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Elements of Earth

% 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% 2

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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 by wind, water and ice.

far away 3

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Parent Rock

~ formed by one of these three different processes igneous sedimentary metamorphic

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formed by cooling of molten magma (lava) e.g., granite formed by gradual deposition, and in layers e.g., limestone, shale formed by alteration of igneous & sedimentary rocks by pressure/temperature e.g., marble

Residual Soils

Formed by in situ weathering of parent rock

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Transported by:



wind



sea (salt water)



lake (fresh water)



river



ice

Transported Soils

Special name: “Aeolian” “Marine” “Lacustrine” “Alluvial” “Glacial”

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Basic Structural Units

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Clay minerals are made of two distinct structural units.

oxygen hydroxyl or oxygen silicon aluminium or magnesium

0.26 nm

Silicon tetrahedron

0.29 nm

Aluminium Octahedron

Tetrahedral Sheet

Several tetrahedrons joined together form a tetrahedral sheet.

tetrahedron hexagonal hole 9

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Tetrahedral & Octahedral Sheets

For simplicity, let’s represent silica

tetrahedral sheet

by:

and alumina

octahedral sheet

by:

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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

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Al Si Al Si Al Si Al Si

0.72 nm joined by oxygen sharing

Kaolinite

 used in paints, paper and in pottery and pharmaceutical industries  (OH) 8 Al 4 Si 4 O 10

Halloysite

 kaolinite family; hydrated and tubular structure  (OH) 8 Al 4 Si 4 O 10 .4H

2 O 14

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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 15

Montmorillonite

 A highly reactive (expansive) clay  (OH) 4 Al 4 Si 8 O 20 .nH

2 O swells on contact with water

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high affinity to water

Bentonite

 montmorillonite family  used as drilling mud, in slurry trench walls, stopping leaks 16

K +

ions fit into the hexagonal holes in Si-sheet

Si Al Si Si Al Si Si Al Si

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Illite

0.96 nm 17

Others… Chlorite

 A 2:1:1 (???) mineral.

Si Al or Mg Al

Vermiculite

 montmorillonite family; 2 interlayers of water

Attapulgite

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 chain structure (no sheets); needle-like appearance 18

A Clay Particle

Plate-like or Flaky Shape 19

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Clay Fabric

face-to-face contact

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Flocculated Dispersed

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 23

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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 24

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Casagrande’s PI-LL Chart

60 50 40 30 20 10 0 0 montmorillonite U-line illite A-line kaolinite halloysite 10 20 30 chlorite 40 50 60

Liquid Limit

70 80 90 100 25

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Specific Surface

 surface area per unit mass (m 2 /g)  smaller the grain, higher the specific surface e.g., soil grain with specific gravity of 2.7

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10 mm cube spec. surface = 222.2 mm 2 /g 1 mm cube spec. surface = 2222.2 mm 2 /g 27

Isomorphous Substitution

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 substitution of Si 4+ and Al 3+ (e.g., Mg 2+ ) cations by other lower valence  results in charge imbalance (net negative) + +

_ _

_ _ _

_ _ _ _ _ _ _

_ _ _ _ _

+ +

_ _ _ _ _ _

positively charged edges negatively charged faces Clay Particle with Net negative Charge 28

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

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 The replacement power is greater for higher valence and larger cations.

Al 3+ > Ca 2+ > Mg 2+ >> NH 4 + > K + > H + > Na + > Li + 29

A Comparison

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Mineral Kaolinite Illite Montmorillonite Chlorite Specific surface (m 2 /g) 10-20 80-100 800 80 C.E.C (meq/100g) 3-10 20-30 80-120 20-30 30

Cation Concentration in Water

 cation concentration drops with distance from clay particle

+ clay particle + + +

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+ + + + + + + + + + + + + + + + + + + + + + + + + + + - - - + - - + + + + + + + - - + + + + cations + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + double layer + + + + + + free water

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

- - - - - - - -

Clay Particle in Water

adsorbed water

- - - - - - - -

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).

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 Bentonite (a form of Montmorillonite) is frequently used as drilling mud.

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