C hapter
1
NATURE OF SOIL SOLIDS
Soil is a dispersed polyphase system consisting of solid, liquid and gaseous phases.
Each phase is physically or chemically different and mechanically separable. Because
of presence of more than one phase, the soil is called a heterogeneous system. Solid
phase consists of soil solids or particles of mineral and organic origin. It forms the
skeleton of the soil system. The liquid phase consists of soil water or soil solution
filling the soil voids and the gaseous phase consists of gases or air occupying the void
space not occupied by water.
The inorganic fraction of the soil solid varies from less than 5 per cent in organic
soils (peat) to more than 90 per cent in sandy soils. In a fertile soil, the mineral matter
may be 95 per cent (Fig. 3.5). The mineral particles are derived from rocks and are of
varying shapes and composition. Their size varies from visible range (stones, gravel
and sand) to those visible under electron microscope. The particles of organic origin
consist of plant and animal residues at various stages of decomposition. The chemical
composition of particles varies from soil to soil.
1.1
MINERALOGICAL COMPOSITION
The mineralogical composition of soil solid is influenced by the rocks from which it is
derived, nature of weathering and the soil forming factors namely climate, biosphere,
topography and time. The soil minerals can be classified into primary minerals, secondary
minerals and accessory minerals.
The sand and silt fractions of the soil solids are mainly made up of the primary
minerals. These minerals generally do not occur in the clay fraction. The important
primary minerals of soil solids (sand and silt fractions) are quartz and feldspars. The
quartz occurs in most of the soils and makes up about 50 to 90 per cent of sand and silt
fractions. The quartz crystal consists of silica tetrahedra in which all four oxygens are
shared and there are no cleavage planes. Figure 1.1 shows the silica-tetrahedra and a
sheet structure of silica tetrahedra arranged in a hexagonal network.
Feldspars are alluminosilicates of potassium, sodium and calcium. The structure of
feldspar consists of silica tetrahedra linked together by sharing each oxygen atom
between adjacent tetrahedra resulting into a three dimensional network or tekto-silicate
structure. The potassium feldspars include orchoclase, microcline, adularia, etc. These
are common minerals of granite and gneiss rocks. The plagioclase feldspars series
consists of albite, oligoclase, andesine, etc.
3
4
SOIL SOLIDS
Silica tetrahedron
and
= Oxygens
Hexagonal network of silica tetrahedrons
and
Silicons
Fig. 1.1
The accessory minerals occur in the sand and silt fractions in appreciable amounts
and include pyroxenes and amphiboles (ferro-magnesiun minerals consisting of long
chains of silica tetrahedra), Olivines [(Mg, Fe)2 SiO4], Apatite [Ca10(F,OH,CI)2 (PO4)6],
Tourmaline [Na(Mg, Fe)3 Al6(BO3)3 Si6O18(OH)4], Zircon (ZrSiO4), Magnetite (Fe3O4),
Ilmanite (Fe Tio3), Rutile (TiO2), Kyanite (Al2SiO5) and Staurolite [2 Al2 SiO5 Fe(OH)2]
etc. Sometimes, carbonate (Dolomite Ca CO3 . Mg CO3) and sulphur bearing (Gypsum
Ca2 SO4 2 H2O, pyrite FeS2) minerals also occur in soils.
The secondary minerals generally occur in the clay fraction of the soil. These minerals
are also known as layer silicate minerals, phyllo-silicates, or clay minerals. They are
primarily responsible for most of the physical properties of soils like soil consistency,
plasticity, swelling and physico-chemical properties, viz., cation-exchange, ion fixation
and release. The nature of these minerals, therefore, will be dealt in detail.
Clay Minerals
According to clay mineral concept, clays are composed of extremely small crystalline
particles of one or more members of a small group of minerals known as clay minerals.
The clay minerals are essentially hydrous aluminum silicates with isomorphous
replacements of aluminum with magnesium and iron with alkalies and alkaline earths
present as essential constituents. Clay minerals also sometimes contain primary and
accessory minerals and organic matter. Ralph E. Grim in 1953 suggested1 the
classification shown in Table 1.1 based on the shape of clay minerals and expandable
and non-expandable character of layer silicate minerals.
Table 1.1. Classification of clay minerals1
I. Amorphous
Allophane group
II. Crystalline
A. Two-layer type (sheet structures composed of units of one layer of silica
tetrahedrons and one layer of alumina octahedrons)
1. Equidimensional
Kaolinite group
Kaolinite, nacrite, etc.
2. Elongate
Halloysite group
1Grim,
Ralph E. (1953), Clay mineralogy, McGraw-Hill Book Company, Inc. New York.
5
NATURE OF SOIL SOLIDS
B. Three-layer types (sheet structures composed of layers of silica tetrahedrons and
one central dioctahedral or trioctahedral layer)
1. Expanding lattice
(a) Equidimensional
Montmorillonite group—Montmorillonite, sauconite, etc.
Vermicultite
(b) Elongate
Montmorillonite group—Montmorillonite, saponite, hectorite
2. Nonexpanding lattice
Illite group
C. Regular mixed-layer types (ordered stacking of alternate layers of different types)
Chlorite group
D. Chain-structure types (hornblende-like chains of silica tetrahedrons linked
together by octahedral groups of oxygens and hydroxyls containing Al and Mg
atoms)
Attapulgite
Sepiolite
Palygorskite
Two structural units are involved in the crystal lattice of most clay minerals. One,
the tetrahedra silica unit which consists of silicon tetrahedron in which a silicon atom is
equidistant from four oxygen atoms. The silica tetrahedra groups are arranged in a
hexagonal network, thereby reducing excessive negative charge of the tetrahedral unit
by an oxygen atom being shared by the two adjacent tetrahedra (Fig. 1.1). In this way a
tetrahedral sheet is formed of the composition Si4O6(OH)4. The structure is sometimes
known as perforated layer. The second unit is the octahedra hydroxyl unit, which consists
of two sheets of closely packed oxygen or hydroxyls (6 Nos.), around an Al, Fe or Mg
ion in an octahedral coordination—Al (OH)6 (Fig. 1.2). The adjacent octahedra shares
three hydroxyls or oxygens forming an octahedral sheet. When Al is present the structure
is Gibbsite and with Mg it is Brucite. When the central atom is trivalent such as Al, the
structure is dioctahedral where only two thirds of the octahedral positions are
filled. On the other hand, when the central atom is divalent, the structure is
trioctahedral and all the positions are normally filled.
Fig. 1.2
The tetrahedral silica sheet is attached to the octahedral hydroxyl sheet with apical
oxygen of the tetrahedra sheet replacing one hydroxyl position of the octahedral sheet.
A different combination of these two sheets results in the formation of two layer or
three-layer type minerals as well as interstratified forms.
6
SOIL SOLIDS
1. Two-layer Type Minerals
In two-layer type minerals, the tetrahedral silica sheet is attached to only one side of the
octahedra hydroxyl sheet. This results into l : 1 layer silicate structure. The unsubstituted
dioctahedral end member is kalionite [SiO4Al4O10(OH)8]. This group includes number
of clays such as kaolinite, halloysite, anauxite, dickite, etc.
Kaolinite is a two-layer silicate mineral commonly occurring in soil. Its structural
formula is Si4Al4O10OH8. It occurs as crystal of hexagonal shape with a triclinic
symmetry. The structure is composed of single silica tetrahedra sheet (Fig. 1.2) and a
single alumina octahedra sheet (Fig. 1.2) combined in a unit so that the tips of the
silicon tetrahedron and one of the layers of the octahedral sheet form a common layer.
The tips of the silicon tetrahedron point toward the centre of the unit made by silica and
alumina sheets. The dimensions of the a and b axes of the two units are similar so that
the composite octahedral-tetrahedral layers are formed. The charges within the structural
units are balanced. As the structure of the mineral (Fig. 1.3) involves hydrogen bonding
between the adjacent layers spaced at intervals of 7.2 Å the expansion of kaolinite is
prevented beyond its basal spacing.
Aluminiums
Hydroxyls
Oxygens
c-axis
Silicons
7.2 Å
2. Three-layer Type Minerals
When the silica sheet is attached to each side of the octahedral sheet, it is known as 2 : 1
layer silicate structure. The unsubstituted end-members of this group are pyrophyllite—
[Si8Al4O20(OH)4] and talc-[Si8Mg6O20OH4].
Substitution of (KAl)2 for SiO2 of the tetrahedral sheets of pyrophyllite yields the
dioctahedral muscovite mica structure. Micas are of common occurrence in soils and
mostly originate from the parent rock. The broad group of micas of argillaceous sediments
are often termed as hydromica or illite, the nonexpandable 2 : 1 layer structure.
b-axis
Fig. 1.3
Kaolinite structure
In expandable 2 : 1 layer structure, the substitution of various cations in 2 : 1 layer
provides a lower charge which results in interaction of water and exchangeable cations
7
NATURE OF SOIL SOLIDS
between the 2 : 1 layer. Such minerals hardly vary in composition and are prominent
members of swelling and shrinking plastic clays. Montmorillonite mineral represents a
typical group (Fig. 1.4).
1. Montmorillonite
The minerals of montmorillonite or smectite isomorphous series are freely expandable
layer silicates. The examples are montmorillonite, beidellite, nontronite and saponite.
These minerals occur as extremely small particles ranging in diameter from 0.01 to 1 μ.
The spacing of the layer ranges from 12 to 18 Å and is variable with exchangeable
cation species and degree of hydration. Complete drying leads to an spacing of less
than 10 Å. Full hydration can float the layers apart independent of each other. The
3 + Mg ) · O (OH) nH O.
general structural formula is X0.8(Al0.3Si7.7) Al2.6Fe0.9
0.5
20
4
2
c-axis
Exchangeable cations nH2O
Oxygen
Hydroxyls
And
Si occasionally Al
Fig. 1.4
Al, Fe, Mg
b-axis
Montmorillonite structure (without substitution)
The montmorillonite is composed of units made up of one Al-octahedral sheet
sandwiched between two Si-tetrahedral sheets. The tips of the tetrahedrons point in the
same direction toward the centre of the unit. The tetrahedral and octahedral sheets are
combined so that the tips of the tetrahedrons of each Si-sheet and one of the OH layers
of the octahedral sheet form a common layer. The atoms common to both, the tetrahedral
and the octahedral layers, become oxygen instead of the hydoxyl. The layers are
continuous in the a- and b-directions and are stacked one over the other in the c-direction.
As a result, oxygen layers of each unit are adjacent to oxygen layers of neighbouring
units causing a weak bond and excellent cleavage between them. This type of structure
causes water and other polar molecules and certain organic molecules to enter between
the unit layers causing the unit to expand in c-direction.
8
SOIL SOLIDS
2. Micas and other Minerals
Micas are of common occurrence in the soils often associated with smectite or
montmorillonite clays. The idealized end member of micas are dioctahedral muscovite
and trioctahedral biotite. The structural formula of muscovite is K2Al2Si6Al4O20(OH)4
and the biotite is K2Al2Si6 (Fe2+, Mg)6 O20(OH)4.
The basic structural unit is a layer composed of two silica tetrahedral sheets with a
central octahedral sheet. The tips of the tetrahedrons in each silica sheet point toward
the centre of the unit and are combined with the octahedral sheet in a single layer with
suitable replacement of OH by oxygen. Unit is similar to Montmorillonite except that
some of the silicons are replaced by aluminum and the resultant charge density is balanced
by potassium. This potassium occurring between the unit layers, exerts a kind of
stabilizing effect on the crystal lattice. As a result, the crystals are less expansive than
those of montmorillonite and physical properties like hydration, shrinkage, swelling,
etc. are low.
During weathering, potassium and other inter-layer cations diffuse out of the interlayer spaces resulting in cleavage at the weathering edges of Mica. As the inter-layer
potassium is subjected to depletion through chemical equilibrium with the soil solution,
the exchangeable ions and inter-layer water is gained and leads to another mineral
according to following relation:
Illite
Vermiculite
Montmorillonite
Micas
Vermiculite also occurs extensively in soils and is a product of hydrothermal alteration
of mica. The structure of vermiculite consists of sheets of trioctahedral mica separated
by layers of two water molecule thickness (4.98 Å).
3. Chlorite
The substitution of a complete charged hydroxyl sheet in 2 : 1 layer silicates gives a
2 : 1 : 1 layer or 2 : 2 layered structure of chlorite. Chlorite occurs extensively in soils
and is a 2 : 2 layer silicate. The structure consists of alternate mica like [trioctahedral
(OH)4 (Si Al)8 (Mg.Fe)6 O20] and hydroxide inter-layer or brucite like [(Mg · AI)6
(OH)12] layers. The layers are continuous in a- and b-dimensions and are stacked in the
c-direction with the basal cleavage between the layers.
A clay sample consisting predominantly of 2 : 1 type mineral is frequently mixed
with 2 : 2 type of mineral resulting in mixed layering such as chlorite with vermiculite
or chlorite with mica. Similarly, interstratifications in layer silicates is also recognized.
A clay sample predominantly containing 2 : 1 or 2 : 2 layer silicate generally contains
some of one or more other silicate phases. The chain structural type mineral, attapulgite
also known as palygorskite consists of duochains of silica joined through the basal
oxygens of the chains. The structure is a modification of 2 : 1 structure of montmorillonite with alternate duochains pointing in the opposite directions giving an open channel
besides each duochains forming a continuous inter-layer space. The duochains coordinate
octahedral cations. The ideal formula of the end member is Si8Mg5 (OH2)4 O20 (OH)24H20.
4. Oxide Minerals
In a highly leached and oxidised soils, as in the tropical soils, silica is removed while
Al, Fe, Ti, tend to accumulate. In such soils, the soil colloid consists of hydrous oxides
NATURE OF SOIL SOLIDS
9
of Al, Fe and Ti. These clays vary from amorphous state to crystalline. Allophanes and
hydrous oxides of Al, Fe, and Ti are called the free oxide minerals of soils.
1. Allophane—(2SiO2 A12O3 · H2O)
Allophane is a general term for amorphous aluminosilicate gels. Allophane minerals
are amorphous to X-ray differaction. They have silicon in tetrahedral coordination and
metallic ion in octahedral coordination with occasional other units such as phosphate
tetrahedrons. The composition of allophane vary widely. Allophanes have been reported
in lateritic soils of India. Allophane gives a stable porous structure to soil. Such soils
are highly permeable. Due to intensive leaching most of the primary minerals and
essential nutrient elements are removed causing allophanic soils to be generally infertile.
High specific surface and high aluminum and iron activity of allophane causes them to
have high phosphate fixation capacity.
2. Hydrous Oxides
Another clay mineral occurring primarily in intensely weathered tropical soils is the
crystalline mineral gibbsite—Al (OH)3 or (AI2O3 · 3H2O), a free hydrous oxide of
aluminium. Gibbsite consists of paired sheets of hydroxyls held together dioctahedrally
by aluminum atom. A series of paired sheets are held together by hydrogen bonding.
Laterites are also rich in iron oxides like haematite (Fe2O3) giving pink to bright red
colour to soil and goethite (Fe2 O3 · H2O) giving rise to brown and dark brown soils.
Iron oxides tend to occur as amorphous coatings gradually transforming to crystalline
forms. Haematite may occur in silt and sand fractions in a coarsely crystalline forms.
Finely divided yellowish hydrous goethite is also known as limonite. Sometimes
magnetite (Fe3 O4) that is magnetic iron oxide occurs in soils in sand fractions. The
titanium oxide minerals like rutile are also of common occurrence in tropical soils.
The distribution of these minerals depends upon the nature of the parent rock and the
soil forming processes like soluviation (dissolution in water of an element from mineral
followed by leaching), illuviation and eluviation. The primary minerals, like quartz,
feldspars, carbonates and gypsum occur in less weathered zonal soils as well as intrazonal soils. The secondary minerals of 2 : 1, 2 : 2 structure occur in moderately weathered
soils and 1 : 1 and 2 : 1 intergrade occur in highly weathered soils. The oxide minerals,
allophane, gibbsite, haematite, goethite are formed in highly weathered laterites.
1.2
SHAPE
In particle size analysis, clay particles are assumed to be of spherical shape. The
experimental evidence, however, indicates that they are distinctly non-spherical and
are disc shaped. The non-spherical or disc shaped character of the clay particle has
been established through at least five major techniques, viz., ultramicroscopic studies,
the double refraction of clay particles, nature of settling of clays during deposition,
nature of crystal lattice of clays and electron microscopic observations.
When a clay soil illuminated by a beam of light is observed under ultra-microscope,
it exhibits scintillating or twinkling effect. This effect is due to sudden appearance and
disappearance of the particle from the visible or non-visible position. Such an effect is
associated with non-spherical particles. The disc shaped character is further confirmed
when the illuminated suspension is rotated and observed through a microscope with
10
SOIL SOLIDS
crossed nicholes. The field being dark when the suspension is viewed at rest and bright
on rotation. This phenomena is known as streaming double refraction. The cubical or
spherical shaped particles do not exhibit such effects.
It is a common observation that when a clay suspension settles the thin deposited
clay layers curl up on drying. The plate like arrangement of these crusted sheets can
form only with disc or rod shaped particles. The crystal lattice structure of clay consisting
of alternating silica and alumina sheets also suggests a disc shaped type of particle. The
electron photomicrographs clearly shown (Fig. 1.5) well defined plate-like particles2.
The shape of the particle also depends upon the nature of the mineral present. For
example, allophane is amorphous and electron micrograph shows it as fluffy aggregate
with a rounded nodular appearance. Halloysite is now known to appear in electron
micrograph as elongate tubular particle with outer diameter ranging from 0.04 to 0.19
μ and the inner diameter from 0.02 to 0.1 μ. The kaolinite on the other hand is seen as
a crystalline mineral showing well formed six sided flakes with a prominent elongation
in one direction. The electron micrograph of montmorillonite shows broad undulating
mosaic sheets which on dispersion breaks into irregular fluffy mass of extremely small
particles. These particles can barely be discerned and are too small to reveal any
characteristic outline.
Fig. 1.5
Electron micrograph of Koolinite2
Weathering and transportation of rock fragments smoothens the sharp edges of the
sand and silt particles. Silt and sand particles are generally spherical and cubical in
shape. The shape of these particles can be expressed in terms of sphericity and roundness.
Sphericity is expressed by the following relation:
S=
3
Volume of particles
Volume of smallest sphere into which the particles would fit
For a perfect sphere S = l
2
Kerr, P.F., Hamilton, P.K., Davis, D.W. Rochow, T.G., Rowe, F.G. and Fuller, M.L. (1950), “Electron
Micrographs of Reference Clay Minerals”, Prelim. Rept. 6, Amer, Petro. Inst, Project 49, Columbia
Univ., New York.
11
NATURE OF SOIL SOLIDS
Roundness which expresses the angularity of the particle is given by the following
relationship:
Average radius of the corners and edges of the particles
Radius of the maximum circle that can be inscribed in
the particle
Several terms are commonly used for describing the particle shape. These are acicular
(needle shaped), angular (sharp edged), platy (plate like) and granular (nearly
equidimensional).
Quantitatively, particle shape can be expressed in terms of volume, mass and surface
area:
...(1.1)
Volume, V = fρd 3
3
...(1.2)
Mass, m = fρd
...(1.3)
Surface area, s = fsd 2
3
where V = volume of particle, cm ; m = mass of particle, g; s = surface area of particle,
cm2; fv = volume shape factor; fρ = density of particle, gcm–3; and fs = surface shape
factor.
For a sphere, the value of fv and fs is given by
...(1.4)
fv = π/6
...(1.5)
and
fs = π
Similarly, for a cube of side length L (here d is replaced by L in Eqns. (1.1, 1.2 and
1.3)
...(1.6)
fv = l
...(1.7)
and
fs = 6
Since particles are non-uniform a mean value of ‘equivalent diameter’ should be
defined. One way of expressing average diameter is by taking the arithmetic mean, da, of
the particles. For example, if d1 and d2 are the diameter of the particles in a mixture then
R=
da =
1
2
(d1 + d2)
...(1.8)
Another way is by finding out geometric mean diameter, dg given by,
d g = d1d 2
...(1.9)
However, one must be careful while finding out these mean values. For example, if
d1 and d2 are widely different, the resulting d will be meaningless.
More widely applicable mean diameters are the volume mean diameter, dv, the surface
mean diameter and the root mean square diameter, expressed as follows:
Volume mean diameter d v =
∑ nd 4
∑ nd 3
...(1.10)
where n = number of particles of diameter, d.
If M be the total mass of all such uniform particles of diameter d and mass m, the
number of particles, n, will be given by
M
...(1.11)
n=
m
For a constant particle density, dv also represents the weight mean diameter.
12
SOIL SOLIDS
Surface mean diameter, d s can be expressed as:
∑ nd 3
∑ nd 2
The root mean square diameter, d R is expressed as:
ds =
dR =
1.3
∑ nd 2
∑n
...(1.12)
...(1.13)
SPECIFIC SURFACE
Physical behaviour of soil, like water retention or cation exchange, depends to a great
extent on the surface area of the soil particles. Soils differ markedly in texture, type of
clay mineral and the amount of organic matter. Larger particles of sand and silt have
much less surface area as compared to clays and other colloids.
Surface area is generally expressed as specific surface. The term specific surface
refers to area per unit weight of soil or clay and is usually expressed in square meters
per gram (m2/g). It is also expressed as area per unit volume. For spherical particles, the
specific surface is thus given by
Surface of a sphere
Volume of sphere
Using Eqns. (1.1) and (1.3) with appropriate values of fv and fs from Eqns. (1.4) and
(1.5), the specific surface of spherical particle can be written as:
πd 2
6
=
Ss =
...(1.14)
π 3 d
d
6
where, Ss = specific surface of the spherical particle, cm2 cm–3 and d = diameter of the
particle, cm.
If diameter, d, is replaced by radius, r, of the particle Eqn. (1.14) can be written as
3
...(1.15)
ss =
r
This shows that specific surface is inversely proportional to the particle diameter or
the radius. Hence, sand and silt fractions having larger diameters have much less specific
surface as compared to clay.
The specific surface for sand particles of 2 mm diameter will be 6/0.2 = 3 × 101 cm2
cm–3, which for a clay particle of 0.002 mm will be 6/0.0002 = 3 × 104 cm2 cm–3.
Similarly, a cube of one centimeter sides will have specific surface of 6 cm2/cm–3 and if
the side is reduced to 1 μ, it will have specific surface of 60 × 103 cm2 cm–3. Similar to
Eqn. (1.14), the specific surface and its relation to the length of the side of the cube is
given by
Ss =
6L2 6
=
...(1.16)
L
L3
where Sc = specific surface of a cube, cm2 cm–3 and L = side length of the cube, cm.
As discussed earlier, because clay particles are not spheres, but plate or disc shaped
their specific surface will be much larger. Considering square platelets with side length
L and thickness L’, the specific surface area of a plate, Sp, can be given by
Sc =