whereas halloysite (0.7 nm) does not. The major 2:1 type aluminosilicates are
smectite, vermiculite, micaceous minerals, and chlorite.
In layer aluminosilicates, the same unit layer is stacked repeatedly. The distance
between one basal plane and the next one, as shown by arrows in Fig. 3.3, is called
the basal spacing. The difference in the two halloysites is indicated by the difference
in the basal spacing as halloysite (0.7 nm) and halloysite (1.0 nm). The differences
among the 2:1 types are characterized by the difference in the basal spacing and
changes in the basal spacing that occur with several treatments. The basal spacing of
these aluminosilicates is determined by XRD, and the changes in the basal spacing
with several treatments (Mg saturation and glyceration, Mg saturation, K saturation,
heating at 300 and 550
C after K saturation, etc.) are used to identify the aluminosilicate clays in soil. As XRD is the most effective tool for identifying the clay
mineral composition of soil, XRD patterns are used along with TEM and SEM to
introduce clay minerals in the following sections.
As shown in Figs. 3.1 and 3.2, there are two types of octahedral layers. One is the
brucite type in which all octahedral sites are occupied by a divalent cation, which is
called the trioctahedral type. The other is the gibbsite type, called the dioctahedral
type, in which two-thirds of the octahedral sites are occupied by a trivalent cation.
In the secondary minerals in soil, cations in the tetrahedral and octahedral sites are
replaced by other cations with similar size and lower valence. This phenomenon is
called isomorphous substitution. For example, at the tetrahedral sites, Si
4+ is partly
replaced by Al
3+ , and at the octahedral sites, Al
3+ is partly replaced by Mg
2+ or Fe
2+ .
Although the isomorphous substitution does not affect the crystal structure very
much, the number of positive charges decreases and a surplus of negative charge
occurs in the aluminosilicate layer. The surplus of negative charge is neutralized by
adsorption of cations, called exchangeable cations.
3.2.3.1 1:1 Type Minerals
The 1:1 type minerals include kaolinite and halloysite. A group name of these
minerals is “kaolin minerals.” Hydrogen bonds connect the 1:1 layers in kaolinite.
In the idealized formula (Table 3.1), there is no isomorphic substitution.
Figure 3.4 shows a reference kaolinite sample, kaolinite No. 9 of the American
Petroleum Institute (A.P.I.) reference clay minerals. The thin-to-thick platy and
partially hexagonal properties of kaolinite minerals can be seen in both the TEM
image (Fig. 3.4a) and the SEM image (Fig. 3.4b). XRD patterns of oriented samples
show a strong diffraction peak at 0.7 nm, a basal spacing of kaolinite (Fig. 3.3).
Although the basal spacing of kaolinite is not affected by the four treatments
(Mg saturation and glyceration, Mg saturation, K saturation, and heating at 300
C
after K saturation), kaolin minerals are converted to an amorphous state by heating at
550
C, and the diffraction peak at 0.7 nm disappears (Fig. 3.4c).
Kaolinite is one of the most frequently found clay minerals in soil. Figure 3.5
shows an example from the Bt2 horizon of an Ultic Palexeralf used for a vineyard in
Nuble Province, Chile. The TEM image (Fig. 3.5c) and SEM image (Fig. 3.5d) show
42
3 Secondary Minerals
smectite, vermiculite, micaceous minerals, and chlorite.
In layer aluminosilicates, the same unit layer is stacked repeatedly. The distance
between one basal plane and the next one, as shown by arrows in Fig. 3.3, is called
the basal spacing. The difference in the two halloysites is indicated by the difference
in the basal spacing as halloysite (0.7 nm) and halloysite (1.0 nm). The differences
among the 2:1 types are characterized by the difference in the basal spacing and
changes in the basal spacing that occur with several treatments. The basal spacing of
these aluminosilicates is determined by XRD, and the changes in the basal spacing
with several treatments (Mg saturation and glyceration, Mg saturation, K saturation,
heating at 300 and 550
C after K saturation, etc.) are used to identify the aluminosilicate clays in soil. As XRD is the most effective tool for identifying the clay
mineral composition of soil, XRD patterns are used along with TEM and SEM to
introduce clay minerals in the following sections.
As shown in Figs. 3.1 and 3.2, there are two types of octahedral layers. One is the
brucite type in which all octahedral sites are occupied by a divalent cation, which is
called the trioctahedral type. The other is the gibbsite type, called the dioctahedral
type, in which two-thirds of the octahedral sites are occupied by a trivalent cation.
In the secondary minerals in soil, cations in the tetrahedral and octahedral sites are
replaced by other cations with similar size and lower valence. This phenomenon is
called isomorphous substitution. For example, at the tetrahedral sites, Si
4+ is partly
replaced by Al
3+ , and at the octahedral sites, Al
3+ is partly replaced by Mg
2+ or Fe
2+ .
Although the isomorphous substitution does not affect the crystal structure very
much, the number of positive charges decreases and a surplus of negative charge
occurs in the aluminosilicate layer. The surplus of negative charge is neutralized by
adsorption of cations, called exchangeable cations.
3.2.3.1 1:1 Type Minerals
The 1:1 type minerals include kaolinite and halloysite. A group name of these
minerals is “kaolin minerals.” Hydrogen bonds connect the 1:1 layers in kaolinite.
In the idealized formula (Table 3.1), there is no isomorphic substitution.
Figure 3.4 shows a reference kaolinite sample, kaolinite No. 9 of the American
Petroleum Institute (A.P.I.) reference clay minerals. The thin-to-thick platy and
partially hexagonal properties of kaolinite minerals can be seen in both the TEM
image (Fig. 3.4a) and the SEM image (Fig. 3.4b). XRD patterns of oriented samples
show a strong diffraction peak at 0.7 nm, a basal spacing of kaolinite (Fig. 3.3).
Although the basal spacing of kaolinite is not affected by the four treatments
(Mg saturation and glyceration, Mg saturation, K saturation, and heating at 300
C
after K saturation), kaolin minerals are converted to an amorphous state by heating at
550
C, and the diffraction peak at 0.7 nm disappears (Fig. 3.4c).
Kaolinite is one of the most frequently found clay minerals in soil. Figure 3.5
shows an example from the Bt2 horizon of an Ultic Palexeralf used for a vineyard in
Nuble Province, Chile. The TEM image (Fig. 3.5c) and SEM image (Fig. 3.5d) show
42
3 Secondary Minerals