Environmetal Soil Properties and Behaviour

Environmetal Soil Properties and Behaviour


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Trioctahedral chlorite
2:1 layer interlayer hydroxide sheet
Repeating 2:1 unit layer,
thickness, 1.4 nm
Interlayer hydroxide sheet
\u2013could be brucite-type or
gibbsite-type
Chlorite
Kaolini
te
FIguRE 2.4
Basic repeating unit layers for 1:1 layer dioctahedral kaolinite shown in the upper portion of 
figure. The trioctahedral chlorite shown in the bottom of the figure has a 2:1 repeating unit 
layer with interlayer hydroxide sheets. The R2+ and R3+ in the ideal structural formula represent 
the sum of divalent and sum of trivalent cations, respectively. Note that the repeating layers are 
stacked vertically and stretched spatially to form the respective mineral particles. (Structural 
formula information from Newman, A.C.D., and Brown, G., 1987, The Chemical Constitution of 
Clays, Mineralogical Soc. Monograph No. 6, Longman Scientific and Technical, pp.1\u2013128.)
33Nature of Soils
the particles (Figure 2.4). As stated previously, the amount of silanol groups on 
the siloxane bounding surface depends upon the crystallinity of the interlinked 
SiO4 tetrahedra. The broken edges of the kaolinite particle expose hydrous 
oxide types of edge surfaces. These surfaces contain both silanol and aluminol 
groups. The broken octahedral sheets provide for Lewis acid sites [Al(III)\u2219H2O] 
that can bind OH groups in single coordination. Note that the gibbsite sheet, 
which acts as the lower bounding surface, will also have aluminol groups.
2.2.4 Chlorites
2.2.4.1 Mineral Structure
The lower schematic drawing in Figure 2.4 shows the basic structure of chlorite. 
This is basically a unit 2:1 layer structure that has a positively charged inter-
layer consisting of a coordinated octahedral hydroxide sheet that balances the 
unit 2:1 layer negative charge. This sheet differs from the octahedral sheet in 
the unit layer in that it does not have a plane of atoms that are shared with the 
adjacent tetrahedral sheet. The coordinated octahedral hydroxide sheets can be 
trioctahedral (brucite-type) or dioctahedral (gibbsite-type). It is not uncommon 
to find chlorite described as a mineral made up of mica layers held together by 
brucite or gibbsite sheets. Whilst cations such as Fe, Mn, Cr, and Cu are some-
times found in the hydroxy sheets, the more common hydroxy sheets are bru-
cite and gibbsite sheets. The typical repeat spacing for the unit layers is 1.4 nm.
The chlorite group includes various types differentiated according to the 
kinds and amount of substitution in the tetrahedral and octahedral sheets (Grim, 
1953). The substitutions in the tetrahedral sheet vary from Si3Al to Si3Al2, and in 
the octahedral sheet the substitutions vary from Mg3Al to Mg4Al2. Additionally, 
Mg2+ and Fe2+ are partially replaced by Fe2+ and Mn2+, and Al3+ is replaced by 
either Fe3+ or Cr3+. There are chlorites that have interlayer montmorillonite as 
part of their structure. These are sometimes called swelling chlorites.
2.2.4.2 Surface Functional Groups
Substitution of other cations for silicon and aluminium in chlorite results in 
development of a net negative charge in the mineral. Substitution of Al3+ for 
Si4+ in the tetrahedral layer results in a negative charge. Substitution of Al3+ by 
Mg2+ in the brucite layer gives a positive charge. The result of these substitu-
tion is a net positive charge that is the cation exchange capacity of the mineral.
2.2.5 Smectites
2.2.5.1 Mineral Structure
The hydrous aluminium silicate clay minerals classified as smectites include 
dioctahedral and trioctahedral structural configurations. The dioctahedral 
group of minerals includes montmorillonite, beidellite, and nontronite. 
34 Environmental Soil Properties and Behaviour
These are generally obtained from transformation and weathering processes 
of volcanic material and igneous rocks. The trioctahedral group of minerals 
includes saponite, sauconite, and hectorite obtained or inherited from the 
parent material. Dioctahedral smectites are 2:1 layer lattice minerals where 
the repeating unit layer consists of two sheets of SiO4 tetrahedrons confining 
a central octahedral layer of hydroxyls with Fe, Mg, or Li ions (Figure 2.5). 
These unit layers are sometime called lamellae in the literature. Notations 
such as lamellar and interlamellar space are sometimes used in place of layer 
and interlayer space.
A characteristic feature of montmorillonites is the presence of exchange-
able cations in the interlayer between unit 2:1 layers. These interlayer cations, 
their hydration characteristics, and the resulting swelling performance of 
montmorillonites, which are discussed in detail in Chapter 4, are significant 
factors in the use of smectites in engineered clay barriers and buffers. As 
opposed to the montmorillonites, beidelites are aluminium-rich smectites 
and nontronites are iron-rich smectites. The following list summarizes the 
substitution in the octahedral sheets.
Montmorillonite: Only Si in the tetrahedrons and Al in the octahedrons
Beidellite: Si and Al in the tetrahedrons and Al in the octahedrons
Nontronite: Si and Al in the tetrahedrons and Fe in the octahedrons
Aluminium dioctahedral smectite
(Montmorillonite)
(Al3.15Mg0.85)Si8O20(OH)4X0.85nH2O
Charge from divalent cations
in octahedral sites
Exchangeable cations
Repeating 2:1 unit layer
thickness, 1 nm
Potassium ion\u2013fits into
hexagonal holes of silica sheet
Illite
(OH)4Ky(Si8\u2013y ·Aly)(Al4 ·Fe4 ·Mg4 ·Mg6)O20 Values for y vary from 1 to 1.5
FIguRE 2.5
Basic repeating 2:1 unit layers for dioctahedral smectite (montmorillonite) and repeating 2:1 
layer dioctahedral mica (illite) with their ideal structural formulas. Note that the repeating 
layers are stacked vertically and stretched spatially to form the respective mineral particles. 
Structural formula information for smectite is from Newman and Brown (1987) and from Grim 
(1953) for illite.
35Nature of Soils
The trioctahedral minerals saponite, sauconite, and hectorite are not com-
monly found as clay fractions.
Hectorite: Si in the tetrahedrons and Mg and Li in the octahedrons
Saponite: Si and Al in the tetrahedrons and Mg in the octahedrons
Sauconite: Zn in the octahedral sheet
The term bentonite has sometimes been used in the literature to mean 
a swelling clay composed of a significant amount of montmorillonite. 
Bentonites are classified as dioctahedral smectites obtained as the altera-
tion products of volcanic ash and are composed of primarily montmoril-
lonite with measurable proportions of beidellite depending on the source 
of the bentonite.
2.2.5.2 Surface Functional Groups
Siloxane surfaces bound the opposite planar faces of each unit 2:1 layer 
of montmorillonites. The source of the charges is primarily from isomor-
phous substitution in the octahedral sheet of the unit layers, with mag-
nesium or iron substituting for aluminium in this sheet. Table 2.1 gives 
a summary of the sources of charges for the minerals discussed in this 
section.
2.2.6 Micas, Illites, and Mixed-Layer Clays
The many minerals constituting the mica group are varied and complex 
because of the different types of substitution in their structures. The various 
types of mica minerals are distinguished by the nature of the isomorphic 
substitution of the octahedral sheet and the distribution of silicon ions in the 
tetrahedral sheet. For this book, we will limit our consideration to the clay 
micas; micas have a 2:1 layer structure for the basic unit layer, and cations in 
the interlayer separating the basic unit layers. These interlayer cations can be 
potassium, sodium, and calcium, with potassium being the more common 
interlayer cation. The lower illustration in Figure 2.5 shows the illite mineral.
Illites are platy in structure with variable thicknesses. They are hydrous 
clay micas that do not ordinarily expand from a 1.0 nm basal spacing