Clay Mineralogy

1
Topic 3: Clay Mineralogy
Hassan Z. Harraz
hharraz2006@yahoo.com
2013- 2014

OUTLINE OF TOPIC 3:
 ORIGIN OF CLAY MINERALS
 CLAY MINERALS
 ATOMIC STRUCTURE
 Basic Structural Units
 TYPES OF CLAY MINERALS:
1) Silicate Clays (crystalline):
a) Kaolinite
b) Halloysite
c) Smectite
d) Illite
e) Vermiculite
f) Chlorite
g) Attapulgite (Chain Structure Clay Minerals)
h) Mixed Layer Clays
2) Sesquioxide/oxidic clays
3) Amorphous clays (non-crystalline)
 “Activity” of silicate clays
 Generalized Chemical Weathering
 Chemical Weathering Products
 Uses of Clay
 Clay Fabric
 IDENTIFIED CLAY MINERALS
 SPECIAL TERMS
21 November 2015 Prof. Dr. H.Z. Harraz Presentation Clay Mnerals 2

Elements of Earth
3
12500 km dia
8-35 km crust
% by weight in crust
O = 49.2
Si = 25.7
Al = 7.5
Fe = 4.7
Ca = 3.4
Na = 2.6
K = 2.4
Mg = 1.9
other = 2.6
82.4%

Soil Formation
4
Parent Rock
Residual soil Transported soil
~ in situ weathering (by physical &
chemical agents) of parent rock
~ weathered and transported
far away
~ formed by one of
these three different
processes
1) Igneous: formed by
cooling of molten magma
(lava) e.g., Granite, Basalt
2) Sedimentary: formed
by gradual deposition, and in
layers e.g., Sandstone,
limestone, shale
3) Metamorphic: formed
by alteration of igneous &
sedimentary rocks by
pressure/temperature e.g.,
schist, marble
Transported by: Special name:
Wind “Aeolian”
Sea (salt water) “Marine”
Lake (fresh water) “Lacustrine”
River “Alluvial”
Ice “Glacial”

Origin of Clay Minerals
 “The contact of rocks and water produces clays, either at or near the surface of the
earth” (from Velde, 1995).
Rock +Water  Clay
 For example,
 The CO2 gas can dissolve in water and form carbonic acid, which will become
hydrogen ions H+ and bicarbonate ions, and make water slightly acidic.
CO2 + H2O  H2CO3  H+ + HCO3
-
 The acidic water will react with the rock surfaces and tend to dissolve the K ion
and silica from the feldspar. Finally, the feldspar is transformed into kaolinite.
Feldspar + hydrogen ions + water  clay (kaolinite) + cations, dissolved + silica
2KAlSi3O8 + 2H+ + H2O  Al2Si2O5(OH)4 + 2K+ + 4SiO2
Note that:
 The hydrogen ion displaces the cations.
The alternation of feldspar into kaolinite is very common in the decomposed
granite.
The clay minerals are common in the filling materials of joints and faults (fault
gouge, seam) in the rock mass.
21 November 2015 Prof. Dr. H.Z. Harraz Presentation Clay Minerals 5

21 November
2015
Prof. Dr. H.Z. Harraz Presentation Clay Minerals 6

CLAY MINERALS
 Clay minerals exhibit colloidal behaviour. That is, their surface forces have greater
influence than the negligible gravitational forces.
 Clay is a particle size
 i.e., Micelle: meaning particle of silicate clay
 Clay particles are smaller than 2 microns. Their shapes can be studied by an electron
microscope.
 Predominant make-up is Secondary minerals
 Clay minerals are Phyllosilicate minerals
 Composed of tetrahedral and octahedral “sandwiches”
 Tetrahedron: central cation (Si+4, Al+3) surrounded by 4 oxygens
 Octahedron: central cation (Al+3,Fe+2, Mg+2) surrounded by 6 oxygens (or
hydroxyls)
 Sheets combine to form layers
 Layers are separated by interlayer space
 Water, adsorbed cations
 Clay particles are like plates or needles. They are negatively charged.
 Clays are plastic; Silts, sands and gravels are non-plastic.
 Clays exhibit high dry strength and slow dilatancy.

A Clay Particle
9
Plate-like or Flaky Shape

Basic Structural Units
All have layers of Si tetrahedra
and
layers of Al, Fe, Mg octahedra,
similar to gibbsite or brucite
SEM view of clay
Connected tetrahedra,
sharing oxygensTetrahedron and Tetrahedral sheets
Connected octahedra,
sharing oxygens or
hydroxyls
Octahedron and Octahedral Sheets
Silicon tetrahedron
silicon
oxygen
hydroxyl or oxygen
aluminium or
magnesium
Aluminium Octahedron
Clay minerals are made of
two distinct structural units
All clay mineral are made of
different combinations of the above
two sheets: tetrahedral sheet and
octahedral sheet.

Basic Unit-Silica Tetrahedra
1 Si
4 O
(Si2O10)-4
Replace four
Oxygen with
hydroxyls or
combine with
positive union
(Holtz and Kovacs, 1981)
Plural: Tetrahedra
 Several tetrahedrons joined together form
a tetrahedral sheet.
 Here is a tetrahedral sheet, formed by
connecting several tetrahedons.
 Note the hexagonal holes in the sheets.
Tetrahedral Sheet
Tetrahedron
hexagonal
hole

13
Basic Unit-Octahedral Sheet
Gibbsite sheet: Al3+
Al2(OH)6, 2/3 cationic spaces are filled
One OH is surrounded by 2 Al:
Dioctahedral sheet
Brucite sheet: Mg2+
Mg3(OH)6, all cationic spaces
are filled
One OH is surrounded by 3 Mg:
Trioctahedral sheet
Different
cations
1 Cation
6 O or OH
(Holtz and Kovacs, 1981)

Tetrahedral & Octahedral Sheets
For simplicity, let’s represent silica tetrahedral sheet by:
Si
and alumina octahedral sheet by:
Al
Mitchell, 1993

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1:1 phyllosilicate
Clay Mineral (e.g.,
kaolinite, halloysite) 2:1 phyllosilicate Clay Mineral
(e.g., montmorillonite, illite)
Different Clay Minerals All clay mineral are made of different combinations of the above two sheets: tetrahedral sheet and
octahedral sheet.
 Different combinations of tetrahedral and octahedral sheets form different clay minerals:

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TYPES OF CLAY MINERALS
1) Silicate Clays (crystalline)
2) Sesquioxide/oxidic clays
3) Amorphous clays (non-crystalline)

1) Silicate Clays (crystalline)
Mitchell, 1993
21 November 2015 Prof. Dr. H.Z. Harraz Presentation
Clay Minerals
17
1:1 one
tetrahedron sheet
to one octahedral
sheet
Different types
of silicate clays
are composed
of sandwiches
(combinations)
of layers with
various
substances in
their interlayer
space.
2:1 two tetrahedral
sheets to one
octahedral sheet

Si
Al
Si
Al
Si
Al
Si
Al
joined by strong H-bond
no easy separation
0.72 nm
Typically 70-
100 layers
joined by
oxygen sharing
a) Kaolinite
layer

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a) Kaolinite
 1:1 phyllosilicate Minerals
 Si4Al4O10(OH)8
 Platy shape
 The bonding between layers are van der Waals
forces and hydrogen bonds (strong bonding).
 There is no interlayer swelling
 Width: 0.1~ 4m
 Thickness: 0.05~2 m
 Hydrogen bonds in interlayer space
 strong
 Nonexpandable
 Low cation exchange capacity (CEC)
 Particles can grow very large (0.2 – 2 µm)
 Effective surface area = 10 – 30 m2/g
 External surface only
 Kaolinite is used for making paper, paint,
pottery and pharmaceutical industries

Trovey, 1971 ( from Mitchell, 1993)
17 m
 Mineral particles of the kaolinite subgroup consists of the basic units
stacked in the c direction.
 The bonding between successive layers is by both van der Waals
forces and hydrogen bonds.
 Kaolinite is the purest of clays, meaning that it varies little in
composition. It also does not absorb water and does not expand when it
comes in contact with water. Thus, kaolinite is the preferred type of clay for
the ceramic industry.
Kaolinite grades
 Clays are categorized into six groups:
1) Kaolin or china clay: white, claylike material composed mainly
of kaolinite industrial applications: paper coating and filling,
refractories, fiberglass and insulation, rubber, paint,
ceramics, and chemicals
2) Ball clay: kaolin with small amount of impurities industrial
application: dinnerware, floor tile, pottery, sanitary ware.
3) Fire clays: kaolin with substantial impurities (diaspore, flint)
industrial applications: refractories
4) Bentonite: clay composed of smectite minerals, usually
montmorillonite industrial applications: drilling muds, foundry
sands
5) Fuller’s earth: nonplastic clay high in magnesia, a similar to
bentonite industrial applications: absorbents
6) Shale: laminated sedimentary rock consisting mainly of clay
minerals mud industrial application: raw material in cement
and brick manufacturing
a) Kaolinite
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Kaolinite "booklets", platelet
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1.Silicate Clays
kaolinite
Kaolinite
• Kaolinite clays have long been used in the ceramic industry, especially
in fine porcelains, because they can be easily molded, have a fine
texture, and are white when fired.
• These clays are also used as a filler in making paper.
 good road base
 good foundation
 good for pottery; China clay (porcelain)
 easy to cultivate, but need manure or fertilizer
 Dominant clay mineral in highly weathered soils

Kaolinite grades
Clays are categorized into six groups:
1) Kaolin or china clay: white, claylike material composed mainly of
kaolinite industrial applications: paper coating and filling,
refractories, fiberglass and insulation, rubber, paint, ceramics, and
chemicals
2)Ball clay: kaolin with small amount of impurities industrial
application: dinnerware, floor tile, pottery, sanitary ware.
3)Fire clays: kaolin with substantial impurities (diaspore, flint)
industrial applications: refractories
4)Bentonite: clay composed of smectite minerals, usually
montmorillonite industrial applications: drilling muds, foundry
sands
5)Fuller’s earth: nonplastic clay high in magnesia, a similar to
bentonite industrial applications: absorbents
6)Shale: laminated sedimentary rock consisting mainly of clay
minerals mud industrial application: raw material in cement and
brick manufacturing
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Blunging :The kaolin is mixed with water and
chemical dispersants, which puts the clay
particles in suspension (slurry).
De-gritting: The slurried kaolin is usually
transported through pipelines to degritting
facilities (rakes), where sand, mica and other
impurities are extracted with the help of
gravity.
Centrifuging: The centrifuge separates the fine
kaolin particles from coarse particles.Fine
particles, still in the form of a slurry, move on
for further processing.
China Clay processing
De-gritting (rake) tables

Brightness enhancement: Undesirable colors are removed through one or more processes
including bleaching, magnetic separation, flocculation, ozonation, flotation, and oxidation,
which will remove iron oxides, titanium oxides, organic, and other undesirable materials.
Delamination :For customers who want a delaminated clay product suited for lightweight
coating applications, coarse kaolinite particles are used as starting material. Delamination
occurs as the coarse particles of kaolin which when magnified appear as "booklets" are
broken into thin platelets by mechanical milling.
China Clay processing (cont.)
Filtering and drying :Large rotary vacuum filters remove water from the slurried kaolin.
Large gas-fired spray dryers remove and evaporate the remaining moisture.
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b) Halloysite
• 1:1 phyllosilicate Minerals
• Si4Al4O10(OH)8·4H2O
• A single layer of water between unit
layers.
• kaolinite family; hydrated and tubular
structure while it is hydrated
• 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 (Irfan, 1966). Note that this process is
irreversible and will affect the results of
soil classifications (GSD and Atterberg
limits) and compaction tests.
• There is no interlayer swelling.
Trovey, 1971 ( from
Mitchell, 1993)
2 m

c) Montmorillonite
Si
Al
Si
Si
Al
Si
Si
Al
Si
0.96
nm
joined by weak
van der Waal’s bond
easily separated
by water
 also called smectite; expands on contact with water
A highly reactive (expansive)
clay
swells on contact
with water
(OH)4Al4Si8O20.nH2O
high affinity to water
Bentonite:
 montmorillonite family
 used as drilling mud, in slurry trench
walls, stopping leaks

 Montmorillonite or smectite is family of expansible 2:1 phyllosilicate clays having permanent
layer charge because of the isomorphous substitution in either the octahedral sheet (typically from
the substitution of low charge species such as Mg2+ , Fe2+, or Mn2+ for Al3+)
 The most common smectite clay is Montmorillinite, with a general chemical formula :
(0.5Ca,Na)(Al,Mg,Fe)4(Si,Al)8O20(OH)4.nH2O
 Montmorillonites have very high specific surface, cation exchange capacity, and affinity to
water. They form reactive clays.
 Montmorillonites have very high liquid limit (100+), plasticity index and activity (1-7).
 Montmorillinite is the main constituent of bentonite, derived by weathering of volcanic
ash. Bentonite has the unsual property gives rise to interesting industrial used.
Montmorillinite 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), in slurry
trench walls, stopping leaks and to plug leaks in soil, rocks, and dams.
 Most important is as drilling mud in which the montmorillonite is used to give the fluid
viscosity several times that of water. It is also used for stopping leakage in soil, rocks, and
dams.
 Montmorillinite, however, is a dangerous type of clay to encounter if it is found in tunnels
or road cuts. Because of its expandable nature, it can lead to serious slope or wall
failures.
c) Montmorillonite

c) Montmorillonite
n·H2O+cations
5 m
Film-like shape.
There is extensive isomorphous substitution for silicon
and aluminum by other cations, which results in charge
deficiencies of clay particles.
Always negative due to isomorphous substitution
Layers weakly held together by weak O-O bonds or
cation-O bonds
Cations adsorbed in interlayer space
Interlayer cations hold layers together:
 In dry soils, bonding force is strong and hard
clods form; deep cracks
 In wet soils, water is drawn into interlayer space
and clay swells.
n·H2O and cations exist between unit layers, and the
basal spacing is from 9.6 Å to  (after swelling).
Maximum Swelling
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).
High Cation Exchange Capacity (CEC)
High effective surface area = 650 – 800 m2/g
 Internal surface area >> external
Expandable……..Most expandable of all clays
Width: 1 or 2 m
Thickness: 10 Å ….. About ~1/100 width
(Holtz and Kovacs,
1981)

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Swelling Clays
The interlayer in montmorillonite or
smectites is not only hydrated, but it is
also expansible; that is, the separation
between individual smectite sheets
varies with the amount of water present
in the soil. Because of this, they are
often referred to as "swelling clays".
Soils having high concentrations of
smectites can undergo as much as a
30% volume change due to wetting and
drying or these soils have a high
shrink/swell potential and upon drying
will form deep cracks.
Bentonite

Plummer et al., Physical Geology 9th edition, McGraw Hill Inc, Fig. 2.19b
Main difference- ions that make up the middle of the sandwich

Plummer et al., Physical Geology 9th edition, McGraw Hill Inc, Box 02.04.f1
Cat-litter in action

d) Illite
Si
Al
Si
Si
Al
Si
Si
Al
Si
0.96
nm
joined by K+ ions
fit into the
hexagonal holes in
Si-sheet
7.5 m
Trovey, 1971
( from
Mitchell,
1993)

d) Illite (Fine-grained micas, mica-like minerals)
 Illite is the most common clay mineral, often composing more than 50 percent of the clay-mineral suite
in the deep sea.
 They are characteristic of weathering in temperate climates or in high altitudes in the tropics, and
typically reach the ocean via rivers and wind transport.
 Illite type clays are formed from weathering of K and Al-rich rocks under high pH conditions. Thus,
they form by alteration of minerals like muscovite and feldspar. Illite clays are the main constituent of
shales.
 The Illite clays have a structure similar to that of muscovite, but is typically deficient in alkalies, with
less Al substitution for Si. Thus, the general formula for the illites is:
Si8(Al,Mg, Fe)4~6O20(OH)4·(K,H2O)2 OR
KyAl4(Si8-y,Aly)O20(OH)4 , usually with 1 < y < 1.5, but always with y < 2.
 Because of possible charge imbalance, Ca and Mg can also sometimes substitute for K.
 The K, Ca, or Mg interlayer cations prevent the entrance of H2O into the structure.
 Thus, the illite clays are non-expanding clays.
1) Fewer of Si4+positions are filled by Al3+ in the illite.
2) There is some randomness in the stacking of layers in illite.
3) There is less potassium in illite. Well-organized illite contains 9-10% K2O.
4) Illite particles are much smaller than mica particles.
5) Ferric ion Fe3+

d) Illite (Fine-grained micas, mica-like minerals)
 2:1 phyllosilicate Minerals
 Flaky shape.
 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 are replaced by the Al3+, and some of the Al3+ in the octahedral
sheet are 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
 As mica crystallizes from magma:
 Isomorphous substitution of Al+3 for Si+4 in tetrahedra
 high net negative charge
 K+ ions in interlayer space (Strongly binds layers)
 Non-expandable
 Minimum Swelling
 Surface area 70 -175 m2/g

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e) Vermiculite Vermiculite is a 2:1 phyllosilicate clay mineral
 The octahedral sheet is brucite.
 Octahedral ions are Al, Mg, Fe
 The basal spacing is from 10 Å to 14 Å.
 It contains exchangeable cations such as Ca2+ and Mg2+ and two layers of water within
interlayers.
 It can be an excellent insulation material after dehydrated.
 It is generally regarded as a weathering product of micas (Forms from alteration of mica):
 Weathering removes some K+ ions
 Replaced by hydrated cations in interlayer space
 Water molecules and cations bridge layers, so not as expandable as smectites
 Still have very high net negative charge
 High Cation Exchange Capacity (CEC) (highest of all clays)
 Expandable
 Surface area 600 – 800 m2/g
 Internal >> external
 Vermiculite is similar to montmorillonite, a 2:1 mineral, but it has only two interlayers of water.
 After it is dried at high temperature, which removes the interlayer water, expanded”
vermiculite makes an excellent insulation material.
 Vermiculite is also hydrated and somewhat expansible though less so than smectite because of its
relatively high charge.

Vermiculite
Vermiculite possesses the special property of
expanding to between six and twenty times its
original volume when heated to ~1,000oC.
This process, called exfoliation, liberates
bound water from between the mica-like
layers of the mineral and literally expands the
layers apart at right angles to the cleavage
plane.
Vermiculite is used to loosen and aerate soil
mixes. Mixed with soil, it improves water
retention and fertilizer release, making it ideal
for starting seeds. Also used as a medium for
winter storage of bulbs and flower tubers.
Illite Vermiculite
Mitchell, 1993

f) Chlorite
 2:1 phyllosilicate Minerals
 Central cations in octahedral sheets
is Fe or Mg
 Interlayer space occupied by a
stable, positively charged octahedral
sheet.
 Non-expandable.
 Minimum Swelling.
 70 -100 m2/g surface area
Gibbsite
or
brucite
The basal
spacing
is fixed
at 14 Å

g) Attapulgite (Chain Structure Clay Minerals)
• chain structure (no sheets); needle-
like appearance
• They have lath-like or thread-like
morphologies.
• The particle diameters are from 50 to
100 Å and the length is up to 4 to 5
m.
• Attapulgite is useful as a drilling
mud in saline environment due to its
high stability
4.7 m
Trovey, 1971 ( from Mitchell, 1993)
Attapulgite

h) Mixed Layer Clays
• Different types of clay minerals have similar structures (tetrahedral and octahedral
sheets) so that interstratification of layers of different clay minerals can be observed.
• Most than one type of clay mineral is usually found in most soils. Because of the great
similarity in crystal structure among the different minerals, interstratification of two or more
layer types often occurs within a single particle
• In general, the mixed layer clays are composed of interstratification of expanded water-
bearing layers and non-water-bearing layers. Montmorillonite-illite is most common, and
chlorite-vermiculite and chlorite-montmorillonite are often found.
(Mitchell, 1993)

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2) Sesquioxides / Oxidic Clays
 Ultimate weathering products
 Ultisols and Oxisols
 Very stable; persist indefinitely
 Yellow, red, brown
 Fe or Al as central cations
 Lack negative charge
 Don’t retain adsorbed cations
 Non-expandable
 Low cation exchange capacity (CEC)
 Low fertility:
 Often are net positive
 Often have enough Al or Mn to be toxic to plants
 High capacity to fix phosphorous so it is not available to plants
 Highly weathered so no more nutrients to release in weathering

21 November 2015 Prof. Dr. H.Z. Harraz Presentation Clay Minerals
42
Ultisol profile
 In heavily leached soils, sheets decompose
into component Si tetrahedral and Al
octahedral.
 Al octahedral often weather into gibbsite Al(OH)3

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3) Amorphous clays (non-crystalline, Allophanes and Imogolite)
 silicates
 These are structurally disordered aluminosilicates.
 They are normally derived from volcanic ash materials and constitute a major component
of volcanic soils.
 Allophane and imogolite
The formation of imogolite and allophane occur during weathering of volcanic ash
under humid, temperate or tropical climate conditions.
Allophane is X-ray amorphous and has no definite composition or shape. It is
composed of hollow, irregular spherical particles with diameters of 3.5 to 5.0 nm.
Allophane is often associated with clay minerals of the kaolinite group
Imogolite has the empirical formula SiAl4O10.5H2O
 High internal negative charge
 High cation exchange capacity (CEC)
 High water-holding capacity
 Surface area 100 – 1000 m2/g

“Activity” of silicate clays
refers to cation exchange capacity (CEC)
Ability to retain and supply nutrients
Fertility
High activity clays:
Less weathered ; high effective surface area
smectite, vermiculite, mica (illite), chlorite
Low activity clays:
More weathered; less effective surface area
kaolinite

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What determines clay minerals in a given soil?
Usually a mixture
Climate
Parent material
Degree of weathering

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Generalized Chemical Weathering
 Temperate Climates
3KAlSi3O8 + 2H+ + 12H2O  KAlSi3O10(OH)2 + 6H4SiO4 + K+
(K-feldspar) (mica/illite) (silicic acid)
 Temperate Humid Climates:
2KAlSi3O8 + 2H+ + 3H2O  3Al2Si2O5(OH)4 + K+
(K-feldspar) (kaolinite)
 Humid Tropical Climate:
Al2Si2O5(OH)4 + 5H2O  2Al(OH)3 + 2K+ + 4H4SiO4
(kaolinite) (gibbsite)

Clays: Important Chemical Weathering Products
 Clay Mineral Species are a function of:
 environmental conditions at the site of weathering
 available cations produced by chemical degradation

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Generalized relationships:
Ultisols
Kaolinite, oxidic clays
Oxisols
Alfisols
Mica, vermiculite, smectiteMollisols
Vertisols
Andisols Amorphous

Chemical Weathering Products
 As the age of
sedimentary rocks
increases clay mineral
assemblages in the
subsurface transform
through diagenesis to
illite + chlorite

Uses of Clay - Drilling Mud
Bentonite and other clays are used in the drilling of oil and water wells.
The clays are turned into mud, which seals the walls of the boreholes,
lubricates the drill head and removes drill cuttings.
Drilling mud slurry
Cooling and
cleaning the drill “Gushers” used to be
common until the use
of drilling mud was
implemented
deep oil is at high pressure

Uses of Clay - Contaminant Removal
Clay slurrys have effectively been used to remove a range of
comtaminants, including P and heavy metals, and overall water
clarification.
Schematic of montmorillonite
absorbing Zn

Clay Fabric
52
Flocculated
Dispersed
edge-to-face contact face-to-face contact
 The term fabric is used to describe the geometric arrangement of the clay particles.
Flocculated and Dispersed are the two extreme cases.
Flocculated fabric gives higher strength and stiffness.
Electrochemical environment (i.e., pH, acidity, temperature, cations present in the water)
during the time of sedimentation influence clay fabric significantly.
Clay particles tend to align perpendicular to the load applied on them.

Scanning Electron Microscope
53
 common technique to see clay particles
plate-like
structure
 qualitative
Clay particles are smaller than 2
microns. Their shapes can be studied
by an electron microscope.

2.1 X-ray diffraction
• The distance of atomic planes d can be determined based on the Bragg’s
equation.
BC+CD = n, n = 2d·sin, d = n/2 sin
where n is an integer and  is the wavelength.
• Different clays minerals have various basal spacing (atomic planes). For
example, the basing spacing of kaolinite is 7.2 Å.
Mitchell, 1993

2.2 Differential Thermal Analysis
(DTA)
For example:
Quartz changes from the  to  form at 573 ºC
and an endothermic peak can be observed.
• Differential thermal analysis
(DTA) consists of simultaneously
heating a test sample and a
thermally inert substance at
constant rate (usually about 10
ºC/min) to over 1000 ºC and
continuously measuring differences
in temperature and the inert
material T.
• Endothermic (take up heat) or
exothermic (liberate heat) reactions
can take place at different heating
temperatures. The mineral types
can be characterized based on those
signatures shown in the left figure.
(from Mitchell, 1993)
T
Temperature (100 ºC)

2.2 DTA (Cont.)
•If the sample is thermally inert,
•If the phase transition of the sample
occurs,
T
Time t
T
Time t
Crystallize
Melt
Endothermic reactions take up
heat from surroundings and
therefore the temperature T
decreases.
Exothermic reactions liberate
heat to surroundings and
therefore the temperature T
increases.
T= the temperature of the sample – the temperature of the thermally inert substance.

Others…
57
1.Specific surface (Ss)
2.Cation exchange capacity (cec)
3.Plasticity chart(Casagrande’s PI-LL Chart)
5. Potassium determination
Well-organized 10Å illite layers contain 9% ~ 10 % K2O.
6. Thermogravimetric analysis
It is based on changes in weight caused by loss of water or CO2 or gain in oxygen.
Sometimes, you cannot identify clay minerals only based on one method.
0
10
20
30
40
50
60
0 10 20 30 40 50 60 70 80 90 100
Liquid Limit
PlasticityIndex
A-line
U-line
montmorillonite illite
kaolinite
chlorite
halloysite

Specific Surface
 surface area per unit mass (m2/g)
 smaller the grain, higher the specific
surface
e.g., soil grain with specific gravity of 2.7
10 mm cube
1 mm cube
spec. surface = 222.2 mm2/g spec. surface = 2222.2 mm2/g

Specific Surface
forcelGravationa
forcerelatedSurface
mass/surfacesurfaceSpecific
volume/surfacesurfaceSpecific


g/m3.2
cm/g65.2m1
m16
S
cm/g65.2,cubem111
g/m103.2
cm/g65.2cm1
cm16
S
cm/g65.2,cubecm111
2
33
2
s
3
24
33
2
s
3











Example:
Surface related forces: van der
Waals forces, capillary forces, etc.
Ss is inversely
proportional to
the particle size
Preferred
Demonstration of capillary force between Large particle and small particle.

Isomorphous Substitution
60
The clay particle derives its net negative charge from the
isomorphous substitution and broken bonds at the boundaries.
 substitution of Si4+ and Al3+ by other lower valence (e.g., Mg2+)
cations, i.e. Lower charge cations replace higher charge cations as
central cation (e.g., Mg+2 replaces Al+3).
 leaves net negative charge (results in charge imbalance (net
negative))
+
+
+
+ +
+
+
__
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
positively charged edges
negatively charged
faces
Clay Particle with Net negative Charge

Cation Exchange Capacity (c.e.c)
61
 capacity to attract cations from the water (i.e., measure of the net
negative charge of the clay particle)
 measured in meq/100g (net negative charge per 100 g of
clay)
milliequivalents
known as exchangeable cations
 The replacement power is greater for higher valence and larger
cations.
Al3+ > Ca2+ > Mg2+ >> NH4
+ > K+ > H+ > Na+ > Li+
 The negatively charged clay particles can attract cations from the water. These
cations can be freely exchanged with other cations present in the water. For
example Al3+ can replace Ca2+ and Ca2+ can replace Mg2+.

A Comparison
62
Mineral Specific surface
(m2/g)
C.E.C (meq/100g)
Kaolinite 10-20 3-10
Illite 80-100 20-30
Montmorillonite 800 80-120
Chlorite 80 20-30

Cation Concentration in Water
63
+++
+
+
++
+
+
+
+
+
+
+
+
+ +
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
++
+
+ + +
+
+ +
+
+
+
+
+
+
+
+
++
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+ +
+
cations
 cation concentration drops with distance from clay particle
- -
- -
- -
- -
- -
- -
- -
clay particle
double layer free water
The negatively charged faces of clay particles attract cations in the water. The concentration of the cations
decreases exponentially with the increasing distance from the clay particle. The negatively charged clay
surface and the positively charged cations near the particle form two distinct layers, known as “electric
double layer” or simply “double layer”.

Adsorbed Water
64
- -
- -
- -
- -
- -
- -
- -
 A thin layer of water tightly held to particle; like a skin
 1-4 molecules of water (1 nm) thick
 more viscous than free water
adsorbed water

Clay Particle in Water
65
- -
- -
- -
- -
- -
- -
- -
free water
double layer
water
adsorbed water
50 nm
1nm

Origins of Charge Deficiencies
21 November 2015 Prof. Dr. H.Z. Harraz Presentation
Clay Minerals
66
1) Imperfections in the crystal lattice -Isomorphous substitution.
• The cations in the octahedral or tetrahedral sheet can be replaced by different
kinds of cations without change in crystal structure (similar physical size of
cations).
For example,
Al3+ in place of Si4+ (Tetrahedral sheet)
Mg2+ instead of Al3+(Octahedral sheet)
unbalanced charges (charge deficiencies)
• This is the main source of charge deficiencies for montmorillonite.
• Only minor isomorphous substitution takes place in kaolinite.
2) Imperfections in the crystal lattice - The broken edge:
• The broken edge can be positively or negatively charged.
3) Proton equilibria (pH-dependent charges):
• Kaolinite particles are positively charged on their edges when in a low pH
environment, but negatively charged in a high pH (basic) environment.
4) Adsorbed ion charge (inner sphere complex charge and outer sphere complex
charge:
• Ions of outer sphere complexes do not lose their hydration spheres. The inner
complexes have direct electrostatic bonding between the central atoms.
)ionDeprotonat(OHOMOHOHM
)otonation(PrOHMHOHM
2
2
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
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

Clay Mineralogy

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