This document is a report submitted by Vimlesh Kumar Verma for the partial fulfillment of the requirements for a Master's degree in Geotechnical Engineering. It discusses the structure of clay minerals. The basic structural units of clay minerals are the tetrahedron unit consisting of silicon or aluminum surrounded by oxygen ions, and the octahedron unit consisting of aluminum, iron, or magnesium atoms surrounded by hydroxyl groups. Clay minerals are classified into groups based on their layered structure, with the main groups being the kaolin group, smectite group, illite group, chlorite group, and others. Properties vary between the mineral groups.

1. A
Report
On
STRUCTURE OF CLAY MINERALS
Submitted in the partial fulfillment of the
requirement for the award of the degree
Of
MASTER OF TECHNOLOGY
IN
“GEOTECHNICAL ENGINEERING”
BY
VIMLESH KUMAR VERMA
(Roll No. 2017GE10)
Department of Civil Engineering
MNNIT ALLAHABAD 211004 (UP)

2. CONTENTS
1. Introduction……………….……………………………………………………………..iii
1.1 Clay and Clay Mineralogy
1.2 Soil Structure
2. Basic Structural unit of Clay Mineral…….……………………………………………iv-v
2.1 Tetrahedron unit
2.2 Octahedron unit
3. Types of prevalent bonds …………………………………………….………………….vi
3.1 Primary valance bonding
3.2 Hydrogen bonding
3.3 Van der waals bonding
4. Clay minerals classification………………………………………….………………vii-xiii
4.1 Kaolin group
4.1.1 Kaolinite mineral
4.1.2 Halloysite mineral
4.2 Smectite mineral
4.2.1 Montmorillonite mineral
4.3 Illite mineral
4.4 Chlorite mineral
4.5 Vermiculite mineral
4.6 Allophane
4.7 Palygorskite and sepiolite
5. Properties of clay minerals…………………….……………………………………..xiv-xv
5.1 Kaolinite mineral
5.2 Montmorillonite mineral
5.3 Illite mineral
6. Conclusion………..……………………………………………………………………..xvi
7. References………………………………………………………………………………xvii

3. 1. INTRODUCTION
1.1 Clay and clay mineralogy- The term "clay" refers to a naturally occurring material
composed primarily of fine-grained minerals (grain size <.002mm) which is generally
plastic at appropriate water contents and will harden when dried or fired. Clay
usually contains phyllosilicates, it may contain other materials that impart plasticity
and harden when dried or fired. Associated phases in clay may include materials that
not impart plasticity and organic matter.
The term “clay mineral” refers to phyllosilicate minerals and to minerals which
impart plasticity to clay and which harden upon drying or firing. Clay minerals are
layer silicates that are formed usually as products of chemical weathering of other
silicate minerals at the earth's surface. They are found most often in shales, the most
common type of sedimentary rock. In cool, dry, or temperate climates, clay minerals
are fairly stable and are an important component of soil. Clay minerals act as
"chemical sponges" which hold water and dissolved plant nutrients weathered from
other minerals. This results from the presence of unbalanced electrical charges on the
surface of clay grains, in which some surfaces are positively charged (and thus attract
negatively charged ions), while other surfaces are negatively charged (attract
positively charged ions). Clay minerals also have the ability to attract water
molecules. Because this attraction is a surface phenomenon, it is called adsorption
(which is different from absorption because the ions and water are not attracted deep
inside the clay grains). Clay minerals resemble the micas in chemical composition,
except they are very fine grained, usually under microscope. Like the micas, clay
minerals are shaped like flakes with irregular edges and one smooth side.
1.2 Soil structure-It means the geometrical arrangement of particle in a soil mass,
relative to each other and the forces acting between them to hold them together in
their positions. In coarse grained soil structure is governed by gravity forces where as
in clayey soil by surface forces are predominant.
It has been observed that, Clay soil is made up of many crystal sheets which have a
repeating atomic structure.
Atomic structure of clay mineral are built of fundamental sheets
a. Tetrahedron or silica sheet
b. Octahedron or alumina sheet
iii

4. 2. BASIC STRUCTURAL UNIT OF CLAY MINERAL
2.1 Tetrahedron unit:
The tetrahedron is one of the solid geometric forms used to represent the
arrangement of atoms in clay mineral crystal structures. It is formed by connecting
the centers of the four oxygen anions surrounding a central cation.
In the clay minerals the predominant central cation of the tetrahedron is silicon. A
limited number of tetrahedral are occupied by aluminium and occasionally ferric iron
or other elements. A silicon, or aluminium, ion is surrounded by four oxygen ions to
form a tetrahedron in the Figure (2a).
The isolated tetrahedron has a net negative charge of -4 (Si with 4+ charges and four
O with 2- charges). The tetrahedral rest on triangular face and the four triangular
faces of the tetrahedron are formed by joining the centers of the anions.
In clay minerals, the three oxygens at the base of the tetrahedron are shared with
adjacent tetrahedral and only the apical oxygen retains a charge of -1.
2.2 Octahedron unit:
The second structural unit is the octahedral sheet, in which the hydroxyl atoms (OH)
in the corners and cations in the center. The cations are usually aluminium (Al), iron
(Fe), and magnesium (Mg) atoms.
The octahedral sheet is comprised of closely packed oxygens and hydroxyls in which
Al, Fe, and Mg atoms are arranged in octahedral coordination. The net charge on an
isolated Al-OH octahedron is -3 (Al 3+ and six OH with 1- charge). In the octahedral
sheet the charge is reduced through the sharing of anions by adjacent octahedral. A
single octahedron can be recognized by following the bonds from the small blue balls
(Al atoms). Three of them are directed upwards and are each connected a hydroxyl
group.
The remaining three bonds are directed downwards to other hydroxyl groups. When
aluminium with positive valence of three (Al +3) is present in the octahedral sheet,
only two-thirds of the possible positions are filled in order to balance the charges.
When only two octahedral sites filled with trivalent cations is a dioctahedral sheet.
When magnesium with a positive charge of two (Mg+2) is present, all three positions
are filled by divalent cations is a trioctahedral sheet.
iv

5. Fig 2a
The octahedral sheet is formed by sharing all hydroxyl groups at the corners of an
octahedron with neighboring octahedral. When you view the octahedral sheet from the
side they contain four aluminium atoms, six lower plane hydroxyls, and six upper plane
hydroxyls. The formula for this unit is: Al4 (OH) 12 and the net charge is ZERO.
Tetrahedron unit Fig2b Octahedron unit
v

6. 3. TYPES OF PREVALENT BOND
3.1 Primary valence bond:
A. Ionic bond:
Ionic bonding is the complete transfer of valence electron(s) between atoms. It is a
type of chemical bond that generates two oppositely charged ions.
In ionic bonds, the metal loses electrons to become a positively charged cation,
whereas the nonmetal accepts those electrons to become a negatively charged anion.
Ionic bonds require an electron donor, often a metal, and an electron acceptor, a
nonmetal. Ex. NaCl
B. Covalent bond:
Covalent bonding is the sharing of electrons between atoms. This type of bonding
occurs between two atoms of the same element or of elements close to each other in
the periodic table.
This bonding occurs primarily between nonmetals; however, it can also be observed
between nonmetals and metals. Ex. O2.
C. Metallic bond:
Metallic bonding is the force of attraction between valence electrons and the metal
atoms. It is the sharing of many detached electrons between many positive ions,
where the electrons act as a "glue" giving the substance a definite structure. Ex. Zn2
3.2 Hydrogen bond:
Hydrogen bonding, interaction involving a hydrogen atom located between a pair of
other atoms having a high affinity for electrons; such a bond is weaker than an ionic
bond or covalent bond but stronger than van der Waals forces.
Hydrogen bonds can exist between atoms in different molecules or in parts of the same
molecule.
Bond b/w hydrogen cation and anions of two atoms of another element
Ex: Bond b/w hydrogen atoms and oxygen atoms in a water molecule.
3.3 Secondary valence bond: Van der waals forces are distance dependent interactions
between atoms or molecules. Unlike ionic or covalent bonds, these attractions are not
a result of any chemical electronic bond, and they are comparatively weak and more
susceptible to being perturbed. Van der Waals forces quickly vanish at longer
distances between interacting molecules.
vi

7. 4. CLAY MINERAL CLASSIFICATION
Fig. 4a Source (Bailey, 1980b; Rieder et al., 1998)
4.1 Kaolin Group:
The kaolin group minerals comprise kaolinite, nacrite, dickite and halloysite, and are
among the most common clay minerals in nature. They have a 1:1 layered structure,
that is, each layer consists of one tetrahedral silicate sheet and one octahedral sheet,
with two-thirds of the octahedral sites occupied by aluminum.
Kaolinite, Nacrite and Dickite all have the ideal chemical composition:
Al2Si205(OH)4, they differ from one another only in the manner in which the 1:1
layers are stacked.
Halloysite, in its fully hydrated form, has the ideal chemical formula
Al2Si205(OH)4.2H20 and the theoretical chemical composition is SiO2, 46.54%;
Al2O3, 39.50%; and H2O, 13.96%. Kaolinite differs from the other three members of
the group by including molecular water in the interlayer. Within the kaolin group
minerals, kaolinite is the most abundant and has received most attention in terms of
its structure, properties and industrial applications.
vii

8. 4.1.1 Kaolinite mineral: (Al4Si4O10OH8)
 Basic structural unit consists of alumina sheet combined with silica sheet.
 Structural units join together by hydrogen bond that develops b/w oxygen
of silica sheet and hydroxyls of alumina sheet.
 Thickness 7 A0
.
 They are formed by typically 70-100 elementary layers with t/l=1/10.
 There is no interlayer swelling.
 Electrically neutral.
 Ex: China clay
Fig. 4b
viii

9. 4.1.2 Halloysite mineral:
 Halloysite, in its fully hydrated form, has the ideal chemical formula
Al2Si205(OH)4.2H20 and the theoretical chemical composition is SiO2, 46.54%;
Al2O3, 39.50%; and H2O, 13.96%.
 A single layer of water between unit layers.
 The basal spacing is 10.1 Å for hydrated halloysite and 7.2 Å for dehydrated
halloysite.
 If the temperature is over 50 °C or the relative humidity is lower than 50%, the
hydrated halloysite will lose its interlayer water.
 There is no interlayer swelling.
 Tubular shape while it is hydrated.
Fig.4c Source Grim 1963
4.2 Smectite mineral:
The major smectite minerals are Na-montmorillonite, Ca-montmorillonite, saponite
(Mg-montmorillonite), nontronite (Fe-montmorillonite), hectorite (Li-montmorillonite),
and beidellite (Al-montmorillonite). Smectite minerals are composed of two silica
tetrahedral sheet with central octahedral sheet and are designated as a 2:1 layer mineral.
Water molecules and cations occupy the space between the 2:1 layers. The theoretical
formula is (OH)4Si8Al4O20.NH2O (interlayer) and the theoretical composition without
the interlayer material is SiO2 , 66.7%; Al2O3, 28.3%; and H2O, 5%. However, in
smectite, there is considerable substitution in the octahedral sheet and some in the
tetrahedral sheet. In the tetrahedral sheet, there is substitution of aluminum for silicon up
to 15% and in the octahedral sheet, magnesium and iron for aluminum (Grim, 1953). If
the octahedral positions are mainly filled by Al, the smectite mineral is beidellite; if
filled by Mg, the mineral is saponite; and if by Fe, the mineral is Nontronite.
4.2.1 Montmorillonite mineral: (OH)12Al4 Si8016 • n H20
Montmorillonite is the main constituent of bentonite, derived by weathering of volcanic
ash. Montmorillonite can expand by several times its original volume when it comes in
contact with water. This makes it useful as a drilling mud (to keep drill holes open), and
to plug leaks in soil, rocks, and dams.
It has highest swelling and shrinkage property.
ix

10. Fig4d. Montmorillonite mineral
4.3 Illite mineral:
 Illite is clay mineral mica, which was named by Grim (1937). The structure is a 2:1
layer in which the interlayer cation is potassium. The size, charge, and coordination
number of K is such that it fits easily in hexagonal ring of oxygen of the adjacent
silica tetrahedral sheets. This gives the structure a strong interlocking ionic bond
which holds the individual layers together and prevents water molecules from
occupying the interlayer position as it does in the smectite.
 Simply it might say that illite is a potassium smectite. Illite differs from well-
crystallized muscovite in that there is less substitution of Al3+ for Si4+ in the
tetrahedral sheet. In muscovite, one-fourth of Si4+ is replaced by Al3+ whereas in
illite only one-sixth is replaced. Also, in the octahedral sheet, there may be some
replacements of Al3+ by Mg2+ and Fe2+. The basal spacing of illite is 10Å.The
largest charge deficiency is in the tetrahedral sheet rather than in the octahedral sheet,
which is opposite from smectite. For this reason and because of the fit, potassium
bonds the layer in a fixed position so that water and other polar compound cannot
readily enter the interlayer position and also the K ion is not readily exchangeable.
 Illite which are the dominant clay minerals in argillaceous rocks, form by the
weathering of silicates (primarily feldspar), through the alteration of other clay
minerals, and during the degradation of muscovite. Formation of illite is generally
favored by alkaline conditions and by high concentrations of Al and K.
x
n.H20+cations
10Å
Å

11. Fig4e Illite mineral
4.4 Chlorite mineral:
The chlorite group members contain a 2:1 layer with variable x and an interlayer
hydroxide sheet. In some references, they are referred to as 2:1:1 mineral. The
octahedral sheets may both be dioctahedral (di/di) or trioctahedral (tri/tri), or mixed
(di/tri, or tri/di). The interlayer hydroxide sheet may have a positive charge.
Fig.4f
xi
10Å

12. 4.5 Vermiculite mineral:
 Vermiculite is a member of clay minerals, produced by the decompositions of
micas and occurs as quite large crystals of mica-like appearance. It has a layer
structure, and the interlayer contains water molecules and exchangeable cations,
mainly Mg2+ ions.
 The structure of Mg-saturated vermiculite resembles talc in that it contains a
central octahedrally-coordinated layer of Mg ions that lie between two inwardly
pointing sheets of linked tetrahedral. These silicate layers are normally separated
by two sheets of interlayer water molecules arranged in a distorted hexagonal
fashion.
 The chemical composition of vermiculite in weight percentage was: SiO2
(44.62); Al2O3 (9.18) Fe2O3 (5.46); CaO (0.78); MgO (20.44); Na2O (0.11);
K2O (0.48) with loss of weight after heating at 1273 K or at 1000°C (18.93).
 Based on the data, the typical structural formula of the vermiculite is
(MgFe,Al)3(Al,Si)4O10(OH)2·4H2O.
 Vermiculites are usually formed in sediments by the alteration of micaceous
minerals (biotite and chlorite to trioctahedral vermiculite; muscovite to
dioctahedral vermiculite. Present, marine vermiculites are probably derived from
volcanic material, chlorite, and hornblende.
Fig.4g
xii

13. 4.6 Allophane mineral:
 Allophane is a series name used to describe clay-sized, short-range ordered
aluminosilicates associated with the weathering of volcanic ashes and glasses.
Allophane commonly occurs as very small rings or spheres having diameters
of approximately 35 - 50 Å. This morphology is characteristic of allophane,
and can be used in its identification.
 Allophanes have a composition of approximately Al2Si2O5·nH2O. Some
degree of variability in the Si: Al ratios is present: Si: Al ratios varying from
about 1:1 to 2:1. Because of the exceedingly small particle size of allophane
and the intimate contact between allophane and other clays (such as smectites,
imogolite, or non-crystalline Fe and Al hydroxides and silica) in the soil, it
has proven very difficult to accurately determine its composition.
Consequently, there is always some potential error associated with the
compositional ratios has reported.
4.7 Palygorskite and sepiolite:
 Palygorskite and sepiolite have similar fibrous or lath-like morphologies, but
palygorskite exhibits more structural diversity and, although both minerals are
Mg silicates, palygorskite has less Mg and more Al than sepiolite.
 Structurally, (palygorskite and sepiolite) consist of blocks and channels
"ribbon-like" sheets extending in the c-axis direction. Each structural unit is
built up of two tetrahedral silicate layers and a central trioctahedral layer. In
the octahedral layer Mg+2 ions occupy two different structural positions: (1)
on the borders of the structural blocks, coordinated to water molecules; and
(2) in the interior of the blocks, linked to hydroxyl groups.
Fig.4h
xiii

14. 5. PROPERTIES OF CLAY MINERAL
5.1 Kaolinite mineral:
 Kaolinite, the main constituent of kaolin, is formed by rock weathering.
 It is white, greyish white, or slightly coloured.
 It is made up of tiny, thin, pseudo hexagonal, flexible sheets of triclinic crystal
with a diameter of 0.2–12 µm. It has a density of 2.1–2.6 g/cm3.
 The cation exchange capacity of kaolinite is considerably less than that of
montmorillonite, in the order of 2– 10 meq/100 g, depending on the particle size,
but the rate of the exchange reaction is rapid, almost instantaneous (Grim, 1953).
 Upon heating, kaolinite starts to lose water at approximately 400 °C, and the
dehydration approaches completeness at approximately 525 °C (Grim, 1953).
The dehydration depends on the particle size and crystallinity.
5.2 Montmorillonite mineral:
 Smectite feels greasy and soap-like to the touch. Freshly exposed bentonite is
white to pale green or blue and, with exposure, darkens in time to yellow, red, or
brown.
 The special properties of smectite are an ability to form thixotropic gels with
water, an ability to absorb large quantities of water with an accompanying
increase in volume of as much as 12–15 times its dry bulk, and a high cation
exchange capacity. Substitutions of silicon by cations produce an excess of
negative charges in the lattice, which is balanced by cations (Na+, K+, Mg2+,
Ca2+) in the interlayer space. These cations are exchangeable due to their loose
binding and, together with broken bonds (approximately 20% of exchange
capacity); give montmorillonite a rather high (about 100 meq/100 g) cation
exchange capacity, which is little affected by particle size.
 There is extensive isomorphous substitution for silicon and aluminum by other
cations, which results in charge deficiencies of clay particles.
 The interlayer bonding is by van der Waals forces and by cations which balance
charge deficiencies (weak bonding).
 There exists interlayer swelling, which is very important to engineering practice
(expansive clay).
 Width: 1 or 2 m, Thickness: 10 Å~1/100 width
xiv

15. 5.3 Illite mineral:
 Illite, together with chlorite, is the main component of common clay and shale. It is
also an important impurity in limestone, which can affect the properties and thus the
value of the stone for construction and other purposes. Despite the widespread
occurrence of illite in nature, large deposits of high purity are quite rare.
 The basic structure is very similar to the mica, so it is sometimes referred to as
hydrous mica. Illite is the chief constituent in many shales.
 Some of the Si4+
in the tetrahedral sheet is replaced by the Al3+
, and some of the Al3+
in the octahedral sheet is substituted by the Mg2+
or Fe3+
. Those are the origins of
charge deficiencies.
 The charge deficiency is balanced by the potassium ion between layers. Note that the
potassium atom can exactly fit into the hexagonal hole in the tetrahedral sheet and
form a strong interlayer bonding.
 The basal spacing is fixed at 10 Å in the presence of polar liquids (no interlayer
swelling).
 Width: 0.1~ several m, Thickness: ~ 30 Å
xv

16. 6. CONCLUSION
The term “clay mineral” refers to phyllosilicate minerals and to minerals which impart
plasticity to clay and which harden upon drying or firing. Clay minerals are layer
silicates that are formed usually as products of chemical weathering of other silicate
minerals at the earth's surface. They are found most often in shales, the most common
type of sedimentary rock. Clay soil is made up of many crystal sheets which have a
repeating atomic structure. Atomic structure of clay mineral is built of fundamental
sheets Tetrahedron or silica sheet and Octahedron or alumina sheet. These sheets are
attached with different type of bonding such as ionic bonding, covalent bonding,
metallic bonding, hydrogen bonding and van der Waals bonding. And these bonding
leads to different type mineral classifications.
There are different types of clay minerals such as Kaolin group, Smectite mineral, Illite,
Chlorite, Vermiculite, etc. Kaolin mineral are also known as 1:1 type mineral, consist of
silica sheet and gibbsite sheet. They have strong bonding in between them so they have
less swelling property. Kaolinite, Halloysite, Dickite, Nacrite are some minerals of
kaolin group. Montmorillonite is the main constituent of bentonite, derived by
weathering of volcanic ash. Montmorillonite can expand by several times its original
volume when it comes in contact with water. Illite is clay mineral mica, which was
named by Grim (1937). The structure is a 2:1 layer in which the interlayer cation is
potassium. The size, charge, and coordination number of K is such that it fits easily in
hexagonal ring of oxygen of the adjacent silica tetrahedral sheets. This gives the
structure a strong interlocking ionic bond which holds the individual layers together and
prevents water molecules from occupying the interlayer position as it does in the
smectite. There are some other minerals like chlorite, vermiculite, palygorskite and
sepiolite which have different properties.
xvi

17. REFERENCES
1. Ralph E.Grim, Clay Mineralogy, 1st
Ed. McGraw-Hill Book Co.: New York, 1953.
2. CLAY AND CLAY MINERALOGY by Dr. Thair Al-Ani and Dr. Olli Sarapää
3. CLAY MINERALS by CD. Barton United States Department of Agriculture Forest
Service, Aiken, South Carolina, U.S.A. A.D. Karathanasis University of Kentucky,
Lexington, Kentucky, U.S.A.
4. SYNTHESIS OF ANIONIC CLAY MINERALS (MIXED METAL HYDROXIDES,
HYDROTALCITE) Walter T. REICHLE Specialty Chemicals Division, Union
Carbide Corporation, USA
5. Structure and mineralogy of clay minerals by M.F.Brigatti, E.Galan and B.K.G.
Theng.
6. Crystal structure of clay mineral and their x-ray identification by G.W.Brindlay and
G.Brown
7. NPTEL- Advanced Geotechnical Engineering
8. Basic and applied soil mechanics by Gopal Ranjan, ASR RAO
xvii