Electronic and structural properties of reduced-charge montmorillonites

Applied Clay Science 16 Ž2000. 257–271 www.elsevier.nlrlocaterclay

Electronic and structural properties of reduced-charge montmorillonites W.P. Gates a

a,)

, P. Komadel b, J. Madejova´ b, J. Bujdak ´ b, a c J.W. Stucki , R.J. Kirkpatrick

Department of Natural Resources and EnÕironmental Sciences, UniÕersity of Illinois, Urbana, IL 61801 USA b Institute of Inorganic Chemistry, SloÕak Academy of Sciences, 842 36 BratislaÕa, SloÕakia c Department of Geology, UniÕersity of Illinois, Urbana, IL 61801 USA

Received 18 January 1999; received in revised form 25 January 1999; accepted 2 November 1999

Abstract Solid state silicon Ž29 Si. and aluminum Ž27Al. nuclear magnetic resonance ŽNMR. spectroscopies were applied to a series of reduced-charge montmorillonites ŽRCMs. to discern changes in electronic and structural properties that are induced by Li fixation. Room temperature 29 Si MAS NMR spectra revealed a consistent chemical shift to more negative values and increased line width of the main Q 3Ž0Al. Si resonance with increasing levels of Li fixation in the RCM series. A decreased line width of the octahedral Al ŽAl w6x . environment was observed and may be attributed to formation of a more uniform electronic environment surrounding Al w6x as charge reduction occurs. No appreciable changes in the tetrahedral Al ŽAl w4x . peak were observed for the series, except for line broadening. Correlations of 29 Si NMR chemical shifts with layer charge and infrared-active structural vibrations indicated that distortions in the Si–O–T bond angles ŽT s tetrahedral Si or Al. occurred, with the mean Si–O–T bond angle increasing, following charge reduction. These results are interpreted as evidence of a redistribution both of layer charge and an abatement of the fit between octahedral and tetrahedral sheets following Li fixation and charge reduction. Our results are consistent with the formation of pyrophyllite-like character in reducedcharge montmorillonite. q 2000 Elsevier Science B.V. All rights reserved. Keywords: reduced-charge montmorillonites; modified clays; Li fixation; layer charge; MAS NMR spectroscopy; infrared spectroscopy

)

Corresponding author. CSIRO Land and Water, PMB No. 2, Glen Osmond, SA 5064, Australia. Tel.: q61-8-8303-8512; Fax: q61-8-8303-8550; E-mail: [email protected] 0169-1317r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 Ž 9 9 . 0 0 0 5 7 - 5

258

W.P. Gates et al.r Applied Clay Science 16 (2000) 257–271

1. Introduction Reduced-charge montmorillonites Ž RCMs. have long been studied to probe layer charge characteristics Ž Green-Kelly, 1953; Brindley and Ertem, 1971; Glasier and Mering, 1971; Clementz and Mortland, 1974; Jaynes and Bigham, ´ . 1987 and to test the dependence of physicochemical properties of clays to layer charge ŽMaes et al., 1985; Bujdak ´ et al., 1992; Madejova´ et al., 1996.. Heating of Li-saturated dioctahedral montmorillonites reduces the net negative layer charge due to fixation of Li within the crystal structure, and has been applied to prepare surface-modified organo-clays with high sorptive capacities for waterborne pollutants ŽJaynes and Boyd, 1991.. While numerous studies have been directed at attempting to discover the mechanism of layer charge reduction and the location of Li within the montmorillonite structure, to date, little work has attempted to reconcile the often conflicting results and interpretations. Application of various spectroscopic techniques has provided strong evidence suggesting that cations are heterogeneously distributed into localized domains within the structure of clays ŽSanz and Stone, 1977, 1983; Stone, 1981; Slonimskaya´ et al., 1986; Decarreau et al., 1987; Madejova´ et al., 1994. . In montmorillonites, substituted Mg 2q octahedra Ž Mg w6x . and Al 3q tetrahedra ŽAl w4x . may be clustered. Analysis of integrated intensities of individual components of the OH stretching band in the IR spectrum of Jelsovy ˇ ´ Potok ŽJP. montmorillonite revealed higher probabilities of octahedral AlAl and FeMg pairs sharing one OH group compared to the values expected from a random distribution ŽMadejova´ et al., 1994. . If Li enters vacant octahedra or ditrigonal cavities of the interlayer siloxane surface upon heating, one would expect it to reside near Mg w6x andror Al w4x , creating new next-nearest-neighbor ŽNNN. environments for tetrahedral Si and Al and octahedral Al that have different electronic environments than the original. The presence of fixed Li within the crystalline structure of a dioctahedral mineral is thus expected to cause noticeable electronic and structural distortions of the clay structure ŽLuca et al., 1989. . Nuclear magnetic resonance Ž NMR. spectroscopic studies have revealed electronic changes of RCMs ŽLuca et al., 1989; Trillo et al., 1993; Alvero et al., 1994; Theng et al., 1997; Alba et al., 1998. , but how this electronic change influences the structure is poorly understood. Both Luca et al. Ž 1989. and Trillo et al. Ž1993. found that the 27Al w6x NMR signal remained unchanged and that the 27 Al w4x peak maxima shifted toward more negative values and was eventually lost with increasing Li fixation. They interpreted these results as being due to distortion of the Al w4x sites caused by the presence of Li at the base of the ditrigonal cavity, which increased the electric field gradient Ž EFG. and thus the quadrupole coupling constant ŽQCC. at Al w4x sites. Sites with large QCCs are often not observed in MAS NMR spectra of quadrupolar nuclei. Alvero et al. Ž1994. and Alba et al. Ž 1998. investigated changes in the structure of two Li-saturated montmorillonites upon heating in air at 3008C for

W.P. Gates et al.r Applied Clay Science 16 (2000) 257–271

259

24 h and subsequent hydrothermal treatment under an atmosphere of water vapor at 8.5 MPa at 3008C for 24 h. The authors observed complete reexpansion of the hydrothermally treated samples. The authors considered the data obtained to be a direct experimental evidence on the location of lithium ions in the hexagonal holes of the collapsed RCM structure. However, Calvet and Prost Ž1971. showed that the octahedral site is not necessarily the most stable site for Li to occupy, inferring that the site occupied depends on charge properties of the montmorillonite Ži.e., magnitude and location of charge. as well as how heat treatments are performed. Green-Kelly Ž 1955. reported in one of the earliest papers ever published on Li fixation in montmorillonite that cation migration can take place from the octahedral sites to the interlayer position when a Li-montmorillonite is heated at 2508C in a 0.1 M NaCl solution. This result helps to explain the reexpandability obtained following NH 3 vapor treatment and increased exchangeable Li content after heat treatment Ž Farmer and Russell, 1967.. Hydrothermal processing of smectites is known to cause severe changes in the structure, therefore liberation of Liq ions and reexpansion of previously collapsed interlayers cannot be expected to provide conclusive evidence concerning the location of Li in the structure of reduced-charge montmorillonites. Infrared spectroscopic analysis of a series of RCM samples prepared from the same parent clay at different temperatures ŽMadejova´ et al., 1996. provided evidence of some trioctahedral character Ž Calvet and Prost, 1971. , and thus the presence of Li within the octahedral sheet, with increased Li fixation. Both Madejova´ et al. Ž1996. and Calvet and Prost Ž1971. noted an appearance of an AlMgLiOH component in the OH stretching region confirming the development of a local trioctahedral character in the coordination of OH groups in the samples heated above 2008C. Moreover, they also observed increased frequency and diminished intensity of OH bending bands associated with octahedral cations with increasing levels of Li fixation. Still unclear, however, is why the expected changes in the 27Al w6x NMR signal are not observed Ž Luca et al., 1989; Trillo et al., 1993. and what alternative mechanism, if any, explains the 27Al w4x NMR results. In the present study, magic angle spinning Ž MAS. NMR experiments, at Ho magnetic field strengths of 7.5–11 T and MAS frequencies of 4.0–10.0 kHz were conducted on a series of RCMs to address these questions and to establish further spectroscopic evidence concerning the possible locationŽ s. of heat-fixed Li in RCMs. 2. Materials and methods 2.1. Samples and charge reduction The set of reduced-charge Jelsovy ˇ ´ Potok ŽJP. montmorillonites ŽSlovakia. was the same as that prepared and described by Madejova´ et al. Ž1996.. The

260

W.P. Gates et al.r Applied Clay Science 16 (2000) 257–271

samples were Li-saturated, heated in air and re-exchanged for Ca2q. These same RCM samples have been characterized by measurements of swelling Ž Bujdak ´ et al., 1992. , thermogravimetry Ž Bujdak ´ and Slosiarikova, ´ 1994., infrared spectroscopy ŽMadejova´ et al., 1996. , surface area, XRD, HRTEM and acid dissolution ŽKomadel et al., 1996.. Sample identification, treatment temperature and selected properties of the parent montmorillonite and the reduced-charge products are shown in Table 1. 2.2. NMR spectroscopy Multinuclear MAS NMR experiments were conducted using approximately 300 mg of air-dried powder packed into either 7-mm Ž29 Si. or 5-mm Ž27Al. zirconia rotors sealed with Vespel endcaps. For 29 Si, 176 acquisitions were collected on a homebuilt spectrometer with an Ho magnetic field strength of 8.45 T Ž71.5 MHz Larmor frequency. using sample spinning frequencies of 4 kHz Ž probe built by Doty Scientific., 8 ms radio frequency Žrf. pulse lengths, and 90 s recycle delay times. A commercial spectrometer ŽGE-400 WB, Ho s 7.0 T. operating at 59.62 MHz was also employed for 29 Si NMR analysis using a 4-kHz spinning frequency, 8-ms rf pulse lengths, and 30-s recycle delays. A total of 180–200 acquisitions were compiled for each sample. A Table 1 Selected properties of the Ca-saturated reduced-charge montmorillonite ŽJelsovy ˇ ´ Potok, Slovakia. series studied Žn.d.s not determined. Samplea

M1 M2 M3 M4 M5 M6 M7 M8 M9

a

Treatment temperature Ž8C.

Layer charge b ŽmEqr100 g.

– 83 105 71 110 65 120 60 130 50 135 46 160 26 210 14 – 83 Correlation coefficient to Layer charge 29 Si NMR Q 3Ž0Al.

IRc AlAlŽOH. bend Žcmy1 .

AlMgŽOH. bend Žcmy1 .

Si–O stretch Žcmy1 .

915 916 916 918 920 922 932 934 n.d. y0.9556

843 843 843 844 849 852 854 854 n.d. y0.9258

1035 1038 1037 1041 1043 1043 1047 1047 n.d. y0.9712

y0.9727

y0.9111

y0.9047

Sample M9 is the Li-form of the Jelsovy ˇ ´ Potok montmorillonite not subjected to heat treatment. b From Komadel et al., 1996. c From Madejova´ et al., 1996.

W.P. Gates et al.r Applied Clay Science 16 (2000) 257–271

261

homebuilt spectrometer based on an 11.7-T superconducting solenoid magnet equipped with a MAS probe Ž Doty Scientific. was employed to collect 27Al spectra at 130.3 MHz. Up to 3300 acquisitions were collected at sample spinning frequencies of about 10.8 kHz, rf pulse lengths of 1 ms, and 300 ms recycle delay times. Chemical shifts for 29 Si and 27Al were referenced to pure tetramethylsilane Ž TMS. and 1 M AlCl 3 ŽAlŽH 2 O. 6 . solutions. Curve-fitting procedures were carried out to quantify the chemical shifts of 29 Si using a spectroscopic analysis program Ž Gramsr32, Galactic Industries, Salem, NH, USA.. The calculated chemical shift and line-width parameters of Gaussian-shaped peaks were allowed to vary unconstrained during the nonlinear least squares fitting procedure. A particular solution was deemed adequate when the simulated curve visually matched the original spectra and the minimized x 2 estimates of three separate, stand-alone trial replications were within 5% of one another. Only the peak maximum and line widths were reported for 27Al due to quadrupolar distortions. Problems with quantitative interpretation of 27Al NMR spectra of phyllosilicates are well documented ŽPlee et al., 1985; Kirkpatrick, 1988..

3. Results 3.1.

29

Si MAS NMR

The 29 Si MAS NMR spectra provide evidence of electronic changes in the tetrahedral sheet of the montmorillonite with heat treatment and Li fixation Ž Fig. 1, Table 2. . A progressively more negative Ž up-field. change of the chemical shift Ž d . and increased line width Ž W — peak width at half height. of the 29 Si Q 3Ž0Al. signal occurred with decreasing layer charge and increasing Li fixation ŽFig. 2.. The Ca Ž sample M1. and Li Ž sample M9. forms of the untreated montmorillonite and the RCMs prepared at the lowest temperatures Ž samples M2–M4. had similar chemical shifts Žy93.6 to y94.0 ppm. and similar line widths Ž 4.2–4.6 ppm.. For samples heated above 1308C, the 29 Si NMR peak maximum progressively moved to more negative values Ž Fig. 2. , and the half-height width increased ŽTable 2. with increasing amounts of Li fixation. The observed 29 Si chemical shift of y95.4 ppm for sample M8 agrees well with the values reported for pyrophyllite Žy95.9 ppm, Weiss et al., 1987; y95.4 ppm, Sanchez-Soto et al., 1993. . Madejova´ et al. Ž 1996. also observed bands near 1120 and 420 cmy1 in the IR spectra of samples M7 and M8, and proposed that these bands indicated the development of pyrophyllite-like character of the layers present in these samples. Komadel et al. Ž 1996. reported that increasing Li contents resulted in increased numbers of nonswelling, pyrophyllite-like layers and decreased specific surface area in the RCM series.

W.P. Gates et al.r Applied Clay Science 16 (2000) 257–271

262

Fig. 1. Selected 29 Si MAS NMR spectra of reduced-charged montmorillonite from Jelsovy ˇ ´ Potok, Slovakia. Spectra were referenced to TMS. Definitions: dashed line — partial least squares fit of central Q 3Ž0Al. peak and spinning side bands; dotted line — partial least squares fit of entire spectrum Žsolid line..

3.2. 27Al MAS NMR The most obvious change in the 27Al MAS NMR spectra was the decreased line width of the Al w6x peak due to apparently diminished quadrupolar distortion Table 2 Room temperature 29 Si and 27Al MAS NMR parameters obtained on the Ca-saturated reducedcharge montmorillonite ŽJelsovy ˇ ´ Potok, Slovakia. series Samplea M1 M2 M3 M4 M5 M6 M7 M8 M9 a

29

Si d Žppm.

27

W Žppm.

peak max.b Žppm.

W Žppm.

27 Al w4x peak max. Žppm.

y93.6 y93.6 y93.8 y94.0 y94.0 y94.4 y94.8 y95.4 y93.6

4.6 4.2 4.4 4.6 5.2 5.3 5.7 6.1 4.6

3.6 3.9 3.3 3.9 3.7 3.8 3.9 4.1 3.6

13.3 13.0 13.7 12.3 12.3 11.4 12.3 10.5 12.8

68.9 68.9 68.2 68.0 68.0 68.1 67.9 67.8 68.9

Al w6x

Sample M9 is the Li-form of the Jelsovy ˇ ´ Potok montmorillonite not subjected to heat treatment. Samples M1–M8 are the Ca-form of samples treated as listed in Table 1. b peak max.s position in parts per million of maximum peak intensity.

W.P. Gates et al.r Applied Clay Science 16 (2000) 257–271

263

Fig. 2. The Q 3Ž0Al. 29 Si MAS NMR chemical shifts as a function of layer charge following charge reduction of Li-montmorillonite from Jelsovy ˇ ´ Potok, Slovakia Ž r 2 s 0.9611. compared 29 with the Si MAS NMR chemical shifts of dioctahedral and trioctahedral smectites studied by Weiss et al. Ž1987.. Layer charge for dioctahedral and trioctahedral smectites estimated from data of Weiss et al. Ž1987..

ŽWoessner, 1989. with increasing Li fixation ŽTable 2, Fig. 3. , which to the best of our knowledge, has never before been noted. The 27Al w6x peak maximum for sample M8 Ž4.1 ppm. observed here is similar to values reported in the literature for pyrophyllite Ž4.0 ppm, Sanchez-Soto et al., 1993. , but no general trend was discernible with increasing Li substitution in the series Ž Table 2. . The 27Al w4x signal did not change significantly, but a slight shift to less positive values at the highest levels of Li fixation Ž Fig. 3, Table 2. was noted.

Fig. 3. Selected 27Al MAS NMR spectra of reduced-charged montmorillonite from Jelsovy ˇ ´ Potok, Slovakia. 10.8 kHz sample spinning speed. Spectra were referenced to 1 M AlCl 3 solution. Definitions: RCM-Al IV — tetrahedral Al signal of RCM; FW-Al IV — tetrahedral Al signal of framework aluminosilicate admixture present in sample; ss — spinning sidebands; Al VI — octahedral Al signal of RCM.

264

W.P. Gates et al.r Applied Clay Science 16 (2000) 257–271

The ratio of tetrahedral to octahedral Al ŽAl w4xrAl w6x . as determined by the NMR peak relative intensities ŽGoodman and Stucki, 1984. also remained unchanged. The behavior of the 27Al w4x signal is in general agreement with the results of Trillo et al. Ž 1993. and Alvero et al. Ž 1994. , except that the previous studies reported a 27Al w4x signal loss with increasing Li fixation. Because our spectral decomposition was complicated by a tetrahedral Al ŽFW-Al w4x . signal due to the presence of a framework aluminosilicate admixture Ž not observed by XRD. resonance near 55 ppm ŽPhillips et al., 1988; Kirkpatrick, 1988. , it is not possible to further interpret the 27Al w4x spectra.

4. Discussion 4.1. Structural Õs. electronic factors The negative shift of the 29 Si Q 3Ž0Al. peak can be readily interpreted as being due to decreased layer charge Ž Weiss et al., 1987; Luca et al., 1989; Trillo et al., 1993; Sanz and Robert, 1992. of the montmorillonite following Li fixation. Weiss et al. Ž1987. and Sanz and Robert Ž1992. observed that an increase in layer charge, regardless of whether the origin is tetrahedral or octahedral isomorphous substitution, resulted in less negative 29 Si chemical shifts ŽFig. 2.. Upon reduction of octahedral FeŽ III. to FeŽ II. , Gates et al. Ž 1996. observed a shift in the 29 Si signal to less negative chemical shifts. They attributed these shifts partly to increased negative charge in the octahedral sheet, and partly to structural changes that occurred during Fe reduction. The results shown here confirm the observation Ž Luca et al., 1989; Trillo et al., 1993. that Li fixation and associated charge reduction affect the 29 Si chemical shifts in the same way. Local structural effects, which are related to isomorphous substitution and charge are known to be correlated to 29 Si chemical shifts, and such considerations are useful for understanding the structural and electronic effects of Li fixation in montmorillonites. Changes in 29 Si chemical shifts of silicates due to cation substitution are normally dominated by subtle changes in electronic states near the observed nuclide. These in turn are often correlated with differences in structural parameters such as bond angles and bond distances Ž e.g., Smith and Blackwell, 1983; Kinsey et al., 1985. . For phyllosilicates, measures of ditrigonal distortion correlate well with 29 Si chemical shifts, with the 29 Si chemical shift becoming more negative with greater amounts of distortion Ž Weiss et al., 1987; Sanz and Robert, 1992; Gates et al., 1996. . This is also the case for our RCMs ŽFig. 2.. With increased Li fixation the IR active Si–O vibrational frequencies increased and approached those of pyrophyllite Ž Madejova´ et al., 1996. . The broad complex Si–O stretching band involves both in-plane Si–O–Si and plane-nor-

W.P. Gates et al.r Applied Clay Science 16 (2000) 257–271

265

mal Si–O stretching modes Ž Farmer, 1974. . The in-plane vibrations absorb in the 1040–1020 cmy1 range for montmorillonites and near 1060 cmy1 for pyrophyllite, whereas the plane-normal mode is located near 1080 cmy1 for montmorillonite, but at lower frequency Ž near 1050 cmy1 . for pyrophyllite. For montmorillonites, a decrease in the layer charge upon Li fixation causes a shift of the in-plane Si–O–Si vibration to higher wave numbers and a shift of the perpendicular Si–O vibration to lower wave numbers Ž Madejova´ et al., 1996. . Consequently, the complex Si–O stretching band of a reduced-charge montmorillonite is narrower and located at higher frequencies than the unheated clay. Similar shifts of the Si–Oapical and Si–O basal vibrations were observed by Kitajima et al. Ž1991. in the spectra of synthetic micas, in which the layer charge resulted from the positive charge deficiency in the octahedral sheets. For layer silicates, the Si–Oapical bond distance increases with decreasing layer charge, while the Si–O basal bond distance decreases because the effective negative charge on Oapical decreases with decreasing layer charge. Thus, the Si–O stretching vibrations reflect the structural aspects of SiO4 tetrahedra, which are determined by the magnitude and location of layer charge Ž Kitajima et al., 1991. . 4.2. Correlations of NMR and FTIR data The frequencies of various IR-active vibrational modes involving both tetrahedral and octahedral components in the RCM structure correlated reasonably well with 29 Si NMR d ŽFig. 4.. While the correlation of the complex band near 1035–1047 cmy1 arising from the Si–O stretching vibrations with the 29 Si chemical shift Ž r 2 s y0.9047. is sufficient to indicate linearity, study of Figs. 2 and 4a and Table 1 reveals that Ž i. at low levels of Li fixation Ž M1–M4. charge reduction influenced the Si–O vibrational frequency to a greater extent than the electronic environment about the Si nuclei, and Ž ii. at higher levels of Li fixation ŽM5–M8. charge reduction perturbed both the electronic environment about Si and the IR active Si–O vibrational frequencies. Thus, while layer charge influences both electronic and structural properties of smectites, during the charge reduction process, Si–O vibrational frequencies appear to be more readily perturbed than the electronic environment about the Si nuclei. The frequencies of the IR-active AlMgŽOH. and AlAlŽ OH. bending vibrations ŽMadejova´ et al., 1996. are also more readily perturbed than the 29 Si NMR chemical shifts of the Q 3Ž0Al. sites ŽFig. 4b, c. , presumably due to the proximity of the proton to the site of Li fixation. A positive shift of the vibrational frequency of AlMgŽOH. centers with charge reduction occurred despite the lower correlation of this vibration with layer charge ŽTable 1.. The correlations of the AlAlŽ OH. bending vibrations with layer charge Ž Table 1. and with 29 Si d for the RCM series were excellent Ž Fig. 4c. , and the greatest changes in the IR vibrations, again, occurred between samples M5 and M8.

266

W.P. Gates et al.r Applied Clay Science 16 (2000) 257–271

Fig. 4. Correlations of infrared active absorption bands associated with lattice vibrations Ža. Si–O stretching mode Ž r 2 sy0.9047., Žb. MgAlOH bending deformation Ž r 2 sy0.9111. and Žc. AlAlOH bending deformation Ž r 2 sy0.9727. with 29 Si MAS NMR chemical shift. Lines are visual aid only. Infrared data from Madejova´ et al. Ž1996..

The 27Al w6x line widths decreased by more than 3 ppm as the amount of Li incorporation into the montmorillonite structure increased Ž Table 2. . The most important effect is probably a decrease in the range of 27Al quadrupole coupling constants Že.g., Woessner, 1989. , which is consistent with decreasing electronic heterogeneity of the Al w6x environment with charge reduction. The decrease in 27 Al w6x W with increasing Li content in the samples may be due to the creation of a more uniform structural andror electronic environment about Al octahedra ŽPlee et al., 1985. . This result is consistent with intensity differences reported by

W.P. Gates et al.r Applied Clay Science 16 (2000) 257–271

267

Madejova´ et al. Ž 1996. for AlAlŽ OH. stretching vibrations at 3635 cmy1 and 3618 cmy1 in the IR spectra of M7 and M8. They observed that the absorbance intensity of the band at 3635 cmy1 increased, while it decreased for the band at 3618 cmy1, indicating different OH dipole environments arising from a decreased negative charge on adjacent Oapical . These results lend further support to the hypothesis that charge reduction due to Li incorporation into the montmorillonite structure caused a redistribution of charge at the apical oxygens joining the tetrahedral and octahedral sheets, and thus influenced the bonding environment of adjacent octahedral and tetrahedral cations. 4.3. Redistribution of layer charge — the pyrophyllite-like character of RCMs For smectites the interatomic distances between cations located within the structure in octahedral, tetrahedral and ditrigonal sites are similar, and a cation in any of these sites is a next-nearest neighbor ŽNNN. to another located in adjacent sites Ž Bailey, 1984. . Changes in charge at the shared apical oxygen and distortions in bonds linking the tetrahedral and octahedral sheets are likely to influence the overall bond character of both the tetrahedral and octahedral cations ŽBrown, 1992. . Farmer and Russell Ž 1966. showed that each Si–O vibration within any one smectite layer is affected by neighboring bond vibrations within that layer. The Si–O bonds resonate within an electric field generated from charges residing on Si and the apical oxygen ions, whereas the Si–O–Si bonds resonate within a field produced by charges residing at Si and the basal oxygen ions. This implies that redistribution of charge at Oapical affects not only the field which Si experiences, but also the field, through the Si ion, in which the Si–O basal –Si vibrate. Incorporation of Li into the montmorillonite structure decreases the negative layer charge and the electric field experienced by individual dipoles and, thus, their vibrational frequencies approach values more typical for pyrophyllite. The different frequencies of Si–O stretching modes for montmorillonite and pyrophyllite suggest that these minerals differ significantly in average local structure ŽFarmer and Russell, 1964; Robert and Kodama, 1988. . A decrease in the charge at Oapical due to incorporation of Li into the structure should, therefore, result in changes in Si–O bond distances and Si–O basal –Si bond angles, similar to that reported by Kitajima et al. Ž1991. for synthetic fluorine micas. The strong correlations between these tetrahedral structural parameters measured by both IR and NMR spectroscopies on this RCM series infer that shifts in Si–O vibrational frequencies and 29 Si chemical shift with charge reduction are influenced as much by structural characteristics as by simple charge neutralization. It follows that changes in charge in the octahedral sheet Ždue to Li incorporation, or oxidationrreduction of structural Fe. influence the electronic environment of tetrahedral Si as much or more than similar changes in the tetrahedral sheet Ž Gates et al., 1996. . Li incorporation either into the

268

W.P. Gates et al.r Applied Clay Science 16 (2000) 257–271

ditrigonal cavities of the siloxane surface or the octahedral sheet could significantly influence the tetrahedral 29 Si chemical shift by increasing the Si–Oapical bond distance and changing Si–O basal –Si bond angles. Thus, the goodness of fit between the octahedral and tetrahedral sheets is diminished following charge reduction, and the RCMs develop significant pyrophyllite-like character. Our current understanding of the process of formation of RCMs presumes that Li fixation occurs adjacent to sites of isomorphous substitution Ž Madejova´ et al., 1996., e.g., in sites near where Mg 2q substitutes for Al w6x , or where Al 3q substitutes for Si w4x. The lack of complete trioctahedral character in the JP montmorillonite studied here, as demonstrated by incomplete loss of OH bending modes associated with dioctahedral cations in the IR spectra Ž Madejova´ et al., 1996. , occurs because there are many more vacant octahedra and ditrigonal cavities available for Li substitution than are necessary to induce complete layer charge neutralization. Thus, Li incorporation into either of the possible sites occurs only to the extent necessary to balance the layer charge in montmorillonites, leaving many octahedral sites vacant, and resulting in an electronic environment surrounding Si that is similar to pyrophyllite Ž Madejova´ et al., 1994; Komadel et al., 1996. . In addition, clustering of Mg w6x and Al w4x substitutions ŽSanz and Stone, 1977, 1983; Stone, 1981; Slonimskaya´ et al., 1986; Decarreau et al., 1987; Madejova´ et al., 1994. may result in domains of residual negative charge surrounded predominantly by domains of no net charge following charge reduction.

5. Conclusions Charge reduction of the Jelsovy ˇ ´ Potok ŽSlovakia. montmorillonite due to Li fixation by heat treatment resulted in increased line width and a progressive negative Žup-field. change in the chemical shift of the 29 Si Q 3Ž 0Al. signal. Little changes were observed for the 27Al w4x and 27Al w6x signals. As expected, correlations between the 29 Si NMR chemical shift and various IR active vibrational modes indicated that both structural and electronic properties were effected by incorporation of Li into the structure of the montmorillonite. Fixation of Li within the montmorillonite structure perturbs the electric field experienced at the apical oxygen that bridges the octahedral and tetrahedral sheets. This in turn influences both Si–O bond distances Ž and thus IR active vibrational frequencies. and the electronic shielding at the Si nucleus. Both IR and NMR spectroscopic evidence indicates that charge reduction due to Li fixation into the montmorillonite structure results in electronic and structural character similar to that of pyrophyllite, despite the existence of trioctahedral character in RCMs. This apparent discrepancy is due to the presence, in the structure of dioctahedral hydrous phyllosilicates, of an excess of octahedral

W.P. Gates et al.r Applied Clay Science 16 (2000) 257–271

269

vacancies Ž and ditrigonal cavities. for Li to possibly occupy than is required for complete charge neutralisation.

Acknowledgements The authors wish to acknowledge Mr. Ben Montez for his able assistance in the fast-MAS NMR experiments, and Dr. C.A. Weiss, Jr., for valuable and critical comments on an earlier version. The authors acknowledge financial support from the Slovak Grant Agency for Science Ž grant No. 2r4042r98., NSF grant EAR 90-04260, and the Illinois Ground Water Consortium.

References Alba, M.D., Alvero, R., Becerro, A.I., Castro, M.A., Trillo, J.M., 1998. Chemical behavior of lithium ions in reexpanded Li montmorillonite. J. Phys. Chem. B 102, 2207–2213. Alvero, R., Alba, M.D., Castro, M.A., Trillo, J.M., 1994. Reversible migration of lithium in montmorillonite. J. Phys. Chem. 98, 7848–7853. Bailey, S.W., 1984. Classification and structures of the micas. In: S.W. Bailey ŽEd.., Micas. Reviews in Mineralogy, Vol. 13. Mineralogical Society of America, Bookcrafters, Chelsea, pp. 1–12. Brindley, G.W., Ertem, G., 1971. Preparation and solvation properties of some variable charge montmorillonite. Clays Clay Miner. 19, 399–404. Brown, I.D., 1992. Chemical and steric constrains in inorganic solids. Acta Crystallogr. B42, 553–572. Bujdak, ´ J., Petrovieova, ` ´ I., Slosiarikova, ´ H., 1992. Study of water-reduced charge montmorillonite system. Geol. Carpathica—Clays 43, 109–111. Bujdak, ´ J., Slosiarikova, ´ H., 1994. Dehydration and dehydroxylation of reduced-charge montmorillonite. J. Thermal Anal. 41, 272–825. Calvet, R., Prost, R., 1971. Cation migration into empty octahedral sites and surface properties of clays. Clays Clay Miner. 19, 175–186. Clementz, D.M., Mortland, M.M., 1974. Properties of reduced-charge montmorillonite: tetra-alkylammonium ion exchange forms. Clays Clay Miner. 22, 223–229. Decarreau, A., Colin, F., Herbillon, A., Manceau, A., Nahon, D., Paquet, H., Trauth-Badaud, D., Trescases, J.J., 1987. Domain segregation in Ni–Fe–Mg-smectites. Clays Clay Miner. 35, 1–10. Farmer, V.C., 1974. Layer silicates. In: V.C. Farmer ŽEd.., Infrared Spectra of Minerals. Mineralogical Society, London, pp. 331–363. Farmer, V.C., Russell, J.D., 1964. The infra-red spectra of layer silicates. Spectrochim. Acta 20, 1149–1173. Farmer, V.C., Russell, J.D., 1966. Effects of particle size and structure on the vibrational frequencies of layer silicates. Spectrochim. Acta 22, 389–398. Farmer, V.C., Russell, J.D., 1967. Infrared absorption spectrometry in clay studies. Clays Clay Miner. 15, 121–142. Gates, W.P., Stucki, J.W., Kirkpatrick, R.J., 1996. Structural properties of reduced Upton montmorillonite. Phys. Chem. Miner. 23, 535–541.

270

W.P. Gates et al.r Applied Clay Science 16 (2000) 257–271

Glasier, R., Mering, J., 1971. Migration of lithium cations in the dioctahedral smectites ŽHoff´ mann–Klemen effect.. C. R. Acad. Sci., Paris 273, 2399–2402. Green-Kelly, R., 1953. The identification of montmorilloniods in clays. J. Soil Sci. 4, 233–237. Green-Kelly, R., 1955. Dehydration of the montmorillonite minerals. Miner. Mag. 30, 604–615. Goodman, B.A., Stucki, J.W., 1984. The use of nuclear magnetic resonance ŽNMR. for the determination of tetrahedral aluminum in montmorillonite. Clay Miner. 19, 663–667. Jaynes, W.F., Bigham, J.M., 1987. Charge reduction, octahedral charge, and lithium retention in heated, Li-saturated smectites. Clays Clay Miner. 35, 440–448. Jaynes, W.F., Boyd, S.A., 1991. Hydrophobicity of siloxane surfaces in smectites as revealed by aromatic hydrocarbon adsorption from water. Clays Clay Miner. 39, 428–436. Kinsey, R.A., Kirkpatrick, R.J., Hower, J., Smith, K.A., Oldfield, E., 1985. High resolution aluminum-27 and silicon-29 nuclear magnetic resonance spectroscopic study of layer silicates, including clay minerals. Am. Miner. 70, 537–548. Kirkpatrick, R.J., 1988. MAS NMR spectroscopy of minerals and glasses. In: Hawthorne, F. ŽEd.., Spectroscopic Methods in Mineralogy and Geology. Reviews in Mineralogy, Vol. 18. Mineralogical Society of America, Chelsea, MI, pp. 341–403. Kitajima, K., Taruta, S., Takusagawa, N., 1991. Effects of layer charge on the IR spectra of synthetic fluorine micas. Clay Miner. 26, 435–440. ˇ Komadel, P., Bujdak, V., Elsass, F., 1996. Effect of non-swelling layers on ´ J., Madejova, ´ Sucha, the dissolution of reduced-charge montmorillonite in hydrochloric acid. Clay Miner. 31, 333–345. Luca, V., Cardile, C.M., Meinhold, R.H., 1989. High-resolution multinuclear NMR study of cation migration in montmorillonite. Clay Miner. 24, 115–119. Madejova, ´ J., Bujdak, ´ J., Gates, W.P., Komadel, P., 1996. Preparation and infrared spectroscopic characterization of reduced-charge montmorillonite with various Li contents. Clay Miner. 31, 233–241. ˇ´ˇ B., 1994. Infrared study of octahedral site populations in Madejova, ´ J., Komadel, P., Cıcel, smectites. Clay Miner. 29, 319–326. Maes, A., Verheyden, D., Cremers, A., 1985. Formation of highly selective cesium-exchange sites in montmorillonite. Clays Clay Miner. 33, 251–257. Phillips, B.L., Kirkpatrick, R.J., Hovis, G.L., 1988. 27Al, 29 Si, and 23 Na MAS NMR study of an Al, Si ordered alkali feldspar solid solution series. Phys. Chem. Miner. 16, 262–275. Plee, D., Borg, F., Gatineau, L., Fripiat, J.J., 1985. High-resolution solid-state 27Al and 29 Si nuclear magnetic resonance study of pillared clays. J. Am. Chem. Soc. 107, 2362–2369. Robert, J.-L., Kodama, H., 1988. Generalisation of the correlations between hydroxyl-stretching wavenumbers and composition of micas in the system K 2 O–MgO–Al 2 O 3 –SiO 2 –H 2 O: a single model for trioctahedral and dioctahedral micas. Am. J. Sci. 288A, 196–212. Sanchez-Soto, P., Sobrados, I., Sanz, J., Perez-Rodriguez, J.L., 1993. 29-Si and 27-Al MAS NMR study of the thermal transformations of pyrophyllite. J. Am. Ceram. Soc. 76, 3024–3028. Sanz, J., Robert, J.-L., 1992. Influence of structural factors on 29 Si and 27Al NMR chemical shifts of phyllosilicates 2:1. Phys. Chem. Miner. 19, 39–45. Sanz, J., Stone, W.E.E., 1977. NMR study of micas: I. Distribution of Fe 2q ions on the octahedral sites. J. Chem. Phys. 67, 3739–3743. Sanz, J., Stone, W.E.E., 1983. NMR study of micas: III. The distribution of Mg 2q and Fe 2q around the OH groups in micas. J. Phys. C: Solid State Phys. 16, 1271–1281. Slonimskaya, ´ M.V., Besson, G., Danyak, L.G., Tchoubar, C., Drits, V.A., 1986. Interpretation of the IR spectra of celadonites and glauconites in the region of OH-stretching frequencies. Clay Miner. 21, 115–149. Smith, J.V., Blackwell, C.S., 1983. Nuclear magnetic resonance of silica polymorphs. Nature 303, 223–225.

W.P. Gates et al.r Applied Clay Science 16 (2000) 257–271

271

Stone, W.E.E., 1981. The use of NMR in the study of clay minerals. In: Fripiat, J.J. ŽEd.., Advanced Techniques for Clay Mineral Analysis. Elsevier, Amsterdam, pp. 77–112. Theng, B.K.G., Hayashi, S., Soma, M., Seyama, H., 1997. Nuclear magnetic resonance and X-ray photoelectron spectroscopic investigation of lithium migration in montmorillonite. Clays Clay Miner. 45, 718–723. Trillo, J.M., Alba, M.D., Alvero, R., Castro, M.A., 1993. Reexpansion of collapsed Li-montmorillonite; evidence of the location of Liq ions. J. Chem. Soc., Chem. Commun. 23, 1809–1811. Weiss, C.A. Jr., Altaner, S.P., Kirkpatrick, R.J., 1987. High resolution 29 Si NMR spectroscopy of 2:1 layer silicates: correlations among chemical shift, structural distortions, and chemical variations. Am. Miner. 72, 935–942. Woessner, D., 1989. Characterization of clay minerals by 27Al nuclear magnetic resonance spectroscopy. Am. Miner. 74, 203–215.