consist of linear chains of SiO 4 tetrahedra. The chains may be single or double. In
phyllosilicates, the SiO 4 tetrahedra are assembled into 6-membered rings, which
connect and spread two-dimensionally into a sheet-like structure. Tectosilicates are
three-dimensional assemblages of SiO 4 tetrahedra. In soils, nesosilicates,
inosilicates, phyllosilicates, and tectosilicates are common to abundant and present
as primary and clay minerals. Allophane and imogolite are also grouped in the
nesosilicates, although these two are non-crystalline.
2.3.2 Examples of Silicate and Silica Minerals in Soil
This section presents examples of the silicate and silica minerals frequently found in
soils. Several examples of un-weathered mineral particles were taken from new
volcanic ash deposits. Partially weathered minerals are so common in soils that a few
examples of them are also included in this section.
2.3.2.1 Silicate Minerals
Silicate minerals are various salts of silicate anions (Fig. 2.3). The cations are Al,
Mg, Fe, Ti, Na, K, Ca, and other elements. In the following discussion, the silicate
minerals are introduced in order of increasing complexity of their silicate framework.
Table 2.1 Grouping of silicate and silica minerals
Subclass
name
Characteristics
Example
Ideal formula
1 Nesosilicates Lone
tetrahedron
[SiO 4 ]
4À
Olivine
(Mg, Fe
2+ ) 2 SiO 4
2 Sorosilicates Double
tetrahedra
[Si 2 O 7 ]
6À
Vesuvianite Ca 19 (Al, Fe) 10 (Mg,
Fe) 3 (Si 2 O 7 ) 4 (SiO 4 ) 10 (O,
OH, F) 10
3 Cyclosilicates Ring silicates [Si n O 3n ]
2nTourmaline NaMg 3 Al 6 (Si 6 O 18 )
(BO 3 ) 3 (OH) 3 (OH, F)
(dravite)
4 Inosilicates
Single chain
silicates
[Si n O 3n ]
2nÀ
Pyroxene
(Ca, Mg, Fe
2+ , Al) 2 (Si,
Al) 2 O 6 (augite)
Double chain
silicates
[Si 4n O 11n ]
6nÀ
Amphibole Ca 2 (Mg, Fe
2+ ) 4 Al
[Si 7 Al]O 22 (OH) 2
(magnesiohornblende)
5 Phyllosilicates Sheet silicates [Si 2n O 5n ]
2nÀ
Muscovite K 2 Al 4 (Si 6 Al 2 )O 20 (OH) 4
Biotite
K 2 (Mg, Fe) 6 (Si 6 Al 2 )
O 20 (OH) 4
Clay
minerals
See Chap. 3
6 Tectosilicates 3D framework [Al x Si y O 2(x + y) ]
xÀ
Quartz
SiO 2
Feldspar
NaAlSi 3 O 8 (albite)
14
2 Primary Minerals
phyllosilicates, the SiO 4 tetrahedra are assembled into 6-membered rings, which
connect and spread two-dimensionally into a sheet-like structure. Tectosilicates are
three-dimensional assemblages of SiO 4 tetrahedra. In soils, nesosilicates,
inosilicates, phyllosilicates, and tectosilicates are common to abundant and present
as primary and clay minerals. Allophane and imogolite are also grouped in the
nesosilicates, although these two are non-crystalline.
2.3.2 Examples of Silicate and Silica Minerals in Soil
This section presents examples of the silicate and silica minerals frequently found in
soils. Several examples of un-weathered mineral particles were taken from new
volcanic ash deposits. Partially weathered minerals are so common in soils that a few
examples of them are also included in this section.
2.3.2.1 Silicate Minerals
Silicate minerals are various salts of silicate anions (Fig. 2.3). The cations are Al,
Mg, Fe, Ti, Na, K, Ca, and other elements. In the following discussion, the silicate
minerals are introduced in order of increasing complexity of their silicate framework.
Table 2.1 Grouping of silicate and silica minerals
Subclass
name
Characteristics
Example
Ideal formula
1 Nesosilicates Lone
tetrahedron
[SiO 4 ]
4À
Olivine
(Mg, Fe
2+ ) 2 SiO 4
2 Sorosilicates Double
tetrahedra
[Si 2 O 7 ]
6À
Vesuvianite Ca 19 (Al, Fe) 10 (Mg,
Fe) 3 (Si 2 O 7 ) 4 (SiO 4 ) 10 (O,
OH, F) 10
3 Cyclosilicates Ring silicates [Si n O 3n ]
2nTourmaline NaMg 3 Al 6 (Si 6 O 18 )
(BO 3 ) 3 (OH) 3 (OH, F)
(dravite)
4 Inosilicates
Single chain
silicates
[Si n O 3n ]
2nÀ
Pyroxene
(Ca, Mg, Fe
2+ , Al) 2 (Si,
Al) 2 O 6 (augite)
Double chain
silicates
[Si 4n O 11n ]
6nÀ
Amphibole Ca 2 (Mg, Fe
2+ ) 4 Al
[Si 7 Al]O 22 (OH) 2
(magnesiohornblende)
5 Phyllosilicates Sheet silicates [Si 2n O 5n ]
2nÀ
Muscovite K 2 Al 4 (Si 6 Al 2 )O 20 (OH) 4
Biotite
K 2 (Mg, Fe) 6 (Si 6 Al 2 )
O 20 (OH) 4
Clay
minerals
See Chap. 3
6 Tectosilicates 3D framework [Al x Si y O 2(x + y) ]
xÀ
Quartz
SiO 2
Feldspar
NaAlSi 3 O 8 (albite)
14
2 Primary Minerals