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Duration = 15 mins.
N. Sivakugan
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Elements of Earth
8-35 km crust
% by weight in crust
O
Si
Al
Fe
Ca
Na
K
Mg
other
= 49.2
= 25.7
= 7.5
= 4.7
= 3.4
= 2.6
= 2.4
= 1.9
= 2.6
82.4%
12500 km dia
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Soil Formation
Parent Rock
Residual soil
~ in situ weathering (by
physical & chemical
agents) of parent rock
Transported soil
~ weathered and
transported far away
by wind, water and ice.
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Parent Rock
~ formed by one of these three different processes
igneous
formed by cooling of
molten magma (lava)
e.g., granite
sedimentary
formed by gradual
deposition, and in layers
e.g., limestone, shale
metamorphic
formed by alteration
of igneous &
sedimentary rocks by
pressure/temperature
e.g., marble
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Residual Soils
Formed by in situ weathering of parent rock
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Transported Soils
Transported by:
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Special name:
 wind
“Aeolian”
sea (salt water)
“Marine”
lake (fresh water)
“Lacustrine”
river
“Alluvial”
ice
“Glacial”
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Basic Structural Units
Clay minerals are made of two distinct structural
units.
hydroxyl or
oxygen
oxygen
aluminium or
magnesium
silicon
0.26 nm
Silicon tetrahedron
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0.29 nm
Aluminium Octahedron
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Tetrahedral Sheet
Several tetrahedrons joined together form a
tetrahedral sheet.
tetrahedron
hexagonal
hole
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Tetrahedral & Octahedral Sheets
For simplicity, let’s represent silica tetrahedral sheet by:
Si
and alumina octahedral sheet by:
Al
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Different Clay Minerals
Different combinations of tetrahedral and octahedral
sheets form different clay minerals:
1:1 Clay Mineral (e.g., kaolinite, halloysite):
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Different Clay Minerals
Different combinations of tetrahedral and octahedral
sheets form different clay minerals:
2:1 Clay Mineral (e.g., montmorillonite, illite)
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Kaolinite
Typically
70-100
layers
joined by strong H-bond
no easy separation
Al
Si
Al
Si
Al
Si
Al
Si
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0.72 nm
joined by oxygen
sharing
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Kaolinite
 used in paints, paper and in pottery and
pharmaceutical industries
 (OH)8Al4Si4O10
Halloysite
 kaolinite family; hydrated and tubular structure
 (OH)8Al4Si4O10.4H2O
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Montmorillonite
 also called smectite; expands on contact with water
Si
Al
Si
easily separated
by water
joined by weak
van der Waal’s bond
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Si
Al
Si
Si
Al
Si
0.96 nm
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Montmorillonite
 A highly reactive (expansive) clay
swells on contact with water
 (OH)4Al4Si8O20.nH2O
Bentonite
high affinity to water
 montmorillonite family
 used as drilling mud, in slurry trench walls,
stopping leaks
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Illite
Si
joined by K+ ions
fit into the hexagonal
holes in Si-sheet
Al
Si
Si
Al
Si
0.96 nm
Si
Al
Si
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Others…
Chlorite
 A 2:1:1 (???) mineral.
Si
Al
Al or Mg
Vermiculite
 montmorillonite family; 2 interlayers of water
Attapulgite
 chain structure (no sheets); needle-like appearance
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A Clay Particle
Plate-like or Flaky Shape
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Clay Fabric
edge-to-face contact
Flocculated
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face-to-face contact
Dispersed
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Clay Fabric
 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.
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Scanning Electron Microscope
 common technique to see clay particles
 qualitative
plate-like
structure
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Others…
X-Ray Diffraction (XRD)
 to identify the molecular structure and minerals present
Differential Thermal Analysis (DTA)
 to identify the minerals present
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Casagrande’s PI-LL Chart
60
U-line
Plasticity Index
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montmorillonite
illite
A-line
40
30
kaolinite
20
halloysite
10
chlorite
0
0
10
20
30
40
50
60
70
80
90
100
Liquid Limit
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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
spec. surface = 222.2 mm2/g
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1 mm cube
spec. surface = 2222.2 mm2/g
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Isomorphous Substitution
 substitution of Si4+ and Al3+ by other lower valence
(e.g., Mg2+) cations
 results in charge imbalance (net negative)
positively charged edges
+ +
+ _ _ _ _+ + _
+ _ _ _
__
+ _ _ _
_ _
_
_ _ _
_ _ _ _
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negatively charged faces
Clay Particle with Net negative Charge
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Cation Exchange Capacity (c.e.c)
known as exchangeable cations
 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
 The replacement power is greater for higher valence and
larger cations.
Al3+ > Ca2+ > Mg2+ >> NH4+ > K+ > H+ > Na+ > Li+
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A Comparison
Mineral
Specific surface
(m2/g)
10-20
C.E.C
(meq/100g)
3-10
80-100
20-30
Montmorillonite
800
80-120
Chlorite
80
20-30
Kaolinite
Illite
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Cation Concentration in Water
 cation concentration drops with distance from clay particle
clay particle
+
+
+
+
+
+
++
+
+
+
+
+
+ +
+
+
+
+ +
+ + +
+
+ + + + + + + +
+ + + ++
++
+
+
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- + cations
+
- ++ +
+
+ +
+
+ +
+
+
-+ + + +
+
- +
+ + +
+
+
+
-+ +
+
+
+
+
-+
+
+
double layer
+
+
+
+
+
+
free water
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Adsorbed Water
 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
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-
adsorbed water
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Clay Particle in Water
adsorbed water
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1nm
50 nm
- double layer
- water
free water
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Summary - Clays
 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.
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Summary - Montmorillonite
 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).
 Bentonite (a form of Montmorillonite) is frequently used as
drilling mud.
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