Bergaya F. Handbook of Clay Science
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The alteration of biotite into kaolinite represents an extreme example of compos-
itional change. Nevertheless, the process occurs frequently, especially under a tropical
climate (White et al., 1998). An understanding of the mechanisms at the atomic scale is
critical for constructing a chemical mass balance (Murphy et al., 1998). Metastable,
moderately altered biotite, and halloysite are formed at low temperatures when re-
actions are sluggish, consistent with the Ostwald step rule (Dong et al., 1998).
By contrast, the reaction kinetics at high temperatures (Ahn and Peacor, 1987)favour
a direct alteration of the reactant (biotite) to the product (kaolinite).
Although there are no consistent differences in the chemical and mineralogical
composition of the original materials, the alteration of pyroclastic deposits over large
areas leads to a great diversity of 1:1 minerals and allophanic phases. Different parts of
the deposits are characterized by a particular particle morphology of kaolinite (platy,
wavy, degree of automorphy) or halloysite (tabular, tubular, spheroidal). These par-
ticles are most often associated with relict minerals (sanidine, plagioclase, micas), an
amorphous Si-rich phase, and/or poorly crystallized silicates (opal, cristobalite, tridy-
mite). The morphological and mineralogical differences suggest changes in physico-
chemical conditions and/or preferential percolations (Adamo et al., 2001). Delvaux
et al. (1992) related the morphology of halloysite in the clay fractions of soil pedons to
weathering stage and the nature of the exchangeable cation. They suggested a weath-
ering sequence for similar volcanic parent materials. Similar differentiation between
1:1 clay minerals are also common in soil profiles (Robain et al., 1990; Romero et al.,
1992b) and toposequences (Hidalgo et al., 1998; Elsass et al., 2000).
12.8.8. CLAYS IN DIAGENETIC ALTERATION
Diagenetic transformations can very often be assessed by HRTE M observations.
The smectite-to-illite transition was extensively studied by XRD. The development
of HRTEM and AEM techniques enabled this transformation to be followed. These
techniques were also used to provide information about the mechanism of illitization
in deep burial diagenesis (Lee et al., 1985; Ahn and Peacor, 1986). Similarly,
Klimentidis and Mackinnon (1986) combined XRD with TEM to study well-known
mixed-layer clay minerals with the aim of verifying current mixed-layer stacking
models. They and others (Eberhart and Triki, 1972; Tchoubar et al., 1973; Brown
and Jackson, 1973; McKee and Brown, 1977 ) used several methods of sample prep-
aration, and avoided collapse of smectitic interlayer spaces.
Different preparation methods give different results, while the apparent discrep-
ancy between XRD and HRTEM results is difficult to reconcile. Nevertheless ex-
perimentally observed layer sequences (illite/smectit e an d corrensite) agree well with
the illite/smectite layer sequences calculated using the Monte Carlo model for ill-
itization. Cluster s of segregated illite and/or smectite layers tend to develop as well as
mixed-layer structures.
Disordered mica structures (Iijima and Buseck, 1978) and perfect mica polytypes
(Amouric et al., 1981) were used to interpret HRTEM images.
12.8.8. Clays in Diagenetic Alteration 957
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Handbook of Clay Science
Edited by F. Bergaya, B.K.G. Theng and G. Lagaly
Developments in Clay Science, Vol. 1
r 2006 Elsevier Ltd. All rights reserved.
965
Chapter 12.9
SURFACE AREA AND POROSITY
L.J. MICHOT AND F. VILLIE
´
RAS
Laboratoire Environnement et Mine
´
ralurgie, BP 40, F-54501 Vandoeuvre Cedex,
France
The specific surface area (SSA) of clay minerals and related materials is one of
their mo st important properties controlling surface phenomena. The SSA of non-
swelling and non-microporous phyllosilicates ranges from a fraction to more than
one hundred square meters per gram. Higher values are obtained with microporous
clay minerals, such as sepiolite and palygorskite, and swelling (expanding) clay
minerals.
Clay minerals play an important role in natural and industrial processes as they
can take up organic and inorganic molecules at solid–liquid or solid–gas interfaces.
The surface properties of these minerals have to be precisely determined if we are to
gain insight into the underlying mechanisms. The present chapter has two parts; the
first is concerned with the determination of the SSA of clay minerals, in general, and
related topics, while the second part is specific to swelling clay minerals.
12.9.1. THE SPECIFIC SURFACE AREA OF CLAY MINERALS
In the case of non-swelling clay minerals, the most widely used technique for
determining SSA is based on gas ad sorption, notably of nitrogen gas at 77 K. Ad-
sorption isotherms, describing the amount of gas adsorbed as a function of relative
pressure (P/P
0
) can exhibit different features depending on the size of particles, the
presence of organized pores (between 0.5 and 50 nm), and the energetic properties of
the mineral surface (Sing et al., 1985).
Fig. 12.9.1 shows the classical shape of a gas adsorption isotherm. In the low -
pressure region, from P=P
0
¼ 0toC0.2, adsorption occurs on the external surface
of the particles and the surface of micropores, if such pores (diameter 0–2 nm) exist.
At larger relative pressures (0.4oP/P
0
o0.95), adsorption of the second and third
layers occurs, accompanied by condensation into mesopores (4–40 nm). Finally, at
relative pressures >0.95, condensation into macropor es (>40 nm) takes place.
DOI: 10.1016/S1572-4352(05)01035-4
Different methods of data analysis are used to derive quantitative information
from experimental adsorption curves of which the Brunauer Emmett and Teller
(BET) analysis is the most common. This is based on the equation proposed by
Brunauer et al. (1938), describing multilayer adsorption. From the linearised form,
expressed by Eq. (1), it is possible to derive the amoun t of gas required to cover the
surface of the particles by a single statistical layer, Q
mono.
(Gregg and Sing, 1982 ).
x
Q
ads:
ð1 xÞ
¼
C 1
Q
mono:
C
x þ
1
Q
mono:
C
ð1Þ
Here x is the relative pressure (P/P
0
), Q
ads.
the adsorbed amount at P/P
0
,andC a
constant related to the interactions between the adsorbed species and the surface.
0
50
100
150
200
0 0.2 0.4 0.6 0.8 1
Vads (cm
3
STP/g)
P/Po
First layer adsorption
and micropore filling
Second layer adsorption
and intermediary pore filling
Multilayer adsorption and
capillary condensation in mesopores
Capillary condensation in macropores
Fig. 12.9.1. Adsorption isotherm, general feature and corresponding phenomena.
Chapter 12.9: Surface Area and Porosity966
The SSA, also called the ‘BET surface area’, can be derived from the monolayer
capacity (Q
mono.
) and the cross-sectional area of the gas used. In the case of mi-
croporous materials, the measured BET surface area describes the combination of
surface phenomena such as adsorption on external particle surfaces, and spatial
phenomena such as condensation into micropores.
A different method should then be used to derive the external surface area and
micropore volume, based on comparing the experimental adsorption isotherm with a
reference isotherm obtained on a non-porous sample. In the case of nitrogen, the
most commonly used is the ‘t-plot method’ (de Boer et al., 1966). For other gases, the
‘a
s
method’ can be used (Gregg and Sing, 1982).
Information on mesoporosity can a lso be obtained from the gas adsorption iso-
therm in the medium and high relative pressure ranges. Several methods were de-
veloped to describe simultaneous multilayer adsorption and capillary condensation.
The most common is the ‘BJH method’ proposed by Barrett et al. (1951).
Although, techniques for gas adsorption and their analysis have long been es-
tablished and tested, the results should always be treated with caution when accurate
values are required. The lack of accuracy of the BET method for microporous
samples has already been remarked. In this instance, the measured adsorption iso-
therms are also strongly dependent on outgassing conditions as illustrated by the
behaviour of fibrous clay minerals such as sepiolite (Grillet et al., 1988) and paly-
gorskite (Cases et al., 1991). The proper outgassing conditions unde r which all
weakly adsorbed (zeolitic) water is removed, while structural water molecules are not
affected, cover a narrow temperature range (25–100 1C for palygorskite and
25–200 1C for sepiolite). In that context, sample-controlled thermal analysis tech-
niques, such as controlled rate thermal analysis (CRTA) (Rouquerol, 1970, 1989;
Rouquerol et al., 1992) should be implemented before gas adsorption isotherms are
determined.
Furthermore, the volume of very narrow micropores may not be totally filled by
the gas, especially when conventional step-by-step adsorption is used. The only way
to assess the total micropore volume of sepiolite, for example, is to use a quasi-
equilibrium adsorption set-up with carbon dioxide gas as a probe (Michot et al.,
1990). In the case of Al
13
-pillared clay minerals with varied layer charges, the fol-
lowing paradox is observed (Bergaoui et al., 1995): the higher the layer charge the
higher the nitr ogen BET surface area. Here again, accurate micropore volumes and
expected changes with layer charge are obtained from quasi-equilibrium carbon
dioxide adsorption. The above paradox arises because gases such as argon and
nitrogen, are adsorbed around the pillars, and do not condense into micropores
(Michot et al., 1998).
For non-microporous clay miner als, other problems arise when the results for the
adsorption of different gases are compared. For instance, the SSA of kaolinites
measured by argon ad sorption is generally lower than that obtained using nitrogen
(Cases et al., 1986). The cross-sectional area of water adsorbed on non-swelling
minerals depends on the crystallo-chemical properties of the minerals and the nature
12.9.1. The Specific Surface Area of Clay Minerals 967