minerals appear to form through polymerization of exchangeable Al in an acid soil,
but the polymerization of Al is not complete as in a gibbsite sheet. The position of
the XRD peak of the basal spacing appears broadly between 1.4 and 1.0 nm after
heating at 300
C. Several researchers have used citrate, fluoride, etc., to remove the
polymerized Al interlayering (Barnhisel 1977).
Acid soils, soils with pH(H 2 O) roughly less than 5.5, have exchangeable or
KCl-extractable Al and cause Al-overload disorders in sensitive plants. Liming is
effective for removing this exchangeable Al. Further, application of other plant
nutrients, such as Mg, K, P, etc., is necessary to improve crop production in acid
soils.
3.2.4 Dioctahedral and Trioctahedral Type
The difference in the octahedral layers between dioctahedral and trioctahedral types
appears to affect the weathering resistance of 2:1 type aluminosilicates. By comparing the muscovite and the weathered biotite in Fig. 2.9, biotite appears to weather
earlier than muscovite in soil. The octahedral layer of biotite is the trioctahedral type,
and that of muscovite is the dioctahedral type.
Figure 3.11 schematically shows the differences between trioctahedral and
dioctahedral micas. Potassium ion locates at the center of two six-membered Si
tetrahedron rings and is sandwiched by both upper and lower phyllosilicate layers, as
shown in Fig. 3.11a, b. In the trioctahedral type, through observation of the lower
aluminosilicate shown in Fig. 3.11a from the plane shown in Fig. 3.11c, the
six-membered Si tetrahedron ring and a part of the lower trioctahedral layer appear
(Fig. 3.11d). At the center of the Si tetrahedron ring, there is a front OH. The
direction of the proton of the front OH is perpendicular to the phyllosilicate plane
because the front OH is surrounded by three Al ions. Due to the difference in
electronegativity between O and H, the H is positively charged slightly. Hence, a
repulsive force occurs between K
+ and H
δ+
. On the other hand, in the dioctahedral
type, one-third of the octahedral sites are vacant, and the proton of the front OH can
be directed to the vacant octahedral site (Fig. 3.11e). Returning to Fig. 3.11a, b, the
repulsion between K
+ and H
δ+ is weaker in the dioctahedral type than in the
trioctahedral type. This is the possible reason why dioctahedral mica is more stable
than trioctahedral mica in soil.
The position of the powder XRD peak of the (060) plane can be used to
distinguish the trioctahedral type from the dioctahedral type. The positions of the
XRD peak of these two types are 1.53 and 1.50 nm, respectively. If a clay sample is a
mixture of the two types, it is difficult to make this distinction. When the mixture is
with kaolin minerals, peaks from the kaolin minerals can be removed by heating at
550
C. If the composition of the remaining mineral is simple enough, distinction
between these two types using the position of the (060) peak may be possible
(Nanzyo et al. 2001).
50
3 Secondary Minerals
but the polymerization of Al is not complete as in a gibbsite sheet. The position of
the XRD peak of the basal spacing appears broadly between 1.4 and 1.0 nm after
heating at 300
C. Several researchers have used citrate, fluoride, etc., to remove the
polymerized Al interlayering (Barnhisel 1977).
Acid soils, soils with pH(H 2 O) roughly less than 5.5, have exchangeable or
KCl-extractable Al and cause Al-overload disorders in sensitive plants. Liming is
effective for removing this exchangeable Al. Further, application of other plant
nutrients, such as Mg, K, P, etc., is necessary to improve crop production in acid
soils.
3.2.4 Dioctahedral and Trioctahedral Type
The difference in the octahedral layers between dioctahedral and trioctahedral types
appears to affect the weathering resistance of 2:1 type aluminosilicates. By comparing the muscovite and the weathered biotite in Fig. 2.9, biotite appears to weather
earlier than muscovite in soil. The octahedral layer of biotite is the trioctahedral type,
and that of muscovite is the dioctahedral type.
Figure 3.11 schematically shows the differences between trioctahedral and
dioctahedral micas. Potassium ion locates at the center of two six-membered Si
tetrahedron rings and is sandwiched by both upper and lower phyllosilicate layers, as
shown in Fig. 3.11a, b. In the trioctahedral type, through observation of the lower
aluminosilicate shown in Fig. 3.11a from the plane shown in Fig. 3.11c, the
six-membered Si tetrahedron ring and a part of the lower trioctahedral layer appear
(Fig. 3.11d). At the center of the Si tetrahedron ring, there is a front OH. The
direction of the proton of the front OH is perpendicular to the phyllosilicate plane
because the front OH is surrounded by three Al ions. Due to the difference in
electronegativity between O and H, the H is positively charged slightly. Hence, a
repulsive force occurs between K
+ and H
δ+
. On the other hand, in the dioctahedral
type, one-third of the octahedral sites are vacant, and the proton of the front OH can
be directed to the vacant octahedral site (Fig. 3.11e). Returning to Fig. 3.11a, b, the
repulsion between K
+ and H
δ+ is weaker in the dioctahedral type than in the
trioctahedral type. This is the possible reason why dioctahedral mica is more stable
than trioctahedral mica in soil.
The position of the powder XRD peak of the (060) plane can be used to
distinguish the trioctahedral type from the dioctahedral type. The positions of the
XRD peak of these two types are 1.53 and 1.50 nm, respectively. If a clay sample is a
mixture of the two types, it is difficult to make this distinction. When the mixture is
with kaolin minerals, peaks from the kaolin minerals can be removed by heating at
550
C. If the composition of the remaining mineral is simple enough, distinction
between these two types using the position of the (060) peak may be possible
(Nanzyo et al. 2001).
50
3 Secondary Minerals