Clay mineralogy sivakugan (Complete Soil Mech. Undestanding Pakage: ABHAY)

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

0

10

20

30

40

50

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)

++

+ + +

+

+

__ _

_ _

_

_

___

_

_

_

_

_

_

_

_

_

__

__

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

+++ +

+

++

+

+

+

+

+

+

+

++ +

+

+

+ +

++

++

+

+

+

+

+

+

++

+

++

+

+

+

++

+

+ + +

+

+ +

+

+

+

+

+

+

+

+ ++

+

+

+

++

++

+

+

+

+

+

+

++

+

++

+

+

+

+ +

+

cations

cation concentration drops with distance from clay particle

- -- -- -- -- -- -- -

clay particle

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

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).