Clays and Clay Minerals, Vol. 52, No. 5, 635–642, 2004.

DEHYDRATION AND REHYDRATION OF PALYGORSKITE AND THE INFLUENCE

OF WATER ON THE NANOPORES

W E NX I NG K U A N G , G L EN N A. F A C E Y AN D C HR I S TI A N D ET E L L IE R *
Center for Catalysis Research and Innovation and Department of Chemistry, University of Ottawa, Ottawa, Ontario,
Canada K1N 6N5

Abstract—The dehydration and rehydration processes of the clay mineral palygorskite (PFl-1) were
studied by textural analysis, thermogravimetric analysis connected with mass spectrometry (TGA-MS),
and 29Si and 1H solid-state NMR techniques. The TGA-MS results clearly reveal weight losses at maxima
of 70ºC, 190ºC, 430ºC and 860ºC. PFl-1 is characterized by a micropore area of 93 m2/g, corresponding to a
micropore volume of 47 mm3/g. These values are also obtained for the sample heated up to 200ºC for 20 h.
Further heating at 300ºC produces a collapse of the structure, as shown by the almost complete loss of
microporosity.
The 29Si NMR spectra of palygorskite show two main resonances at 92.0 and 97.5 ppm, attributed to
one of the two pairs of equivalent Si nuclei in the basal plane. A minor resonance at 84.3 ppm is
attributed to Q2(Si-OH) Si nuclei. The resonance at 92.0 ppm is assigned to the central Si position, while
the resonance at 97.5 ppm is assigned to the edge Si sites. It is confirmed by solid-state 29Si and 1H NMR
that nearly complete rehydration is achieved by exposing palygorskite samples that have been partially
dehydrated at 150ºC and 300ºC, to D2O or water vapor at room temperature. When the rehydration is
accomplished with D2O, the atoms are disordered across all the protons sites.
Key Words—Clay Minerals, Microporosity, Palygorskite, Sepiolite, Textural Analysis, TGA-MS,
29
Si NMR.

INTRODUCTION water, the structural water. The nanopores run parallel to

Palygorskite is an important clay mineral with the fiber axis and are filled at room temperature by
industrial applications, due to its unique crystal structure zeolitic water molecules that are hydrogen bonded to the
and microfibrous nature (Serratosa, 1979; Jones and structural water. The zeolitic water is easily lost at
Galán, 1988; Galán, 1996). The name palygorskite was relatively low temperature, <120ºC. The cross-sections
first used by von Ssaftschenkov in 1862, from the name of the tunnels are ~3.766.4 Å. They are responsible for
of the locality where it was first found, Palygorsk (Jones the large specific surface area and excellent sorptive
and Galán, 1988, and references therein). A fascinating properties of palygorskite, once the zeolitic water has
application of palygorskite was its use centuries ago by been removed by thermal treatment.
the Mayas to prepare a blue pigment known as ‘Maya Besides the monoclinic structure model for palygors-
Blue’ (van Olphen, 1966; Kleber et al., 1967; Yacamán kite (Bradley, 1940; Preisinger, 1963), an orthorhombic
et al., 1996; Polette et al., 2002; Chiari et al., 2003; Fois structure model has also been proposed (Chisholm,
et al., 2003; Hubbard et al., 2003). 1990, 1992; Artioli and Galli, 1994; Artioli et al., 1994;
Palygorskite and sepiolite are 2:1 type phyllosilicates Chiari et al., 2003). An intermediary structure between
(Jones and Galán, 1988, and references therein). The dioctahedral and trioctahedral has been proposed (Galán
structure of palygorskite is derived from talc-like T-O-T and Carretero, 1999). Recently, Chahi et al. (2002)
ribbons that expand along the axis of the fibers, with a found evidence of trioctahedral Mg3OH features.
width of two pyroxene chains. The octahedral sites are Palygorskite is progressively dehydrated with
principally occupied by Mg(II) cations, with some increase in temperature (Preisinger, 1963). Heating
replacement principally by Al(III) or Fe(III) cations. palygorskite in air at relatively low temperatures
Each ribbon is connected to the next through an inverted removes selectively water adsorbed on external surfaces
Si O Si bond, resulting in a continuous tetrahedral and the zeolitic water from the nanoporous tunnels,
sheet and a discontinuous octahedral sheet. Rectangular, while leaving the structural water and the Mg-OH groups
tunnel-like nanopores are formed. The terminal cations unaffected. The elimination of coordinated, structural
that are located at the edges of the octahedral sheets water starts when the zeolitic water is lost and ends
complete their coordination shells with two molecules of when dehydroxylation begins. Folding of the palygors-
kite crystals occurs when some structural water has been
removed, which allows the terminal Mg(II) to complete
* E-mail address of corresponding author: its coordination with the oxygen of the neighboring
dete@science.uottawa.ca silica surface. Structural folding is nearly completely
DOI: 10.1346/CCMN.2004.0520509 reversible provided that the treatment temperature does

Copyright # 2004, The Clay Minerals Society 635

636 Kuang, Facey and Detellier Clays and Clay Minerals

not exceed 350ºC, but becomes irreversible once all the (1.89), Na2O (0.05), K2O (0.87), P2O5 (0.92) (Mermut and
structural water molecules are removed and partial Cano, 2001); structure: (Mg0.33Ca0.62Na0.04K0.13)[Al1.50
dehydroxylation has occurred, forming an anhydride Fe3+ 2+
0.52Fe0.01Mn0.01Mg1.91Ti0.06][Si7.88Al0.22]O20(OH)4. As
form. Finally, the remaining Mg-OH hydroxyl groups reported by Güven et al. (1992), palygorskite makes up
are released at ~850ºC (VanScoyoc et al., 1979; Frost ~85% of the fine (<2 mm) fraction. Smectite (10%), quartz
and Ding, 2003). (1 3%), apatite (2%), carbonates (1%) and mica (<1%)
Given the large number of applications of palygors- were identified as the main impurities in the fine fraction.
kite (Murray, 1991, 1999, 2000), it is of great interest to These data are close to those reported by Chipera and
fully characterize and to understand its properties. In Bish (2001).
recent years, there have been numerous studies of Deuterium oxide (>99.9%) was obtained from
palygorskite (e.g. Augsburger et al., 1998; Frost et al., Cambridge Isotope Laboratories, Inc.
1998, 2001; Fernandez et al., 1999; Galán and Carretero,
1999; Borden and Giese, 2001; Madejová and Komadel, Rehydration of samples
2001; Wu, 2001; Birsoy, 2002; Chahi et al., 2002; Palygorskite was heated with a ramp rate of 1ºC/min
McKeown et al., 2002; Sidheswaran, 2002). A variety of to 150ºC (or 300ºC) and then kept at 150ºC (or 300ºC)
physicochemical techniques such as X-ray diffraction for 20 h under air in a baffle furnace to eliminate
(XRD), thermal analysis, infrared (IR) spectroscopy, selectively the surface-bound water, and zeolitic water
solid-state nuclear magnetic resonance (NMR) spectro- (and partially structural water) from the nanoporous
scopy, scanning electron microscopy (SEM) and textural tunnels. The vials containing ~0.1 g of the partially
analysis have been performed to characterize palygors- dehydrated palygorskite were immediately transferred
kite (Jones and Galán, 1988; Costanzo and Guggenheim, from the furnace into capped bottles containing a few
2001), mL of water or D2O and then remained in contact with
There are some contradictory reports in the literature, the vapor at room temperature.
e.g. the assignment for the resonances in the 29Si NMR
spectrum of palygorskite (Barron and Frost, 1985; Nuclear magnetic resonance spectroscopy
Komarneni et al., 1986; d’Espinose de la Caillerie and Solid-state 1H magic-angle spinning (MAS) NMR,
Fripiat, 1994), the BET surface area determined from and 29Si MAS and cross-polarization (CP)/MAS NMR
textural analysis, which can range from 50 83 m2/g to spectra were recorded at 200.10 and 39.75 MHz,
222 m2/g for palygorskite (PFl-1) (van Olphen and respectively, at room temperature, on a Bruker ASX-
Fripiat, 1979; Shariatmadari, et al., 1999) and thermal 200 spectrometer. Typical spinning rates of 6 kHz (1H)
analysis (Jones and Galán, 1988; Shuali et al., 1988, and 4 kHz (29Si) were used. The excitation pulse and
1990; Frost and Ding, 2003). recycle time for 1H NMR were 3.5 ms (p/2 pulse) and 2 s
In previous studies, the 29Si MAS NMR spectrum of (16 scans), respectively. A ramped CP pulse sequence
sepiolite was assigned using cross-polarization techni- was used for all 29Si cross-polarization experiments. The
ques (Barron and Frost, 1985; Sanz, 1990) and two- recycle delay time was 2 s, which was found to be
dimensional 1H-29Si HETCOR and 29Si-COSY pulse adequate for proton relaxation based on proton MAS
sequences (Shore et al., 1998; Weir et al., 2002). As signal intensity measured as a function of delay time.
sepiolite and palygorskite are chemically and structu- The proton 90º pulse was 4 ms. The contact time to allow
rally similar, differing mainly by the length of their unit- the transfer of magnetization between 1H and 29Si nuclei
cell, the previous results from sepiolite are helpful for was 10 ms. The direct 29Si MAS spectra were collected
the peak assignments of palygorskite. In the current using 4 ms pulses (90 deg) and high-power proton
work, the 29Si MAS and CP/MAS NMR spectra of decoupling. The recycle delay was 2 s. The 1H NMR
palygorskite were analyzed at room temperature and signals were externally referenced to the CH3 reso-
upon thermal treatment. It is also confirmed by solid- nance of acetone at 2.06 ppm, corresponding to tetra-
state NMR that nearly complete rehydration is achieved methylsilane (TMS) at 0 ppm. The 29Si NMR signals
at room temperature by exposing palygorskite samples were externally referenced to the Si(CH3)3 resonance
that had been partially dehydrated up to 300ºC to D2O or of tetrakis trimethylsilylsilane at 9.9 ppm, correspond-
H2O vapor. ing to TMS at 0 ppm.

Thermal analysis
EXPERIMENTAL
Differential thermal analysis (DTA), thermogravi-
Materials metric analysis (TGA), and derivative thermal gravi-
Palygorskite (PFl-1) from Gadsden County, Florida, metric (DTG) analysis were performed on a TA SDT
was obtained from the Source Clays Repository of the 2960 instrument. Approximately 10 20 mg samples
Clay Minerals Society (Purdue University), with a were placed in a platinum crucible on the pan of a
chemical composition (%) of SiO2 (60.35), Al 2 O3 microbalance, and then heated from room temperature to
(11.13), Fe2O3 (3.74), TiO2 (0.48), MgO (10.58), CaO 1000ºC at a heating rate of 10ºC/min while being purged
Vol. 52, No. 5, 2004 Structural studies of palygorskite 637

with He at a flow rate of 100 mL/min and constantly

weighed. The gases were drawn down a capillary in the
SDT furnace. Mass spectra were recorded on a Pfeiffer
GSD 301 instrument. The mass spectrum was run in the
scanning mode from mass 10 to mass 80, at a scan rate of
0.2 s per mass.

Texture analysis
The BET surface area and micropore measurements
were performed on a Micromeritics ASAP-2010 instru-
ment (N2 or Ar adsorption at 196ºC). The samples
were pre-degassed at room temperature under vacuum on
the apparatus. The degas process was terminated when
the vacuum pressure decreased to 3 mm Hg. The Figure 1. DTG and mass spectrometry of water (m/e = 18) of
molecular cross-section of N2 used in the data analysis palygorskite heated from room temperature to 1000ºC in He
was 0.1620 nm2. The typical range of thickness chosen with a ramp of 10ºC/min.
for t-plot measurements was 3.5 to 5 Å.
experimental conditions (rate of heating and nature and
RESULTS AND DISCUSSION flow rate of the flowing gas). As a likely interpretation
of the shoulder observed at 370ºC, the same authors have
Structure of palygorskite proposed that the completion of the dehydration at that
Four major weight-loss steps were observed on the temperature is accompanied by the folding of the
TGA curve for palygorskite in the range of temperatures tunnels. A partial dehydroxylation occurs at 430ºC and
from room temperature to 1000ºC in He gas. These steps is completed at 860ºC (Frost and Ding, 2003). It has
are similar to the thermal behavior of palygorskite under been proposed that the loss of the first structural water is
N2 (Guggenheim and Koster van Groos, 2001; Frost and accompanied by a partial collapse of the structure
Ding, 2003). The four steps, which correspond to weight (Preisinger, 1963). This is apparent in Table 1: when
losses of 4.8% (room temperature to 130ºC), 3.3% the sample is heated at 300ºC for 20 h, the micropore
(130 270ºC), 5.1% (270 580ºC), and 1.2% area decreases from 93 m 2 /g to 22 m 2 /g, leaving
(580 950ºC), were assigned to the release of zeolitic unaffected the external surface area. Also shown in
water, the release of the first structural water, the release Table 1, the median pore diameter increases from 6.1 Å
of the second structural water, and the dehydroxylation to 7.9 Å with the decrease of cumulative pore volume,
of the Mg-OH groups, respectively (Shuali et al., 1988). because of the loss of the microporosity.
This is in agreement with the in situ mass spectrometry The nitrogen adsorption-desorption isotherm of PFl-1
analysis: gaseous water is evolved from the material at is of type I at lower relative pressures, characteristic of
the same temperatures. Figure 1 shows the correspond- microporous solids, and of type IV at higher relative
ing DTG and mass spectrometric analyses of water. It is pressures (Webb and Orr, 1997). This is in good
interesting to note that the DTG curve is almost identical agreement with previously reported measurements for
to the water mass trace. Similar results were reported for palygorskite samples of other origins (Cases et al.,
PFl-1 (Frost and Ding, 2003) or for palygorskite samples 1991). A hysteresis is observed at higher relative
from other origins (Shuali et al., 1990; Artioli et al., pressures, indicative of the presence of interparticle
1994). Four major weight-loss events are clearly mesopores. The BET surface area is 196 m2/g, a value in
observed at maxima of 70ºC, 190ºC, 430ºC and 860ºC, good agreement with previously reported measurements
with an additional shoulder at ~370ºC. The difference for palygorskite samples of other origins (Barrer and
between these maxima and those recently reported by Mackenzie, 1954: 195 m 2 /g; Serna et al., 1977:
Frost and Ding (2003) may be due to the different 195 m2/g; Gonzalez et al., 1989: 208 m2/g; Suárez

Table 1. Textural analysis of palygorskite heated at various temperatures, and then degassed at room temperature.

BET surface —————— T-plot —————— —— Horvath-Kawazoe ——

Heating area Micropore External surface Micropore Cumulative pore Median pore
temperature (m2/g) area (m2/g) area (m2/g) volume (mm3/g) volume (mm3/g) diameter (Å)

No heating 196 93 103 47 89 6.1

150ºC 206 93 113 46 93 6.2
200ºC 205 95 110 47 93 6.3
300ºC 130 22 108 10 55 7.9
638 Kuang, Facey and Detellier Clays and Clay Minerals

29
Barrios et al., 1995: 186 m2/g). This measurement was Si NMR spectra of palygorskite
confirmed by performing a congruent analysis using Ar
as the adsorbed gas, yielding a BET surface measure- Figures 3a and 4a give the 29Si MAS and CP/MAS
ment of 197X2 m2/g. A t-plot analysis of the isotherm NMR spectra of palygorskite, with two well-resolved
gives a micropore area of 93 m2/g, a micropore volume resonances at 92.0 and 97.5 ppm. A significantly less
of 47 mm3/g (Table 1), and an external surface area of intense resonance around 84.4 ppm is also observed.
103 m2/g. Estimation of the micropore volume was Barron and Frost (1985) reported the 29Si MAS and CP/
enabled through desorption of the internal zeolitic water MAS NMR spectra of palygorskite, with two major
molecules during degassing. resonances at 92 and 98 ppm, and a broad shoulder
Figure 2 shows the differential pore-volume plot of in the vicinity of 85 ppm. They assigned those
palygorskite (Horvath and Kawazoe, 1983), giving a resonances at 85, 92 and 98 ppm, respectively, to
median pore diameter of 6.1 Å (Table 1), with a sharp Q2(Si-OH), to the center Si sites, and to the edge Si site.
maximum at 4.9 Å. A median pore diameter of 6.3 Å is Different assignments of those resonances were reported
obtained for the sample heated up to 200ºC for 20 h, still later (Komarneni et al., 1986; d’Espinose de la Caillerie
keeping the microporosity of the original palygorskite and Fripiat, 1994), under the assumption that for
sample. These values are in good agreement with the sepiolite and palygorskite, polarization transfer is
idealized crystallographic structure of palygorskite, and possible only from the non-mobile protons of the
the dimension of its tunnels. Further heating at 300ºC structural water molecules and not from Mg-OH protons.
produces a collapse of the structure as evidenced by the In a previous study (Weir et al., 2002), the 29Si NMR
loss of microporosity, and a modification of the median signals of sepiolite were assigned with the aid of two-
pore diameter (7.9 Å) for the remaining micropores. As dimensional 29 Si COSY and 1 H- 29 Si heteronuclear
shown in Figure 2, the microporosity is almost com- correlation (HETCOR) NMR pulse sequences. It was
pletely lost for a palygorskite sample heated at 500ºC shown that the Mg-OH protons are principally respon-
and then degassed at 200ºC. sible for the polarization transfer to 29Si, and conse-

Figure 2. Horvath-Kawazoe differential pore-volume plot of palygorskite degassed at room temperature (open circles) and of
palygorskite heated at 500ºC then degassed at 200ºC (filled circles).
Vol. 52, No. 5, 2004 Structural studies of palygorskite 639

quently for the increase of intensity observed for the 29Si Si is replaced by Q2(SiOH) at the borders of external
NMR signals of the Si atoms close to the Mg-OH unit. surfaces, the sum of the integrals of the edge and of the
Due to the existence of relatively large amounts of Q2 resonances should be equal to the integrals for the
Fe, the 1H NMR spectrum of the palygorskite sample is center signals. This is an additional argument to assign
composed of a series of very broad and strongly the resonances at 92.0, and 97.5 ppm to the center
overlapping peaks (see Figure 5a). It is thus not possible and edge silicon nuclei, respectively. The experimental
to apply 2D techniques for an unambiguous assignment quantitative ratio of Q 2 over Q3 is 1:13, in excellent
of the two palygorskite 29Si NMR peaks. Moreover, the agreement with the morphology model of palygorskite
29
Si NMR signals of palygorskite are much weaker than proposed by Serna and VanScoyoc (1979), giving an
those of sepiolite. idealized ratio of 1:14. This result also indicates that
A comparison of the 29Si MAS and CP/MAS NMR ~8% of the silicon nuclei in palygorskite exist as SiOH
spectra shown in Figures 3 and 4, repectively, permits us groups.
to assign the resonance at 92.0 ppm to the center Si 29
nuclei, which are located very close to the Mg-OH Si NMR spectra of partially dehydrated palygorskite
groups and are expected to cross-polarize efficiently The 29 Si MAS and CP/MAS NMR spectra of
with the hydroxyl protons. The resonance at 97.5 ppm palygorskite previously heated in air at 150 and 300ºC
can be assigned to the edge Si nuclei, situated further are presented in Figures 3(b,c) and 4(b,c), respectively.
away from the Mg-OH groups. The 29Si NMR spectra are sensitive to the changes in the
By analogy with sepiolite (Weir et al., 2002), the palygorskite structure occurring when zeolitic water
signal at 84.4 ppm can be assigned to the Q2(SiOH) Si molecules are removed from the tunnels. After heating at
site. The 29Si MAS NMR spectrum of palygorskite given 150ºC for 20 h, the two main resonances shift to 93.1,
in Figure 3a is quantitative, and the integral ratio is and 96.9 ppm, respectively. As mentioned above, only
~1:7:6 for the resonances at 84.4, 92.0 and the mobile zeolitic water molecules are removed after
97.5 ppm, respectively. The sum of the integrals of heating at 150ºC for 20 h, while the Mg-OH groups and
the resonances at 84.4 and at 97.5 ppm is equal to the the coordinated water molecules remain in the structure.
integral for the resonance at 92.0 ppm. Since the edge After heating at 300ºC for 20 h, the resonances overlap,

Figure 3. 29 Si MAS NMR spectra of samples: (a) palygorskite; Figure 4. 29 Si CP/MAS NMR spectra of samples: (a) palygors-
(b) palygorskite heated at 150ºC for 20 h; (c) palygorskite heated kite; (b) palygorskite heated at 150ºC for 20 h; (c) palygorskite
at 300ºC for 20 h; (d) palygorskite heated at 150ºC for 20 h then heated at 300ºC for 20 h; (d) palygorskite heated at 150ºC for
exposed to D 2 O; (e) palygorskite heated at 300ºC for 20 h then 20 h then exposed to D 2 O; (e) palygorskite heated at 300ºC for
exposed to D 2 O. 20 h then exposed to D 2 O.
640 Kuang, Facey and Detellier Clays and Clay Minerals

giving several broad signals, with a peak maximum similar to that reported for sepiolite (Weir et al., 2002).
around 92.8 ppm. This results from the partial removal The resonance at lower frequency produces only a few
of structural water molecules from the coordination shell spinning side bands, and can be attributed to Mg-OH
of the edge cations and the folding of the palygorskite groups, while the resonance at higher frequency can be
structure, with a loss of symmetry causing unresolved attributed to structural water. The spectra are similar
multiplicity of the Si sites. when the palygorskite sample is heated at 150ºC and at
300ºC. Upon exposure to D2O, a strong signal with no
Rehydration of palygorskite previously heated to 150ºC spinning side bands develops. It can be attributed to the
and 300ºC mobile, zeolitic water. Upon rehydration with D2O,
As discussed above, after heating at 150ºC for 20 h, protons and deuterons undergo statistical exchange
the zeolitic water molecules are selectively removed, the among the various proton sites, resulting in a large
microporosity is maintained, the tunnels remain acces- proton signal for the zeolitic water. The peak of the
sible, and the presence of the structural water molecules hydroxyl groups can be observed as a shoulder at lower
coordinated to terminal Mg(II) prevents folding of the frequency. The dilution of the 1H nuclei in the D2O-
structure. Heating at 300ºC for 20 h produces a folding exchanged palygorskite results in a spectrum (Figure 5f)
of the structure due to the partial removal of the better resolved than the spectrum of the original
structural water molecules. The microporosity of the palygorskite sample (Figure 5a) due to significantly
material is then largely lost. reduced dipolar interactions between protons which can
The 29Si MAS and CP/MAS-NMR spectra of a be more easily averaged by magic angle spinning.
partially dehydrated palygorskite sample subsequently This study confirms that the dehydration of paly-
exposed to D2O vapor are shown in Figures 3(d,e) and gorskite is fully reversible when it is heated at
4(d,e). They are almost identical to the spectra of the temperatures up to 300ºC, even if, at that temperature,
original clay mineral, indicating that exposure to D2O ~50% of the structural water molecules have been
vapor restores the original structure when palygorskite removed, causing a folded structure with an associated
was previously heated at 150 or 300ºC. This implies that loss in microporosity. 29Si solid-state NMR proves to be
the D2O molecules have filled the microporous tunnels a very sensitive probe of the structural changes of
and reversed the structural changes that were caused by
partial dehydration. Heating to 150ºC for 20 h results in
a partially dehydrated clay mineral in which the
microporous tunnels are accessible to D2O. In the case
of heating to 300ºC for 20 h, despite the observed
folding of palygorskite and the concurrent loss of
microporosity, D2O molecules can access the interior
of the structure, restoring the nanotunnel structure, in a
perfectly reversible manner. This reversibility was not
observed when palygorskite was heated at 400ºC for
20 h, a temperature at which the second structural water
is also removed and some partial dehydroxylation can
occur. At this temperature, the folding appears to be
irreversible.
The relative intensities of the two peaks in the
CP/MAS spectrum are identical to those of the original
sample. This shows that, upon rehydration with D2O, the
D atoms are scrambled across the various proton sites,
zeolitic water, coordinated water and hydroxyls, in a
statistical manner. As a result, the distribution of 1H in
the structure is identical for the original and for the D2O-
exchanged samples, resulting in equivalent relative
cross-polarization effects on the two sites. This H-D
exchange was also shown by IR spectroscopy (Serna et
al., 1977).
Figure 5 gives the corresponding 1H NMR spectra.
While the spectrum is characterized by very broad lines
Figure 5. 1 H MAS NMR spectra of samples: (a) palygorskite;
in the original palygorskite sample, the removal of the
(b) palygorskite heated at 100ºC for 20 h; (c) palygorskite
zeolitic water molecules results in a much better- heated at 150ºC for 20 h; (d) palygorskite heated at 300ºC for
resolved spectrum. Two signals are apparent, with an 20 h; (e) sample d exposed to D2 O for one week; (f) sample d
intense set of spinning side bands. This spectrum is exposed to D 2 O for 3 months. * denotes spinning side bands.
Vol. 52, No. 5, 2004 Structural studies of palygorskite 641

palygorskite and sepiolite, and is particularly responsive 758 766.

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events of PFl-1 at maxima of 70ºC, 190ºC, 430ºC and and Yvon, J. (1991) Evolution of the porous structure and
860ºC, attributed to the release of zeolitic water, the surface area of palygorskite under vacuum thermal treat-
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The Natural Sciences and Engineering Research and palygorskites. Thermochimica Acta, 397, 119 128.
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thanked for financial support. Mountain leather’, sepiolite and attapulgite an infrared
emission spectroscopic study. Vibrational Spectroscopy, 16,
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