Clay Minerals (1995) 30, 89-98
CHARACTERISTICS
OF F I N E P O R E S
IN SOME HALLOYSITES
G . J. C H U R C H M A N ,
T . J. D A V Y * , L . A . G . A Y L M O R E * ,
AND P. G . S E L F
R . J. G I L K E S *
CSIRO Division of Soils, Glen Osmond, South Australia and *Soil Science and Plant Nutrition, Faculty oJA griculture,
The University o f Western Australia, Nedlands, Western Australia
(Received 22 July 1993; revised 8 September 1994)
A B S T R A C T : Isotherms were obtained for nitrogen adsorption and desorption on seven halloysiterich samples from New Zealand and Western Australia. Calculations from these isotherms indicate
that halloysites with mainly small particles (< c. 0.08 I~m in width) had abundant cylindrical pores
with narrow size distributions in the 5-15 nm range. They also indicate that halloysites with mainly
large particles (> c. 0.1 ~tm in width) had few if any pores in the mesopore range (2-50 nm).
Transmission electron microscopy (TEM) shows that cylindrical pores originate from the central
holes in tubular particles. The TEM also suggests that slit-shaped pores can originate from the
shrinkage of blocks of layers upon dehydration of halloysite.
Halloysites include a significant concentration of
fine pores (Diamond, 1970; Jackson et al., 1971;
Churchman & Payne, 1983; McCrea & Gilkes,
1987; McCrea et al., 1990). Diameters recorded for
the largest concentration of pores have depended
upon the resolution of the techniques used for the
measurement. Diamond (1970), Churchman &
Payne (1983), McCrea & Gilkes (1987) and
McCrea et al. (1990), all using mercury intrusion
porisimetry, found that pores in the < 50 nm range
provided most of the pore volume. Jackson et al.
(1971), using isotherms for nitrogen and also for
water sorption, found a sharp peak at 2.3 nm in the
pore-size distribution.
The high concentration of very small pores in
halloysites contrasts with the situation in kaolinites.
These tend to show larger pores without a marked
concentration in any particular size range (Aylmore
& Quirk, 1967; Diamond, 1970; Sills et al., 1973a;
1974; Aylmore & Sills, 1978). Generally, kaolinites
comprise platy particles and pores in clay samples
with platy particles are likely to form from the
interleaving of the plates and should be similar in
size to the thickness of the particles in the samples
(Aylmore & Quirk, 1960; I967; Sills et al., 1973a)
A review of the literature (Churchman & Carr,
1975) revealed that halloysites can be distinguished
from kaolinites most reliably by the presence of
interlayer water or by evidence for its prior
occurrence in the structure. Halloysite particles
often occur as non-platy shapes, and a tubular shape
is common. The concentration of fine pores
observed for halloysites has often been attributed
to the tubular shape of many halloysite particles.
The tubes have been considered to provide the
characteristic small pores, both directly, from their
hollow interiors, and also indirectly, from the voids
created when they pack together with other tubes
(Churchman & Payne, 1983; McCrea & Gilkes,
1987). Halloysite particles, however, can occur in
shapes other than tubes, e.g. spheroidal and blocky
shapes (Churchman & Carr, 1975; Churchman &
Theng, 1984; Dixon, 1989).
Until now, each study of pore-size distribution in
halloysite has been carried out on only one sample of
the mineral (Diamond, 1970; Jackson et al., 1971),
on a single halloysite-containing soil (Churchman &
Payne, 1983), or, at most, on a small number of
halloysite-containing soils, but all from the same
locality (McCrea & Gilkes, 1987; McCrea et al.,
I990). Particle shapes provided the explanations both
for the apparently characteristic pore-size distribu-
9 1995 The Mineralogical Society
90
G . J . Churchman et al.
tions of halloysite and also for the differences
between pore-size distributions of halloysites and
kaolinites. Consequently, we considered it worthwhile to examine the relationships between pore-size
distributions and particle shapes (and sizes) for
halloysites with different shapes.
Gas sorption was used because comparisons of
gas sorption measurements with those made using
mercury intrusion porisimetry (e.g. Sills et al.,
1973b) indicate that only gas sorption, which
measures smaller pore sizes than mercury intrusion
porisimetry, would enable determination of the
sizes of the dominant fine pores in at least some
of the halloysites.
By obtaining information on the shapes of the
pores from the gas sorption measurements and also
from transmission electron microscopy (TEM), we
were able to examine the origins of the fine pores
in halloysites.
MATERIALS
Table 1 gives the characteristics of the seven
samples used. Four of the samples studied
(Dunedin, Te Akatea, Hamilton and Te Puke)
were from the same sites as the halloysites
studied as clay fraction samples by Churchman &
Theng (1984). Two samples (Opotiki fine and
Opotiki coarse) were from the Opotiki area of
New Zealand. Churchman & Theng (1984) studied
the clay fraction of just one sample from the same
area. The remaining sample, from Jarrahdale,
Western Australia, is sample D9 in Churchman &
Gilkes (1989).
Kaolin mineral contents given in Table 1 were
determined by differential thermal analyses
(Whitton & Churchman, 1987) of samples that
had previously been dried at 105~ for several
hours. Previous work (Churchman & Theng, 1984;
Churchman & Gilkes, 1989) had shown virtually all
of the kaolin present in these samples, or in samples
from the same areas, to be halloysite. Transmission
electron microscopy confirmed that halloysite was
the predominant kaolin mineral in the samples.
Contents of allophane and ferrihydrite were
determined from acid oxalate analyses (Blakemore
et al., 1987), leading to estimates of allophane by
Parfitt & Wilson's (1985) method and ferrihydrite
following Parfitt & Childs (1988). Organic matter
contents were invariably low. Only the brownishyellow Hamilton soil sample was not white in
colour. Nevertheless, the organic carbon content of
the Hamilton soil IIC horizon, from which this
sample was taken, is only 0.1% (New Zealand Soil
Bureau, 1968).
METHODS
Approximately 1 g of each sample, except the
Hamilton soil sample, was sealed into a sample
bulb as an intact block, as taken from the field.
Since a mismatch between the isotherms for
adsorption and desorption at low relative pressures
implied the possible perturbation of blocks of the
Hamilton soil sample by the analytical procedure,
this sample was examined as a powder formed by
light hand-grinding and 0.2 g was used. Each
sample was then heated at 105-110~ to degas in a
vacuum of better than 10-4 tort until the pressure
remained below 10 -3 torr for 15 min after the
vacuum pumps were isolated. Complete adsorptiondesorption isotherms to saturation were obtained at
78~ using a liquid nitrogen bath. The apparatus
used for all except the Hamilton soil sample is
described in Sills et al. (1973a,b). An automated
volumetric instrument, Omnisorp 100, was used for
the Hamilton soil sample.
The specific surface areas of the heated samples
were derived from the low pressure range of the
isotherms by the BET method (Brunauer et al.,
1938) and differential pore-size distributions for
both branches calculated using the Kelvin equation
modified to account for physical sorption on
exposed surfaces (Aylmore, 1974a). Cumulative
specific surface a r e a s ( S a d s and Saes) were
calculated from pore filling and emptying on the
adsorption and desorption branches respectively.
The extent of the agreement between the BET area
and Sd~s or Saas is indicative of the extent to which
the assumption of a predominance of slit-shaped or
cylindrical shaped pores respectively is valid
(Aylmore, 1974b).
Plots of V vs. n (Lippens & de Boer, 1965;
Aylmore, 1974a) were obtained, where the volume
adsorbed, V, is plotted against n, a statistical
average of the number of layers which would be
adsorbed onto a similar but non-porous material at
the same relative vapour pressure. The usefulness of
V-n plots derives from observations that nitrogen
adsorption on different non-porous materials gives
essentially the same isotherm when plotted in the
'reduced' form of n(=V/Vm) against P/Po, where Vm
is the volume adsorbed in a monolayer and P/Po is
Characteristics of pores in halloysites
v
v
91
v
~8
,~
~
,:5
~
~
V
V
<8
o
o
o
o.
o
o
o
o
o
~
o
c5
o.
o
,.t5
~3
9
"d
O
e-,
t~
~D
on
e-,
r,..9
,...;
,-A
~D
.<
[-.,
9
t~
~D
cD
O
o~
o
"~
9~
~
~
~
~
-~
~
O
~ ~.~
8
~
G.J. Churchman et al.
92
180
300
Opotikl
280F
fine
J
I
250F
240F
220F
200~-180
i
160
140
~:n
OS
rI'-
120
100
~
6o
8O
~
40
-e
~
2o
"o
t~ Hamilton ..........
E
80
141)
n.
i-r
160
120
100
80
Jarrahi~
5O
40
o
"o
oa
20-
4O
__
---"
0potiki (2} - -
"-
~
'
Te Pulll
2O
Jarrahdale
I
I
I
I
~L
I
I
I
I
I
0.1
0.2
0.3
0.4
0:5
0.6
0.7
0.8
0,9
1.0
60
~
P/Po
4o --
..Of ~"
20
TePuke- 80
6O
40
2O
TeAkatea
Dunedin
0.1 0.2 0.3 0.4 0,5 0.6 0.7 0,8 0.9 1.0
P/Po
FIG. 1. Nitrogen adsorption-desorption isotherms on
Dunedin, Te Akatea, Te Puke, Opotiki coarse and
Opotiki fine halloysite samples. The closed circles
indicate adsorption and the open circles, desorption.
Volume adsorbed is plotted to the same scale but from
different origins for each sample. The origins for each
are indicated on the ordinates.
Fio. 2. Nitrogen adsorption-desorption isotherms on
Hamilton soil sample and Jarrahdale sample. The
closed circles indicate adsorption and the open circles,
desorption.
the suspension to dry on a carbon support film or
by embedding in Spurr's resin (Spurr, 1969) and
sectioning using a Riechert Ultramicrotome, In
addition, intact blocks of air-dried samples of
each of the clays and soils were examined by
scanning electron microscopy, using a Cambridge
Stereoscan instrument.
RESULTS
AND
DISCUSSION
Isotherms
the relative pressure (Shull, 1948; Lippens & de
Boer, 1965; Pierce, 1968). Deviations from linear
V-n plots, which indicate simple monolayer-multilayer adsorption, enable the occurrence of pore
filling to be detected and the nature of such
deviations indicate the general shape of the
relevant pores (Aylmore, 1974a 1974b, 1977).
X-ray diffraction analyses of all of the samples
had been obtained in previous studies. Transmission
electron microscopy was carried out using a Philips
400 electron microscope. Samples for TEM were
prepared by either dispersing in water and allowing
The nitrogen-sorption isotherms for most of the
samples from New Zealand are shown in Fig. 1 and
those for the New Zealand soil sample (Hamilton)
and the Jarrahdale sample in Fig. 2. The isotherms
in Fig. 1 show either very little or no hysteresis
whereas those in Fig. 2 show substantial hysteresis.
Differential pore-size distributions
Figure 3 shows differential pore-size distributions
corresponding to the isotherms which showed
substantial hysteresis, i.e. those in Fig. 2. The
Characteristics of pores in halloysites
r
~:
r
480 I
460
440
400
360 m
320
280
240
200
160
120
E
8O
also studied (Te Puke). However, the reported
closure of many isotherms at relative pressures of
0.42-0.50 (work cited by Gregg & Sing, 1967),
means that the Kelvin equation may not apply at
the small pore sizes i.e. < c. 2 nm which are
equivalent to these relative pressures.
Specific surface areas
4O
m Hamilton
640
o
600
Hamilton
560
~-~
52O
48O
440
400
360
320
28O
240
2OO
160
120
80
4O
t
Jarrahdale _1
0
93
Jarrahdale
1 I I 1 I
20
40
60
80
Pore size (rim)
\..j__-~.
,
100
FIG. 3. Differential pore-size distributions from the
desorption isotherms (Fig. 2) for Hamilton soil sample
and Jarrahdale sample.
distributions in Fig. 3 each show a single sharp
peak; these are at 7 nm for Hamilton and at 12 nm
for Jarrahdale.
Although the isotherms in Fig. 1 show little or no
hysteresis, differential pore-size distributions
derived from them suggested that the five
halloysites represented could have pores that have
narrow size distributions, with sharp peaks in the
2.4-2.8 nm size range. The calculated distributions
were very similar to that shown by Jackson et al.
(1971) for a sample of one of the halloysites we
Values for specific surface areas of the various
samples, as determined by the BET equation (SBET),
are compared in Table 2 with the cumulative
specific surfaces obtained from the isotherms for
both adsorption (Sad~) and desorption (Sde~) of
nitrogen on each sample.
Among the samples giving isotherms with
considerable hysteresis (Fig. 2), the Saes value is
substantially higher than the corresponding BET
area for the Jarrahdale sample, indicating that
retention of capillary condensate by narrownecked pores controls desorption from larger
volumes (Aylmore, 1974a). There is close agreement between Sads and the BET area. Following de
Boer et al. (1964) and Aylrnore & Quirk (1967),
this sample is likely to contain mainly 'bottleshaped', tubular or cylindrical pores (hereafter
referred to as 'cylindrical pores') of varying radius.
In the case of the Hamilton soil sample, while
Sdes>SBET, there is a similar discrepancy between
Sde~ and SBET, on the one hand, and Sads and SBET,
on the other. This suggests that there are both
cylindrical and slit-shaped pores in this sample.
Slit-shaped pores give rise to Sads values in excess
of SBET and equal values for So~ and S~ET (de Boer
et al., 1964; Aylmore & Quirk, 1967).
For the samples represented in Fig. 1, the criteria
for surface areas (Table 2) indicate that pores in only
the Te Akatea halloysite could be largely cylindrical.
The Opotiki fine sample has a very high specific
surface (Table 2). It has significant allophane
(Table 1), and also iron oxides (Churchman &
Theng, 1984).
The Hamilton sample also has a relatively large
specific surface (Table 2). It contains considerable
Fe (Churchman & Theng, 1984), including
significant ferrihydrite (Table 1). These amounts
are insufficient to explain completely its high
surface area (Churchman & Burke, 1991). The
high concentration of very small halloysite particles
could explain much of the high specific surface of
this sample.
94
G. J. Churchman et al.
TABLE 2. Comparison of various measures of specific surface areas of samples.
Sample
Specific surface (m2g -1)
From BET
equation
Cumulative, from
adsorption isotherm
Cumulative, from
desorption isotherm
(aBET)
(aads)
(ades)
Dunedin
Te Akatea
Hamilton
Te Puke
Opotiki fine
Opotiki coarse
Jarrahdale
30.3
46.1
93.8
35.4
169.2
42.4
56.2
26.2
51.3
87.9
27.5
127.5
25.4
53.7
~ro
160
150
.
o Opol~kl
..
H~mllton
140
0
li~
130
b~
n"
- 1.3
+9.1
+4.9
+3.0
-16.1
-5.2
+25.1
The samples differ in the extent of change of
slope at their inflection points. The V-n plots for the
Jarrahdale, Hamilton and Te Akatea samples exhibit
the greatest changes of slope. These materials
apparently contain the greatest proportion of
cylindrically-shaped pores.
Electron microscopy
'~ 100
80 r-
Jarrahdale
"~ 7o
~
-4.1
+5.2
-5.9
-7.9
-41.7
-17.0
-2.5
Sdes--SBE T
120
I-" 110
rj)
0~
29.0
55.2
98.7
38.4
153.1
37.2
81.3
Sads--SBET
go
le A~lea
~- 50-'~
20
//
i //i
/, //
.Ig~,,
Te Puke
....
if
10
1
2
3
n, n u m b e r of a d s o r b e d layers
Fio, 4. Plots of V vs. n from the adsorption isotherms
(Figs. 1 and 2) for all samples.
V-n plots
Plots of V vs. n for the different samples (Fig. 4)
all show an increase in slope with increasing values
of n. This indicates the occurrence of some
cylindrical pores in all samples (Aylmore,
1974a,b; 1977).
Transmission electron microscopy confirmed the
general shapes of the halloysite particles that have
been recorded in the relevant publications, i.e.
Churchman & Theng (1984) for Dunedin, Te
Akatea, Hamilton, Te Puke and Opotiki fine
halloysite samples and Churchman & Gilkes
(1989) for the sample from Jarrahdale. The
Opotiki coarse sample also contained spheroidal
particles. Particles in all halloysite samples were of
four main types. These are represented in the
micrographs of particles shown in Fig. 5. The main
types are: thick tubular (Fig. 5a, Dunedin);
spheroidal (Fig. 5b, Opotiki); blocky (Fig. 5c, Te
Puke); thin tubular (Fig. 5d, Jarrahdale). 'Tubular'
particles may also include laths and many tubes
have a polyhedral rather than a circular crosssectional shape.
Particles of these different types were often
found together in the same sample, suggesting a
range of particle shapes for halloysites.
Effect of particle shapes and sizes on shapes
and sizes o f pores in halloysites
Table 3 details the particle shapes and sizes in the
samples which give rise to dominantly cylindrical
Characteristics of pores in halloysites
95
Fla. 5. Transmission electron micrographs of cross-sections of halloysites with (a) thick tubular (Dunedin); (b)
spheroidal (Opotiki fine); (c) blocky (Te Puke); and (d) thin tubular (Jarrahdale) particles. Note the gel
(allophane) also present in the Opotiki sample. Particles in a and c were embedded in resin while those in b and d
were dispersed in water before examination by TEM. Bars indicate 0.1 Ixm. See text for description and labelling
of pores.
pores according to the
measurements of surface
plots (Fig. 4). These are
characteristics of particles
comparisons of different
area (Table 2) and V-n
compared with the same
in the other samples.
Table 3 shows that, when there was a substantial
volume of fine pores (or 'mesopores' - - Gregg &
Sing, 1967) present in the halloysites (Hamilton and
Jarrahdale), a high proportion of these pores were
G. J. Churchman et al.
96
T~I~E 3. Shapes and mean sizes of dominant types of particles in relation to occurrence of cylindrical
pores in halloysites.
Sample
Dominant
shape
A. Abundant pores
1. Cylindrical pores dominant
Jarrahdale
Tubular
2. Abundant cylindrical pores
Hamilton
Tubular
B. Few, if any, pores
1. Cylindrical pores dominant
Te Akatea
Tubular
2. Cylindrical pores not dominant
Dunedin
Tubular
Te Puke
Tubular
+ blocky
Opotiki fine
Spheroidal
Opotiki coarse
Spheroidal
Average length
(gm)
Average width
(gm)
Number
counted
0.29 (-t-0.19)
0.05 (___0.01)
54
0.15 (+0.09)
0.07 (+0.07)
23"
0.51 (+0.27)
0.08 (+0.05)
32"
0.52 (___0.30)
0.27 (+0.12)
0.12 (-t-0.06)
0.08 (+0.05)
26*
25"
0.30 (+0.10)
0.24 (-t-0.06)
0.30 (+0.10)
0.24 (-t-0.06)
52
45
* Data from Churchman & Theng (1984)
cylindrical in shape. The dominant tubular particles
in these halloysites were either the shortest
(Hamilton) or the narrowest (Jarrahdale) among
those studied.
Location o f pores in halloysites
Representative transmission electron micrographs
of the cross-sections of the halloysites (Fig. 5) show
that particles of all shapes generally are made up of
blocks of layers bent around one another. Only the
smaller, tubular particles shown in Fig. 5d, contain
a cylindrical or near-cylindrical pore in the centres
of their tubes.
In the larger particles, i.e. the particles in Fig.
5 a - c , there are gaps between successive blocks
which constitute virtually all of the visible pores.
The intra-crystal pores in these larger particles tend
to be both narrow and slit-shaped. The small tubular
particles (F) show few visible slit-shaped pores
from gaps between blocks of layers, but slightly
larger tubes, such as those labelled I, can show both
slit-shaped and cylindrical pores.
While the presence of central holes may explain
the occurrence of cylindrical pores in samples
containing small tubular particles, the large pore L
shown in Fig. 5a of a large tube in Dtmedin halloysite
may also exhibit some cylindrical character.
Using an electron microscope with an environmental cell, Kohyama et al. (1978) showed that a
hydrated halloysite did not contain pores within the
tubes. Pores within the tubes were created by
dehydration of the 10 ,~ form of the mineral to give
the 7 A form. Groups of layers apparently shrink as
a block, creating pores between each group. Such
pores may arise from dehydration occurring in the
field or during preparation of samples for analyses
in the laboratory, including gas sorption and
electron microscopy.
Singh & Gilkes (1992) have also shown that
wedge-shaped voids can form by the folding of
layers of kaolinites to form tubular halloysite.
However, there is no evidence that the halloysite
samples in this study had originated from kaolinite.
Primary minerals (quartz, ferromagnesians,
cristobalite, feldspars, mica), and also vermiculite
(altered from, but pseudomorphous with, mica)
should not contribute to the pores <c. 15 nm
which are of interest in this study. Gibbsite is only
ever present in trace amounts in the Jarrahdale
samples (Churchman & Gilkes, 1989). Allophane
tends to give rise to cylindrical pores with radii
near 1 nm (Rousseaux & Warkentin, 1976).
In general, pore sizes and shapes derived from
the sorption of nitrogen on a number of halloysites
can be explained as intra-crystalline phenomena.
Characteristics of pores in halloysites
Pores resulting from the packing together of
different particles have also been suggested as
explanations of the characteristic pore-size distributions for halloysites (Churchman & Payne, 1983;
M c C r e a & Gilkes, 1987). S c a n n i n g electron
micrographs of the samples used in this study
suggest that this explanation is unlikely as pores
from packing would be neither small enough nor
concentrated enough in narrow ranges to provide
the types of pore size distributions shown in Fig. 3.
CONCLUSIONS
(1) Halloysites with mainly small particles (< c.
0.08 Ixm in width) had many cylindrical pores.
(2) These cylindrical pores showed a narrow
distribution of sizes (from c. 5 - 1 5 nm).
(3) Halloysites with mainly large particles (> c.
0.1 I.tm in width) had few, if any, pores in the
mesopore size range ( 2 - 5 0 nm).
(4) Cylindrical pores probably derive mainly
from the central holes in tubular particles.
(5) Slit-shaped pores in halloysites may arise
from the shrinkage on dehydration of blocks of
layers.
ACKNOWLEDGMENTS
GJC is grateful to the University of Western Australia
for the award of a Gledden Senior Visiting Fellowship
which provided financial support for this work and his
former employers, New Zealand Soil Bureau, DSIR,
for study leave in order to carry it out. We thank Dr
Rob Hayes, of the University of South Australia for the
determination of the isotherms for the powdered
Hamilton halloysite sample, Kay Card, formerly of
the Physics and Engineering Laboratories, DSIR, New
Zealand for scanning electron microscopy and Adrian
Beech, Trevor Cock and Jim Thompson, of CSIRO
Division of Soils, for oxalate-extractable cation
determinations, TEM plate preparation and differential thermal analyses, respectively. We also thank Dr
Chris Pope of the University of Otago, New Zealand
for helpful comments on the manuscript.
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