Clay Minerals (t 999) 34, 543-549
Interpretation of the infrared
spectrum
of the NH]-clays: application to the
evaluation of the layer charge
S. P E T I T ,
D. RIGHI,
J. M A D E J O V A *
AND A . D E C A R R E A U
Universitk de Poitiers, CNRS UMR 6532 "HydrASA ; 40, avenue du Recteur Pineau, 86022 Poitiers Cedex, France,
and *Institute of Inorganic Chemistry, Slovak Academy of Sciences, 842 36 Bratislava, Slovakia
(Received 10 October 1998; revised 15 June 1999)
A B S T R A C T : The IR spectra of NH4-saturated
+
smectites were examined in terms of their charge
characteristics. The u4 NH~ band near 1440 cm -1, observed in the DRIFTS spectra (obtained without
use of a KBr matrix), was assigned to the vibrations of NH~ ions compensating the negative charge
of the clays. When KBr was used as a diluting matrix, the u4 NH~ band was located at 1400 and/or
1440 cm -1. The band at 1400 cm -1, related to NH4Br, originated from the replacement of NH~ in
the clay by K + from the KBr. For swelling clay minerals this band indicates that layers have
permanent low charge density and/or variable charge. For non-swelling clay minerals, the 1400 cm I
band characterizes the presence of variable charges only. The u4 NH~ band at 1440 cm -1 suggests
that NH~ in the clay was not replaced by K + from KBr and remains in the interlayer space of the clay
minerals. This absorption is due to NH~ compensating only permanent charge in the interlayers, or
part of the interlayers with a high charge density. The presence of both bands at 1400 cm 1 and
1440 cm ~ in the IR spectrum suggests that the clays studied have a heterogeneous interlayer charge.
Previous studies show that NH~ bearing 2:1 clays
have two types of ammonium vibrational spectra.
The first one, mainly observed for KBr pellets of
NH4-smectites, gives absorption bands at 3150,
3020, 2840 and 1400 cm -~ (Petit et al., 1998).
These NH~ bands, observed in the 1R spectra of
many NH4 salts, correspond to vibrations of NH~
ions in the tetrahedral symmetry group (Nakamoto,
1963). According to Nakamoto (1963), Sherman &
Smulovitch (1970) and Ryskin (1974), the intense
absorption bands at 3150 cm 1 and 1400 cm -1 are
due to u3 (stretching) and u4 (deformation)
vibrations of the NH~ ion, respectively. The
2840 cm -a and the 3020 cm 1 bands are assigned
to the overtone 2 ~2 and the (u4 + u2) combination
vibration, respectively.
The second type of ammonium vibrational
spectrum shows bands in the 3300-3250 cm t
region and near 1440 cm -1. Chourabi & Fripiat
(t981) assigned these bands to the adsorbed NH~
ion in tetrahedral symmetry as well, but due to H
bonding of NH~ with the clay structure, the bands
are in slightly different positions compared to those
o b s e r v e d for NH4 salts (i.e. at 3150 and
1400 cm-1). This second type of ammonium
vibrational spectrum is observed in the case of:
(1) self-supporting films of clays (Mortland et al.,
1962; Mortland & Raman, 1968; Chourabi &
Fripiat, 1981; Srasra et al., 1994) and (2) nonexpandable NH4-clay minerals such as illites or
micas (Vedder, 1965; Sucha et al., 1998).
Moreover, this second type of ammonium vibrational spectrum is also present as a superimposed
component on the first type of spectrum and was
observed in the spectra of undried or dried KBr
pellets of some smectites (Petit et al., 1998).
No detailed discussion of IR spectra of NH4-clay
minerals is available in the literature, therefore the
purpose of this paper is to explain the occurrence of
these two types of IR ammonium vibrational
9 1999 The Mineralogical Society
544
S. Petit et al.
spectra for NH4-saturated smectites and to interpret
this occurrence. The effect of layer charge on
exchangeability of NH~ ions in KBr-clay mixtures
is also discussed.
MATERIALS
AND METHODS
Three synthetic saponites and one smectite obtained
from the Source Clay Repository of the Clay
Minerals Society (SAz-1) were used (Table 1).
The sample SAz-1 is a Mg-rich montmorillonite
with the majority of layer charge in the octahedral
sheet. The samples SAP0.35, SAP0.5, and SAP0.8
are synthetic saponites with layer charge arising
from tetrahedral substitutions only. The range of
their tetrahedral substitution varies from 0.35 to 0.8
atoms of A1 substituted for Si per half unit-cell.
These were the samples used by Petit et al, (1998).
Suspensions of the samples in 1 M ammonium
acetate solution were obtained by agitation and
were left to stand for 2 h. After centrifugation, this
process was repeated once to achieve complete NH~
exchange. The samples were then washed until free
of ammonium salts (until negative reaction with the
Nessler reagent).
The NH~-saponites were further re-saturated with
Ca2+ using solutions of 1 M CaCI2. This exchange
permits identification of the irreversibly collapsed
layers of high-charged smectites by XRD (Righi et
al., 1998). In the present work it was used to
distinguish between fixed and exchangeable NH~
ions by Fourier transform infrared (FTIR)
spectroscopy.
A Nicolet 510 FTIR spectrometer was used to
record FTIR spectra at 4 cm 1 resolution in the
4000 400 cm -1 range. The spectrometer was
continuously purged with dry air during scanning
of the transmission spectra of KBr pellets. The
pellets of 2 cm diameter were prepared by mixing
3 mg of sample with 300 mg of KBr.
Diffuse reflectance infrared spectra (DRIFTS)
with and without dispersion in a KBr matrix were
recorded using a DRIFTS accessory 'Collector'
from Spectra-Tech. The samples, analysed at room
temperature, were placed loosely into a sample cup
of -1 mm depth and 3 mm diameter. Diluting the
sample in a non-absorbing matrix, the proportion of
the IR beam that is diffusely reflected by the
sample increases. However, the DRIFTS spectra of
saponites and SAz-1 obtained without dilution were
of good quality in the regions of NH4 absorption,
and dilution was used to study whether KBr affects
position of NH4 bands in the spectra. To compare
samples quantitatively, the spectra were normalized
using an internal reference band, either the main
S i - O band near 1000 cm -1 or the OH-stretching
band depending on which was more appropriate for
the transmission or reflectance measurements.
RESULTS
AND DISCUSSION
The study was focused on the ua NH~ band near
1400 cm l only, because the intensity of this NH~
band is linked quantitatively to the N content of the
analysed samples (Ferriso & Hornig, 1959; Vedder,
1965; Petit et al., 1998). Unlike the NH~- stretching
region, where the bands are overlapped with
O H - s t r e t c h i n g v i b r a t i o n s of water (near
3400 cm i), the NH~ deformation region is free
of other absorptions, and no decomposition is
necessary to extract the u4 NH~ signal (Petit et
al., 1998). The important point is that the u4 NH~
band characterizes unambiguously the type of NH~
vibrational spectrum. If the u4 NH4 band is at
TABLE 1. Mineralogy, source, and cation exchange capacities (CEC) of the samples used.
Samples
Mineralogy
Source
CEC
(cmol(+).kg i)
References
SAz-I
Montmorillonite,
minor silica phase
Apache County,
Arizona, USA
125
Jaynes & Bigham
(1987)
SAP0.35
SAP0.5
SAP0.8
Synthetic saponites
S14_xAlxMg3010(OH)eNax
CRSCM, Orl6ans, France O.-L. Robert)
9
83.8
+
109.0
163.0
Bergaoui et al. (1995); Petit et al. (1998)
545
IR spectra of NH~4-smectites
1400 cm 1, the other bands are located at
3150 cm 1, 3020 cm -I and 2840 cm -1, while if it
is near 1440 cm 1, the other main band is near
3300-3250 cm -1.
Attribution o f the 1400 cm -1 and 1440 Cm -1
bands
The transmission spectrum of the NH~ saturated
SAz-1, obtained from a KBr pellet, is presented in
Fig. 1, curve f. The u4 NH~ band at 1400 cm -1
indicates the first type of ammonium spectrum
which is typical of NH4 salts. Since the clay
samples used were carefully washed until free of
any NH4 salts, the band at 1400 cm -1 suggests an
exchange of interlayer NH] for K + from KBr
according to the reaction:
NH4-clay + KBr --, K-clay + NH4Br
The DRIFTS spectra of SAz-1 montmorillonite
and the saponites were used to confirm this
hypothesis.
v 4 NH4
SAz-1
1400 cm -1
O
i
|
1600
Wavenumber (em -1)
1400
FI~. 1. The evolution of the u4 NH~ band for the NH4-saturated SAz-1 sample. (a) DRIFTS measurement,
undiluted sample; (b) DRIFTS measurement, sample diluted with KBr and slight grinding; (c) DRIFTS
measurement, 2 h later than for b; (d) DRIFTS measurement, 12 h later than for c and second grinding;
(e) DRIFTS measurement, 2 h later than for d; (f) undried KBr pellet in transmission mode.
546
S. Petit et al.
A DRIFTS spectrum of sample NH4-SAz-1,
recorded without any dilution in KBr, (Fig. la)
shows a broad band at 1440 cm -1. After mixing the
sample with KBr and gentle grinding to homogenize it, the band shifted slightly to lower
wavenumbers (Fig. lb). No change occurred after
storing the clay/KBr mixture for several hours
(Fig. lc). After overnight storage and supplementary mild grinding of the sample, the maximum of
the band shifted again and a new band at
1400 cm -] appeared (Fig. ld). After two additional
hours, the intensity of the band at 1400 cm 1
clearly increased, and the DRIFTS spectrum
(Fig. l e) is comparable to that obtained in the
transmission mode from a KBr pellet (Fig. lf).
This experiment provides direct evidence for the
exchange reaction between NH~ present in the
interlayers of the clay and K+ from the KBr. The
1400 cm ~ band is attributed to the vibrations of
NH~ present in the pellets as NH4Br as a result of
the ion exchange, while the 1440 cm ] band is
assigned to the NH~ not replaced by K+, i.e. to the
NH~ still remaining in the interlayers of the clay.
Petit et al. (1998) found that after overnight drying
of KBr pellets of various clay samples at ll0~
including SAz-1, no more changes were observed in
the intensities ratio of the 1400 and 1440 cm -1
bands, and the replacement of NH~ by K+ was
completed. Thus, after drying the pellet overnight,
the 1400 cm -1 band is due to the NH~ ions
replaced by K+, while that at 1440 cm -j corresponds to the NH~ in the clay structure which is not
replaceable by K+ under the experimental conditions used.
Influence o f clay layer charge
To investigate the influence of clay layer charge
on the possible exchange of NH~ and K+, a series
of synthetic saponites with a gradual increase of the
tetrahedral charge was used (Table 1). The
Na-saponites were NH~ saturated, and in a second
step, these NH4-samples were back-saturated with
Ca2+. The KBr pellets of both NH4- and Casaturated samples were prepared. The transmission
spectra of KBr pellets dried overnight are given in
Fig. 2.
The 1400 cm - l band is present for all NH4saponites with almost the same intensity, while the
intensity of the broader band at 1440 cm -I
increases with the layer charge of the sample, as
was previously reported (Petit et al, 1998). For
,.Q
<
!
1500
!
1400
Wavenumber (cm -1)
FIG. 2. The FTIR spectra in the u4 NH~ region,
obtained in transmission mode of KBr pellets, dried
overnight, of the saponite samples described in
Table 1. (a) NH4-saturated samples; (b) Ca backsaturated samples.
the NHa/Ca-saponites, no NH~ absorption is
shown for the samples of lower layer charges
(SAP0.35 and SAP0.5). The 1440 cm -1 band is
observed only for that with the highest charge
(SAP0.8, Fig. 2b). The absence of the NH~ band
in the IR spectra of the two lower charged NH4/
Ca-saponites indicates that all exchangeable NH~
was removed by the Ca 2+ exchange. Therefore, the
1440 cm -1 band observed for the NH4-forms of
SAP0.35 and SAP0.5 (Fig. 2, spectra a) corresponds to the NH] not replaced by K+ from KBr,
but replaced by Ca 2+, as indicated by the absence
of this band in spectra b of Fig. 2. For SAP0.8,
some NH~ ions remained as interlayer cations
even after Ca2+ saturation (Fig. 2, spectra b). The
remaining NH~ in NH4/Ca-SAP0.8 is considered
to be non-exchangeable, i.e. fixed, as in vermiculite (Mermut, 1994; Shen et al., 1997).
The NH~ signals of NH4-SAP0.8 and NH4/CaSAP0.8 samples obtained by DRIFTS and KBr
547
IR spectra of NH~4-smectites
SAP0.8
1440 cm -1 signal is due to N H ; occurring as an
interlayer cation in the clay.
CONCLUSIONS
a
ID
o
r~
,.Q
<
b
C
I
|
1500
1400
Wavenumber (cm -1)
Fie. 3. The FTIR spectra in the u4 NH~ region, of the
SAP0.8 saponite. (a) DRIFTS measurement, undiluted
NH4-saturated sample; (b) transmission mode of a KBr
pellet, dried overnight, of the NH4-saturated sample;
(c) DRIFTS measurement, undiluted NH4/Ca-saturated
sample; (d) transmission mode of a KBr pellet, dried
overnight, of the NH4/Ca-saturated sample.
pellet methods are shown in Fig. 3. The spectra of
NH4-samples are similar to those of SAz-1
montmorillonite (Fig. 1), i.e. a sharp band at
1400 cm -1 with a shoulder near 1440 cm 1
occurs in the KBr pellet spectrum (Fig. 3b), and
a single broader band near 1440 cm -1 in DRIFTS
(Fig. 3a). However, no difference was observed
between these two types of spectra for the NH4/Casamples (Fig. 3c,d), thus confirming the presence
of NH~ ions even after Ca 2§ exchange. If the 1440
cm -1 band appears in the spectra collected using
KBr as the diluting matrix, either by KBr pellet or
the DRIFTS technique, it shows that some NH~
ions remain fixed in the clay and that they cannot
be replaced by Ca 2+ or K +. It confirms that the
From the present study, it becomes possible to
interpret the published data on the ammonium
signal in NH~-bearing clay minerals.
If FTIR measurements are performed without a
diluting matrix (clay films, DRIFTS, IR-spectroscopy), the D4 NH~ band observed is rather broad
and located near 1440 cm 1. This band is due to
NH~ ions present as compensating cations of the
permanent and/or variable charges. In FTIR spectra
obtained using KBr as a diluting matrix, either for
transmission measurements using pellets or for
DRIFTS, the ammonium signal is usually more
complex, frequently with two superimposed u4 NH~
bands at 1400 and 1440 cm -1.
The position and the shape of the u4 NH~ bands
are clearly indicative of the exchangeability of the
NH~ present in the clay. After complete replacement of the NH~ in the clay by K § from KBr, the
following three situations can occur.
(1) The u4 NH~ band is sharp and appears at
1400 cm -1 without any other band in this region.
This means that all the NH~ in the clay was
replaced and is now present as NH4Br only. In the
case of swelling clay minerals, such an exchange
indicates that the layers have a low charge density.
Conceivably, ammonium ions compensating the
variable charge are also (and probably more
readily) replaced. In the case of non-swelling clay
minerals (kaolinite, mica, etc.), the u4 NH~ band at
1400 cm 1 represents the ammonium compensating
the variable charge only, because there is either no
permanent charge (e.g. kaolinite), or the permanent
charge is compensated by non-exchangeable cations
other than NH~ (e.g. mica). The ammonium
saturation can then be used to measure quantitatively by IR, the variable charge of these minerals.
Fialips et al. (1999) used this method to study the
surface charge of synthetic kaolinites.
(2) The presence of only a broad band near
1440 cm 1 means that ammonium from the clay is
not replaced by K from KBr, and is still present in
the interlayer space of clay minerals. This band
suggests that NH~ is saturating permanent charges
in the interlayers (or some of the interlayers) of
clays with high charge density (e.g. Ca/NH 4SAP0.8). This is in good agreement with spectra
obtained from KBr pellets of natural NH4-illites and
S. Petit et al.
548
micas which showed only this band at 1440 cm -1
when variable charges were compensated by other
cations (Vedder, 1965; Lindgreen, 1994; Sucha et
al., 1998).
(3) Both bands at 1400 and 1440 cm -1 are
present in the spectrum. This situation reflects
heterogeneity in the clay charge (variable charge/
permanent charge; low/high layer charge or
heterogeneous layer charge) compensated by NH~
ions. The integrated intensities of the bands at 1400
and 1440 cm -1 can be used to measure quantitatively the proportions of the different types of
charge.
The NH~ bands were previously used to quantify
ammonium in phyllosilicates (Lindgreen, 1994),
illite crystallization ratio (Sucha et al., 1998),
surface acidity of smectites (Mortland & Raman,
1968), and also tetrahedral and octahedral charges
(Chourabi & Fripiat, 1981; Ben Hadj-Amara et al.,
1987; Srasra et al., 1994; Petit et al., 1998). This
study showed that, using KBr pellets, the ammonium signal can, in addition, be used to indicate
NH~ fixation which generally occurs on clay layers
with high charge density (Sawhney, 1972). This is
due to the extent of exchange between cation from
the diluting salt matrix and cations compensating
the permanent and/or variable charges of clay
minerals. The present study showed that K+ from
KBr can easily replace NH~ in some clays, and that
Ca2+ is able to replace even those NH~ ions which
are not replaceable by K+. By comparing various
FTIR techniques (transmission, DRIFTS, and
microscopy), Pelletier et al. (1999) proved that
Na + from saponite samples could be replaced by K+
from KBr. All these phenomena are closely
connected to different hydration energies of
exchangeable cations (Giiven, 1992) and to
distributions of the interlayer charge (Ci~el &
Machajdik, 1981). The use of various salts as
matrices and/or back-saturating cations, as well as
different techniques for studying ammoniumexchanged clay minerals by IR spectroscopy,
promises to be a very useful way to characterize
the charge of clay minerals.
ACKNOWLEDGMENTS
The authors wish to thank Dr J.L. Robert for supplying
the synthetic saponite samples. M. Garais and D.
Paquet are also acknowledged for their help in
preparing the samples. Critical comments by P.
Komadel helped to improve the paper. We also
acknowledge Drs G. Lagaly and A.R. Mermut for
their critical reviews. The financial support of the
CNRS (project No. 4474) and of the Slovak Grant
Agency (grant No. 2/4042/98) is gratefully acknowledged.
REFERENCES
Ben Hadj-Amara A., Besson G. & Tchoubar C. (1987)
Caract~ristiques stmcturales d'une smectite diocta6drique en fonction de l'ordre-d~sordre dans la
distribution des charges ~lectriques: I. Etudes des
r~flexions OOl. Clay Miner. 22, 305-318.
Bergaoui L., Lambert J.F., Vicente-Rodriguez M.A.,
Michot L.J. & Villi~ras F. (1995) Porosity of
synthetic saponites with variable layer charge
pillared by Al13 polycations. Langmuir, 11,
2849-2852.
Chourabi B. & Fripiat J.J. (1981) Determination of
tetrahedral substitutions and interlayer surface heterogeneity from vibrational spectra of ammonium in
smectites. Clays Clay Miner. 29, 260-268.
Ci~el B. & Machajdik D. (1981) Potassium- and
a m m o n i u m - t r e a t e d m o n t m o r i l l o n i t e s . I.
Interstratified structures with ethylene glycol and
water. Clays Clay Miner. 29, 40-46.
Ferriso C.C. & Hornig D.F. (1959) Absolute infrared
intensities of the ammonium ion in crystals. J. Chem.
Phys. 32, 1240-1245.
Fialips C.I., Petit S., Decarreau A. & Beaufort D. (1999)
Influence of synthesis pH on kaolinite "crystallinity"
and surface properties. Clays Clay Miner. (in press).
Giiven N. (1992) Molecular aspects of clay-water
interactions. Pp. 2-79 in: Clay-water Interface and
its Rheological Applications, (N. Giiven & R.M.
Pollastro, editors). The Clay Minerals Society,
Boulder, Colorado.
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.
Lindgreen H. (1994) Ammonium fixation during illitesmectite diagenesis in upper Jurassic shale, North
Sea. Clay Miner. 29, 527-537.
Mermut A.R. (1994) Problems associated with layer
charge characterization of 2:1 phyllosilicates. Pp.
106-122 in: Layer Charge Characteristics of 2:1
Silicate Clay Minerals, (A.R. Mermut, editor). CMS
Workshop Lectures, 6, The Clay Minerals Society,
Boulder, CO.
Mortland M.M. & Raman K.V. (1968) Surface acidity of
smectites in relation to hydration, exchangeable
cation, and structure. Clays Clay Miner. 16,
393-398.
Mortland M.M., Fripiat J.J., Chaussidon J. &
Uytterhoeven J. (1962) Interaction between ammo-
IR spectra of NI~4-smectites
nia and the expanding lattices of montmorillonite
and vermiculite, a~ Phys. Chem. 6"7, 248-258.
Nakamoto K. (1963) InJ?ared Spectra of Inorganic and
Coordination Compounds'. 2nd ed., Wiley, New
York.
Pelletier M., Michot L.J., Barf,s O., Humbert B., Petit S.
& Robert J.-L. (1999) Influence of KBr conditioning
on the IR hydroxyl-stretching region of saponites.
Clay Miner. 34, 439-445.
Petit S., Righi D., Madejovfi J. & Decarreau A. (1998)
Layer charge estimation of smectites using infrared
spectroscopy. Clay Miner. 33, 579 591.
Righi D., Terribile F. & Petit S. (1998) Pedogenic
formation of high-charge beidellite in a vertisol of
Sardinia (Italy). Clays Clay Miner. 46, 167-177.
Ryskin Y.I. (1974) The vibrations of protons in
minerals: hydroxyl, water and ammonium. Pp.
137-181 in: The Infrared Spectra of Minerals,
(V.C. Farmer, editor). Monograph No. 4,
Mineralogical Society, London.
549
Sawhney B.L. (1972) Selective sorption and fixation of
cations by clay minerals: a review. Clays Clay
Miner. 20, 93-100.
Shen S., Tu S-I. & Doral Kemper W. (1997) Equilibrium
and kinetic study of ammonium adsorption and
fixation in sodium-treated vermiculite. Soil Sci. Soc.
Am. J. 61, 1611-1618.
Sherman W.F. & Smulovitch P.P. (1970) Pressure
scanned Fermi resonance in the spectrum of NH~
isolated in CsBr. J. Chem. Phys. 52, 5187-5193.
Srasra E., Bergaya F. & Fripiat J.J. (1994) Infrared
spectroscopy study of tetrahedral and octahedral
substitutions in an interstratified illite-smectite clay.
Clays Clay Miner. 42, 237-241.
Sucha V., Elsass F., Eberl D.D., Kuchta L., Madejovfi J.,
Gates W.P. & Komadel P. (1998) Hydrothermal
synthesis of ammonium illite. Am. Miner. 83,
58-67.
Vedder W. (1965) Ammonium in muscovite. Geochim.
Cosmochim. Acta 29, 221-228.