Geology Reference
In-Depth Information
Table 4 Structures of common secondary soil minerals
Clay minerals (phyllosilicates)
Kaolinite
Al
4
Si
4
O
10
(OH)
8
Illite
K
1.5
(Al
3.5
Mg
0.5
)(Si
7
Al)O
20
(OH)
4
Vermiculite
M
0.7
Al
2
(Al
0.7
Si
3.3
)O
10
(OH)
2
M
¼
Ca
21
,Mg
21
,K
1
, etc.
Smectite
M
0.3
Al
2
(Al
0.3
Si
3.7
)O
10
(OH)
2
M
¼
Ca
21
,Mg
21
,K
1
, etc.
Chlorite
(Mg, Fe)
3
(Si, Al)
4
O
10
(OH)
2
(Mg, Fe)
3
(OH)
6
Amorphous aluminosilicates
Allophane
xSiO
2
Al
2
O
3
yH
2
O(x
¼
0.8-2, y Z 2.5)
SiO
2
.
Al
2
O
3
.
2.5H
2
O
Imogolite
Hydrous oxides
Goethite FeOOH
Haematite Fe
2
O
3
Ferrihydrite Fe
5
O
7
(OH)
.
4H
2
O
Gibbsite
Al(OH)
3
(Na, Ca)(Mn
31
,Mn
41
)
7
O
14
.
2.8H
2
O
Birnessite
Carbonates
Calcite
CaCO
3
Dolomite
CaMg(CO
3
)
2
Others
Gypsum
CaSO
4
.
2H
2
O
the primary minerals derived from the parent material (see section 5.2.4).
These may be formed by alteration of a primary mineral - e.g. the
conversion of mica into a clay mineral - or by reactions of soluble
products released into the soil environment - e.g. precipitation of iron
oxide. The most important groups of secondary minerals are: (a) the
aluminosilicate clay minerals; (b) short-range order (amorphous) alumi-
nosilicates; and (c) the hydrous oxides of Al, Fe, and Mn (Table 4).
In the study of soil science, most attention has historically been paid
to the aluminosilicate clays, which dominate the properties of temperate
soils, the first to be scientifically studied. More recently, the importance
of the amorphous aluminosilicates has been shown in young soils, in
soils derived from volcanic ash and in leached, acidic soil (e.g. podzols or
spodosols). The hydrous oxides are especially important components of
old, highly weathered soils, such as those found in the tropics (e.g.
oxisols). This is an important distinction as the charge on the alumino-
silicate clays is predominantly a permanent negative charge, while
the amorphous aluminosilicates and hydrous oxides have a variable,
