High-field region Chemical data Transition ion content (%) Sample Character g4 I4 g2 I~ H(G) Mn Cu Fe N O R M A L A B N O R M A L Resonance at high field The line-shapes of the high-field resonance differ between the two types of montmorillonite. Thus for abnormal samples the intensity of the signal which appears in the g = region (Table 1) is much higher than for normal samples. Some abnormal samples (13, 14 and 15) show a very intense resonance at g = , while others show a narrower range of values ( ). In these latter samples the resonance intensity is also lower. All normal samples show a very low, broad and diffuse resonance in this region and thus it is difficult to determine precise positions and intensities of the signal. Of the reported assignments of this feature (for details see the comprehensive paper of

3 ESR studies of montmorillonites 283 Hall (1980) and references cited therein), the most reasonable seems to be that it arises from Fe 3+ ions. Indeed, of the transition metal ions which might be expected to produce a resonance of this type (Table 1) Cu 2+ and Mn 2+ may be ruled out since: (i) Cu 2+ is not present in sufficient concentration to account for the intensity of this feature, (ii) the hyperfine structure characteristic of Mn 2+ could not be observed in the spectra of the present montmorillonites. In order to establish by ESR whether this feature arose from an iron-rich phase within the silicate framework or a separate iron-rich phase attached to the surface of the montmorillonite, the following experiments were carried out. 1. Investigation of the 'Fe3+-exehanged ' montmorillonites. Generally, interpretation of the results obtained on the Fe3+-exchanged montmorillonites prepared by the method 10/.6 G f ' / ~" 2,38 A 8go 6 B 840 G l 4.30 ~ II II I I 1 i ~ t 1 s FIG. 1. The effect of FeCI 3 treatment on the g ~ 2.0 resonance on ESR spectra of montmorillonites. (A) Abnormal montmorillonite (sample 13) before (I) and after (II) treatment. (B) Normal montmorillonite (sample 2) before (I) and after (II) treatment. (C) ESR spectrum of limonite for comparison.

4 284 C. Crhciun and A. Meghea described previously is subject to uncertainty as to whether Fe 3+ precipitates as a separate phase (Herrera & Peech, 1970). After this treatment the spectral differences between normal and abnormal samples became very small. All normal samples showed a resonance at g = 2.09 similar to that of the abnormal samples (Fig. 1A, B). This resonance closely resembles that which appears in the spectra of limonite (Fig. 1C). The Fe3+-treated abnormal samples showed an increase in resonance intensity, and for samples 13, 14 and 15 this increase was accompanied by a displacement of the signal position from g = to g = The results of this experiment indicate that the broad resonance with g ~ 2.0 are due to clustered Fe 3+ ions. The ferromagnetic character of these resonances results from the presence of iron-rich phases on the montmorillonite surface (Goodman, 1978). The higher intensity of this feature for the abnormal samples is in agreement with their higher Fe 3+ content. A 4,30 A II 4,30~ 2 38 B ~ ' I Z':,39/I~ 2 21 C 14'39,/'~ 2 21 II I i m I L i i i i a FIG. 2. The effect of free iron oxide removal on the g ~ 2.0 resonance on ESR spectra of some abnormal montmorillonites. (A) Sample 12 before (I) and after (II) treatment. (B) Sample 13 before (I) and after (II) treatment. (C) Sample 15 before (I) and after (II) treatment.

5 ESR studies of montmorillonites Investigation of the treated samples for removal of free iron. Removal of free iron (Mehra & Jackson, 1960) had no influence on the spectra of normal samples, while for the abnormal samples the resonance intensity varied as detailed below (Fig. 2). Samples 9, 11 and 12 show a marked decrease of this resonance, such that its intensity approaches that of the normal samples (Fig. 2A). For sample 13 the resonance intensity is partially reduced (Fig. 2B), while sample 15 shows no modification of the signal intensity (Fig. 2C). A possible explanation of these results is that the iron-rich phases (oxyhydroxides) are present either as discrete particles or as coatings on the surface of the montmorillonite particles, as this can influence the effectiveness of free iron removal (Schwertmann et al., 1968; Angel A B 1046 G I i 4,36 2,08 ' I / j..2,26 4,37 //~ II I I 996,6, i III _ ~ ~ 2,0 6 I 1240 G I III I.,, I I i I, ~ }, I I I I L FIG. 3. The influence of heating on the g ~ 2.0 resonance on ESR spectra of abnormal montmorillonites. (A) Sample 11 before (I) and after heating at 600~ (II) and 800~ (III). (B) Sample 13 before (I) and after heating at 600~ (II) and 800~ (III).

286 C. Crgteiun and A. Meghea & Vincent, 1978). However, the results of deferration indicate that the removal of free iron by chemical treatment is in some cases incomplete or very weak.

6 286 C. Crgteiun and A. Meghea & Vincent, 1978). However, the results of deferration indicate that the removal of free iron by chemical treatment is in some cases incomplete or very weak. Thus Fe 3+ resistant to this treatment is not in an identical environment to that which could be reduced. In these circumstances it can be assumed that this resonance could have a structural component. 3. The influence of thermal treatment on the g ~ 2.0 resonance. Changes of this resonance on heating indicated differences between abnormal samples (Fig. 3). On heating to temperatures below that of montmorillonite dehydroxylation (600~ no significant modification of the resonance was observed for samples 13, 14 and 15 (Fig. 3B). After dehydroxylation of the montmorillonite (heating to 800~ the intensity of the resonance decreased (Fig. 3B). This factor could be used as an argument for the existence of a structural component which contributes to resonance. Under the same heating conditions the intensity of this resonance for samples 9, 11 and 12 increases just above 600~ (Fig. 3A). This increase could be due to the thermal transformation of the iron oxyhydroxides (goethite or lepidocrocite) to hematite. Results of the thermal treatment suggest that the resonance with g ~ 2.0, which is due to an iron-rich phase associated to montmorillonite surface, could have a structural component. Resonance at low field The ESR spectra of all the samples contained a peak with g which can be interpreted as arising from Fe 3+ in sites of near rhombic symmetry (Angel & Hall, 1972; McBride et al., 1975; Goodman, 1978). In a few cases (samples 4, 6, 11 and 12) this signal was accompanied by another one with g In the spectrum of kaolinite such a signal has been ascribed to Fe 3+ occupying a site of partially orthorhombic symmetry (Angel & Hall, 1972). Goodman (1978) reported a resonance with g = 3.67 in the spectra of montmorillonite which was interpreted in the same way as the resonance with g = 4.3. The intensity of the g--4-3 signal varies between the montmoriuonites and is an adequate criterion for the differentiation of normal and abnormal samples. The normal samples exhibit higher intensities of this resonance than abnormal samples in spite of a lower structural Fe 3+ content. A decrease in the octahedral Fe 3+ content accompanies an increase in the absolute intensity of the g = 4.3 resonance, as shown in Fig. 4. Interpretation of the data in this figure can be made following Goodman (1978), namely in the abnormal samples which are richer in iron than the normal samples, the g signal due to isolated Fe 3+ ions is smaller since most of the iron contributes to the ferromagnetic resonance. In this way the smaller intensity of the same resonance in nontronites compared with montmorillonites can be explained. The temperature-dependence of the g = 4.3 signal (Table 2) is difficult to interpret. In general, the intensity of this resonance increases on heating. Matyash et al. (in Hall, 1980) attributed such an increase following heating of illitic mica to the oxidation of Fe E+ to Fe 3+. Although Fe 2+ contents of the present montmorillonites are low (<0.35%) such an explanation is possible. It is interesting to note differences which appear between the normal and abnormal samples after heating to temperatures below (600~ and above (800~ the montmorillonite dehydroxylation temperatures. Enhancement of the resonance intensity for abnormal samples occurs after heating to 600~ and a decrease follows further heating to 800~ The behaviour of the normal samples does not follow any identifiable trend, as shown in Table 2.

7 ESR studies of montmorillonites 287 1/,,0" ~120 ~100' 8o- "~ 60- ~ 40- ~ 20" 0 O O O y: 88./, x r = -0.60*' S 012 0'4 0:6 0:8 Octohedrctt Fe 3 FIG. 4. Relationship between the intensity of the g = 4.3 resonance and octahedral Fe 3+ content of the montmorillonites. O, normal samples; 0, abnormal samples. TABLE 2. The influence of heating on the resonance at g = 4.3 of some of the montmorillonites. 20oC 600oc 800~ Sample g, 14 g4 I4 g I It should also be mentioned that the abnormal samples give spectra with smaller g = 4-3 resonances and higher g ~ 2.0 resonances than the normal samples. The smaller Fe3+-for-A1 octahedral substitutions characteristic of the normal samples give rise to a more intense signal with g = 4.3 due to isolated Fe 3+ ions. The g ~ 2-0 resonance arising from Fe 3+ in a different type of environment can be due to exchange interactions between clusters of Fe 3+ ions. Thus, as expected, this resonance is found in the abnormal samples which are iron-rich. The results of additional experiments suggest that this signal could have a structural component and it appears possible (see p. 284) to distinguish by g-value between structural iron and iron phases external to the structure. However, the existence of a separate iron-rich phase which is quantitatively differentiated for the two varieties fails to explain the differences revealed by X-ray diffraction between these. In fact, X-ray diffraction investigation of some normal and abnormal samples after iron removal showed the same variation in intensity and width at half-intensity of the 001 basal reflection between the two varieties (Table 3). The ESR results do indicate that differences between normal and abnormal

8 288 C. Crhciun and A. Meghea TABLE 3. The effect of free iron removal on the height and width at half-height of the 001 basal reflection of some of the montmorillonites.* Untreated samples Treated samples Sample Height Width Height Width 5 (normal) (normal) (abnormal) (abnormal) * Values are expressed in arbitrary units. montmorillonites could be related to structural and compositional differences, especially those involving Fe 3+. In this respect the ESR and chemical data recorded here confirm the results obtained previously by infrared spectroscopy (Cr~ciun, 1984b). The influence of Fea+-for-A1 substitutions on decrease in dehydroxylation temperature of dioctahedral smectites is well known (Mackenzie, 1970). Fig. 5 shows the relationship between dehydroxylation temperature and intensity of the g= 4.3 signal of the montmorillonites. This follows from the relationship between this resonance and structural Fe 3+ (Fig. 4). The differences between the two varieties with regard to the intensity and width of the 001 basal reflections could be due to different factors such as crystallite size, lattice distortions, distribution of ions in the octahedral and tetrahedral sheets, etc. Fig. 6 suggests that octahedral Fe3+-for-A1 substitutions can influence the intensity of the 001 basal reflection of the montmorillonites, i.e. the higher octahedral Fe 3+ content could be responsible for the decrease in intensity of the 001 basal reflection of abnormal o 720q,~ " 680- '~ 670- X o.x O 650" y = 675/ x r = * 6~ 0 20 if0 60 eb 160 Intensity of g= 4.3 signq[ FIG. 5. Relationship between the intensity of the g = 4.3 resonance and dehydroxylation temperature of the montmorillonites. O, normal samples; O, abnormal samples.

9 ESR studies of montmorillonites 289 -~ 200_ ~, 180 o 160 ~- 140 O ~ 120 dj t_ 1oo_ 8O -~ 60 " y x r- 0.62"* octahad ra I. Fe t FIG. 6. Relationship between the intensity of the 001 basal reflection of glycolated montmorillonites and their octahedral Fe 3+ contents. O, normal samples; O, abnormal samples. montmorillonite. For Cu radiation the mass attenuation coefficient of Fe is about 6 times higher than that of A1 and Mg (Brindley, 1980). CONCLUSIONS 1. The ESR spectra of Gurasada montmorillonites contain two features, a g = 4.3 signal ascribed to isolated Fe 3+ ions and a broad resonance at g ~ 2.0 which can be interpreted as arising from exchange interactions between clusters of neighbouring Fe s+ ions. The results of some additional experiments suggest that the last resonance could have a structural component. 2. The lineshape of the two specified resonances differs from abnormal to normal montmorillonites indicating that the Fe 3+ distribution between internal phase (montmorillonite lattice) and external phase (montmorillonite surface) is different for these two varieties. The normal variety is characterized by a higher g = 4.3 signal intensity and a lower g ~ 2.0 resonance intensity. 3. The relationships established between ESR, X-ray, thermal and chemical data indicate that Fe s+ plays a role in the differences between normal and abnormal montmorillonites. 4. The ESR results obtained demonstrate the ability of this technique to distinguish between the normal and abnormal montmorillonites from Gurasada bentonite deposits. ACKNOWLEDGMENTS The authors are deeply indebted to Dr P. L. Hall (Schlumberger Cambridge Research) and Dr B. A. Goodman (The Macaulay Institute for Soil Research) for supplying reprints of their papers and commenting on earlier drafts of this paper.

290 C. Cr?wiun and A. Meghea REFERENCES ANGEL B.R. & HALL P.L. (1972) Electron spin resonance studies of kaolins. Proc. Int. Clay Conf. Madrid, 47-59. ANGEL B.R. & VINCENT W.E.J.

10 290 C. Cr?wiun and A. Meghea REFERENCES ANGEL B.R. & HALL P.L. (1972) Electron spin resonance studies of kaolins. Proc. Int. Clay Conf. Madrid, ANGEL B.R. & VINCENT W.E.J. (1978) Electron spin resonance studies of iron oxides associated with the surface of kaolins. Clays Clay Miner. 26, BRINDLEY G.W. (1980) Quantitative X-ray mineral analysis of clays. Pp. 411~t38 in: Crystal Structures of Clay Minerals and their X-ray Identification (G. W. Brindley & G. Brown, editors). Mineralogical Society, London. CR~,CIUN C. (1978) The abnormal montmorillonite in bentonites from the Vica-Gurasada region. Trans. 2nd Nat. Clay Conf. Bucharest 1975, Studii Tehn. Ec. Seria I 14, Bucharest. C R~CIUN C. & G3~T~ G. (1981) Use of X-ray diffraction for the characterization of Gurasada montmorillonite. Abstracts of 7th Int. Clay Conf. Bologna-Pavia II pp CR~.CIUN C. (1984a) Mineralogical study of the Gurasada bentonite deposits. PhD thesis, University of Bucharest. CR~CIUN C. (1984b) Influence of the Fe3+-for-A1 octahedral substitutions on the IR spectra of montmorillonite minerals. Spectroscopy Letters 17, GOODMAN B.A. (1978) An investigation by Mrssbauer and EPR spectroscopy of the possible presence of iron-rich impurity phases in some montmorillonites. Clay Miner. 13, HALL P.L. (1980) The application of electron spin resonance spectroscopy to studies of clay minerals: I Isomorphous substitutions and external surface properties. Clay Miner. 15, HELSEN J.A. & GOODMAN B.A. (1983) Characterization of iron(ii) and iron(iii)-exchanged montmorillonite and hectorite using the MOssbauer effect. Clay Miner. 18, HERRERA R. St, PEECH M. (1970) Reaction of montmorillonite with iron(iii). Soil Sci. Soc. Am. Proc. 34, MACKENZIE R.C. ( 1970) Differential Thermal Analysis II. Academic Press, London. MCBRIDE M., PINNAVAIA T.J. & MORTLAND M.M. 0975) Perturbation of structural Fe a+ in smectitcs by exchange ions. Clays Clay Miner. 23, MEHRA O. P. & JACKSON M.L. (1960) Iron oxide removal from soils and clays by a dithionite citrate system buffered with sodium bicarbonate. Clays Clay Miner. 7, SCHWERTMANN U., FISHER W.R. 8~, PAPENDORF H. (1968) The influence of organic compounds on the formation of iron oxides, Trans. 9th Int. Cong. Soil Sci. Adelaide 1, WHITTIG L.D. & PAGE X. (1961) Iron adsorption by montmorillonite systems: I. Preliminary studies. Soil Sci. Soc. Am. Proc. 25,

THE USE OF PIPERIDINE AS AN AID TO CLAY-MINERAL IDENTIFICATION

THE USE OF PIPERIDINE AS AN AID TO CLAY-MINERAL IDENTIFICATION By J. M. OADES* and W. N. TOWNSEND Department of Agriculture, The University of Leeds. [Received 30th August, 1962] ABSTRACT It is suggested