clay mineralogy sivakugan
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1N. N. SivakuganSivakuganN. N. SivakuganSivakugan
Duration = 15 mins.
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Elements of Earth
12500 km dia
8-35 km crust % by weight in crust
O = 49.2Si = 25.7Al = 7.5Fe = 4.7Ca = 3.4Na = 2.6K = 2.4Mg = 1.9other = 2.6
82.4%
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Soil Formation
Parent Rock
Residual soil Transported soil
~ in situ weathering (by physical & chemical agents) of parent rock
~ 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 sedimentary metamorphic
formed by cooling of molten magma (lava)
formed by gradual deposition, and in layers
formed by alteration of igneous & sedimentary
rocks by pressure/temperature
e.g., limestone, shale
e.g., marble
e.g., granite
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Residual Soils
Formed by in situ weathering of parent rock
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Transported Soils
Transported by: Special name:
wind “Aeolian”
sea (salt water) “Marine”
lake (fresh water) “Lacustrine”
river “Alluvial”
ice “Glacial”
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Basic Structural Units
0.26 nm
oxygen
silicon
0.29 nm
aluminium or magnesium
hydroxyl or oxygen
Clay minerals are made of two distinct structural units.
Silicon tetrahedron 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
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
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Kaolinite
used in paints, paper and in pottery and pharmaceutical industries
Halloysite kaolinite family; hydrated and tubular structure
(OH)8Al4Si4O10.4H2O
(OH)8Al4Si4O10
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Montmorillonite
Si
Al
Si
Si
Al
Si
Si
Al
Si
0.96 nm
joined by weakvan der Waal’s bond
easily separated by water
also called smectite; expands on contact with water
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Montmorillonite
A highly reactive (expansive) clay
montmorillonite family
used as drilling mud, in slurry trench walls, stopping leaks
(OH)4Al4Si8O20.nH2O
high affinity to waterBentonite
swells on contact with water
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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
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Others…
A 2:1:1 (???) mineral.
montmorillonite family; 2 interlayers of water
chain structure (no sheets); needle-like appearance
Chlorite
Vermiculite
Attapulgite
Si Al Al or Mg
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A Clay Particle
Plate-like or Flaky Shape
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Clay Fabric
Flocculated Dispersed
edge-to-face contactface-to-face contact
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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
plate-like structure
qualitative
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Others…
X-Ray Diffraction (XRD)
Differential Thermal Analysis (DTA)
to identify the molecular structure and minerals present
to identify the minerals present
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Casagrande’s PI-LL Chart
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10
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60
0 10 20 30 40 50 60 70 80 90 100
Liquid Limit
Pla
stic
ity
Ind
ex
A-line
U-line
montmorillonite illite
kaolinite
chlorite
halloysite
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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 1 mm cube
spec. surface = 222.2 mm2/g 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)
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positively charged edges
negatively charged faces
Clay Particle with Net negative Charge
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Cation Exchange Capacity (c.e.c)
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+
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A Comparison
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
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Cation Concentration in Water
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cations
cation concentration drops with distance from clay particle
- -- -- -- -- -- -- -
clay particle
double layer free water
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Adsorbed Water
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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
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Clay Particle in Water
- -- -- -- -- -- -- -
free water
double layerwater
adsorbed water
50 nm
1nm
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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.
Bentonite (a form of Montmorillonite) is frequently used as drilling mud.
Montmorillonites have very high liquid limit (100+), plasticity index and activity (1-7).