Soil MineralogyThe solid phase of a soil may contain various amounts of:
• Crystalline clay and nonclay minerals• Noncrystalline clay material• Organic matter• Precipitated salts.
The range of solid particle size in soil can be great, from gravel and cobbles to very small collodial particles.
Particle size range according to ASTM Standards:
Gravel Sand Silt Clay
75.0 mm 4.75mm 0.074 mm 0.002 mm
Clay is both referred to:• a size (< 2 µm)• a mineral type.
As a mineral term:It refers to specific clay minerals which are distinguished by :
1. Small particle size2. A net negative electrical charge3. Plasticity when mixed with water4. High weathering resistance.
Not all nonclay particles are coarser than 2 µm.
Particles defined as clay on the bases of their size are not necessarily clay minerals. Clay particles possess the tendency to develop plasticity when mixed with water; these are clay minerals.
It is useful to use the terms:• Clay size and• Clay mineral content.
Important Difference between Clay and Nonclay Minerals
• Nonclays →bulky shape par cles
• Clay minerals → platy and in a few cases they are needle shaped or tubular.
Clay MineralogyMineralogy is the primary factor controlling• the size,• shape, and• physical and chemical properties of soilparticles.
Clays are small crystalline particles of one ormore members of a small group of minerals.
They are primarily hydrous aluminum silicates.
Nonclay Minerals in Soil
The physical characteristics of cohesionless, nonclay soils are determined primarily by:
particle size, shape, surface texture, and size distribution.
For non-clay particles, mineralogy is not that much important because they are considered to be relatively inert materials and their interaction is basically physical in nature.
In cohesionless soils, mineralogy is important for:
Hardness,
Cleavage, and
Resistance to chemical attack.
The most abundant nonclay minerals in most of the soils are:
• Quartz→ persistent.
Spiral combination of silica tetrahedra groups,
Structure with high stability, without cleavage plane, no weakly bond, and high hardness.
There are no weakly bonded ions in the structure and the mineral has high hardness.
• Feldspar → easily broken down: Part of the silicon is replaced by
aluminum.
The excess negative charge resulting from this replacement creates an open structure with low bond strength between units.
This mineral has:• cleavage planes, • moderate hardness, and is • relatively easily broken down.
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• Carbonate minerals → calcite and dolomite mainly.
Can occur as bulky particles, shells, precipitates, or in solution.
• Iron and aluminum oxides → are abundant in residual soils of tropical regions.
• Sulfates → gypsumFound primarily in soils of semiarid and
arid regions with gypsum.
Mica Sheet structure composed of tetrahedraland octahedral units.
Electrostatic bond of moderate strengthdue to presence of potassium ions betweensheets;
Perfect cleavage planes.
Classification of Clay MineralsThree criteria can be considered:
• The height of the unit cell or “thickness of layer”.
• Composition, whether dioctahedral or trioctahedral, and ionic content of layer.
• Stacking sequence of layers or degree of orderliness of stacking.
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Silica tetrahedral sheet:
Si
Alumina octahedral sheet:
Al
Types of Silicate Clay Minerals:
On the basis of the number and arrangement of
tetrahedral (silica) and octahedral (alumina-magnesia) sheets
contained in the crystal units or layers, silicate clays are classified into three different groups:
Three different groups of silicate clays
• 1 : 1 Type clay minerals• 2 : 1 Type clay minerals• 2 : 1: 1 Type clay minerals
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Different Clay MineralsDifferent combinations of tetrahedral and octahedral
sheets form different clay minerals:
1:1 Clay Mineral (e.g., kaolinite, halloysite):
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2:1 Clay Mineral (e.g., montmorillonite, illite)
The 1:1 MineralsMinerals of this groups are 1:1 layer
silicates.
Their basic unit of structure consists of:tetrahedral and octahedral sheets
in which the anions at the exposed surface of the octahedral sheet are hydroxyls.
The 1:1 Minerals-KaoliniteKaolinite occurs when the
Si-tetrahedral and gibbsite sheet
are brought together with the oxygen ions of the tetrahedral layer also being in the octahedral layer.
KAOLINITE
Si
Al
Si
Al
Si
Al
Si
Al
joined by strong H-bondno easy separation
0.72 nm
Typically 70-100 layers
joined by oxygensharing
Al2Si2O5(OH)4
Basal spacing, d= 7.2 A˚
, hydrogen bonds
It has a two–sheet layer structure, one silica tetrahedral sheet and one alumina octahedral sheet.
There is no tendency for substitution in these layers. Therefore they have no charge, and no cations are found between the layers.
Because there is no substitution, the mineraldeposits tend to be quite pure.
Kaoline is used extensively as a raw material for ceramics, especially fine china.
Kaolinite Bonding: hydrogen bonds and Vanderwaals
forces. Strong enough that there is no interlayer swelling.
Kaolinite is triclinic. Unit cell dimensions are:
a= 5.16 A˚ b= 8.94 A˚ c= 7.37 A˚α= 91.8 ˚, β= 104.5 ˚, γ= 90 ˚
Since Al is in the octahedral layer, kaolinite is a di-octahedral mineral.
Due to this strong attraction these platelets do not expand when hydrated and kaolinite only has external surface area.
Also, kaolinite has very little isomorphic substitution of Al for Si in the tetrahedral layer.
Therefore, it has a low Cation Exchange Capacity
(1-15 meq/100g).
KaoliniteRocks that are rich in kaolinite are known as china
clay or kaolin.
• Kaolinite is one of the most common minerals; it is mined, as kaolin, in Brazil, France, United Kingdom, Germany, India, Australia, Korea , the People's Republic of China, and the USA.
• It is a soft, earthy, usually white mineral , produced by the chemical weathering of aluminium silicate minerals like feldspar.
• In many parts of the world, it is colored pink-orange-red by iron oxide.
Kaolinite
Structure of 1:1 layer silicate (kaolinite) illustrating the connection between tetrahedral and octahedral sheets.
Kaolinite
• Morphology: particles are six-sided thick plates.
• Isomorphous substitution: replacement of only one Si in every 400 Al.
• Cation exchange capacity: 3-15 meq/100g
• Specific surface area: 10-20 m2/g.
HalloysiteHalloysite is a 1:1 aluminosilicate clay mineral .
Its main constituents are • aluminium (20.90%), • silicon (21.76%), and • hydrogen (1.56%).
Halloysite typically forms by hydrothermal
alteration of alumino-silicate minerals.
Halloysite
(OH)8Si4Al4O10
d=7.2 A˚
Halloysite Think of this mineral as a kaolinite layer with a
layer of water (2.9Å) in the interlayer space.
The layer thickness is therefore, 10Å. There is also lots of disorder within and between layers.
Fe substitutes for Al in the octahedaral sheet.
Often occurs as cylinders or spheroidal shapes (due to hydrogen bonding with water molecules).
Two common forms are found,
• when hydrated the clay exhibits a 10 A˚spacing of the layers and
• when dehydrated (metahalloysite) the spacing is 7.2 A˚.
Halloysite naturally occurs as small cylinders which average 30 nm in diameter with lengths between 0.5 and 10 micrometres.
Halloysite
i. Non-hydrated halloysite
(OH)8Si4Al4O10
d=7.2 A˚
Hydrated halloysite ii. Hydrated halloysite10.1-7.2= 2.9 A˚ is the thickness of a single layer of water molecules.
(OH)8Si4Al4O104H2O
A partially hydrated form may also occur (d= 7.4-7.9 A˚).Hydrated halloysite can dehydrate irreversibly to
metahalloysite (OH)8Si4Al4O102H2O
d=10.1 A˚
• Morphology: hydrated halloysite as tubes.
• Cation exchange capacity: 5-40 meq/100g
• Specific surface area: 35-70 m2/g
Scanning electron image of halloysite
2:1 Smectite MineralsThe basic structure of 2:1 clay minerals is
two silicon tetrahedral layers and one aluminum octahedral layer.
There is much substitution of Mg+2 for Al+3 in the octahedral layer of this clay.
The negative charge produced by this substitution is balanced by an interlayer cation, sometimes Ca+2 and sometimes Na+.
2:1 SMECTITE MINERALS
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Different combinations of tetrahedral and octahedral sheets form different clay minerals:
2:1 Clay Mineral (e.g., montmorillonite, illite)
MONTMORILLONITE
44Si
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
MONTMORILLONITE
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A highly reactive (expansive) clay
(OH)4Al4Si8O20.nH2O
high affinity to water
swells on contact with water
2:1 Smectite Minerals
This layer is weakly held to another 2:1 layer to make the 2:1 family of clay minerals.
Montmorillonite and Vermiculite are two kinds of 2:1 clay minerals.
Montmorillonites: (Saponite) (OH)4Si8Al4O20.n(interlayer)H2O
Montmorillonite Structure
Schematic representation of montmorillonite structure.
9.6 A˚→∞
and n.H2O
A sodium–montmorillonite may be converted to a calcium–montmorillonite by exposing it to a solution containing calcium ions.
• Another interesting feature of sodium montmorillonite is its ability to absorb water into the interlayer region, thereby causing the clay toswell.
• This swelling can be observed by X–raydiffraction experiments because it causes thecrystal to expand in the c–direction.
• Calcium montmorillonite does not swell in this way, only sodium montmorillonite shows this behavior.
Montmorillonite particles are considerablysmaller.
The sodium form has a very small grainsize, typically a single unit cell in thickness, about 1 nm.
The calcium form is larger, perhaps 8–10 unit cell layers or 20–30 nm in thickness.
Montmorillonite • Bonding: by vander Waals forces and by
cations that may be present to balance charge deficiencies in the structure.
Bonds are very weak and unit cells can be easily separeted by cleavage or adsorption of water.
• Complete separation of unit cells is possible.
• Very expansive in character.
• Extension substitution for Al and Si within the lattice by other cations.
Al by Mg, Fe, Zn, Ni, li or others.
• Replacement of every 6Al by a Mg.
• Replacement of 15% of Si by Al.
• CEC: 80-150 meq/100g
Morphology and Surface Area
• Particle shape: very thin flakes like films.
• The primary surface, that is, surface due to particle surfaces exclusive of interlayer zones is in the range of 50-120 m2/g.
• The secondary specific surface after expansion and penetration of water into interlayer region: 700-840 m2/g.
Bentonite• Highly collodial, a very highly plastic expansive
clay and it is an alteration product of volcanic ash.
• LL ≥ 500.
Usage:• As a backfill during the construction of slurry
trench wall,• As a grout material• As a sealent for piozometer installation• Some other special applications.
BentoniteThe term "bentonite" is ambiguous(confusing, not clear).
As defined by geologists,
it is a rock formed of:• highly colloidal and • plastic clays
composed mainly of montmorillonite, a clay mineral of the smectite group.
It is produced by in situ devitrification of volcanic ash.
Devitrification: to change from a vitreous(glassy) state to a crystalline state
or to cause a glassy material to
become crystalline and brittle.
The transformation of ash to bentonite apparently takes place only in water:
(certainly seawater, probably alkaline lakes, and possibly other fresh water)
during or after deposition.
By extension, the term bentonite is
applied commercially to any plastic,
colloidal, and swelling clay regardless of its geological origin.
Such clays are ordinarily composed largely of minerals of the montmorillonite group.
Bentonite is • a rock or • a clay base industrial material.
It is therefore a mixture of minerals.
No "molecular" formula can be given.
BENTONITE
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montmorillonite family
used as drilling mud, in slurry trench walls, stopping leaks.
There are different types of bentonite, each named after the respective dominant element, such as:
• potassium (K) bentonite, • sodium (Na) bentonite, • calcium (Ca) bentonite.
Sodium bentonite
Sodium bentonite expands when wet, absorbing as much as several times its dry mass in water.
Because of its excellent colloidalproperties, it is often used in drilling mudfor oil and gas wells and for geotechnical and environmental investigations.
The property of swelling also makes sodium bentonite useful as:
a sealant, especially for the sealing of subsurface disposal systems
for spent nuclear fuel and for isolating metal pollutants of groundwater.
Sodium bentonite can also be "sandwiched" between synthetic materials to create geo-synthetic clay liners (GCL).
Similar uses include making:• slurry walls, • waterproofing of below-grade walls,
and • forming other impermeable barriers,
e.g., to seal off the annulus of a water well, to plug old wells, or to line the base of landfills to prevent migration of leachate.
2:1 The Micalike Clay MineralsMuscovite and Illite (white micas)
= 10 A˚
ILLITE
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Si
Al
Si
Si
Al
Si
Si
Al
Si
0.96 nm
joined by K+ ions
fit into the hexagonal holes in Si-sheet
• Bonding: by potassium (K) ions.
• Radius of K ion is 1.33 A˚.
• K fits in those holes and it helps balancing the
charge defficiency also.(OH)4K2(Si6Al2)Al4O20
• Muscovite and illite (white mica):¼ of Si by Al balanced by K.
The cation exchange capacity (CEC) of illite is smaller than that of smectite but higher than that of kaolinite, typically around 20 – 30 meq/100 g.
• Phlogopite (brown mica)with trioctahedral units of Mg.
(OH)4K2(Si6Al2)Mg6O20
• Biotites (Black micas)with trioctahedral units of Mg and Fe.
(OH)4K2(Si6Al2)(MgFe)6O20
CEC: 10-40 meq/100g (If no K→ 150 meq/100g)
Specific surface: 65-100 m2/g (illite)
VermiculateThe second most unstable mineral
10 to 14 A°
Brucite
Double molecular layer of water with cations.
It consists a regular interstratification of biotite mica layers and double molecular layer of water with cations.
(OH)4(MgCa)x(Si8‐xAlx)(MgFe)6O20y.H2Ox~ 1.0‐1.4y ~ 8
CEC: 100‐150 meq/100g
Primary surface: 40‐80 m2/gSecondary surface: 870 m2/g
2:1:1 The Chlorite Minerals
Brucite
2:1:1 The Chlorite Minerals
Structure is similar to vermiculate instead of water molecules,
brucite layer appears, better bonding.
CEC: 10-40 meq/100g
Platy morphology, occurs in mixture with other clay minerals.
Secondary mineral Type Interlayer condition /
BondingCEC [cmol/kg]
Swelling potential
Specific surface area [m2/g]
Basal spacing [nm]
Kaolinite 1 : 1 (non-expanding)
lack of interlayer surface, strong bonding
3 - 15 almost none 5 - 20 0.72
Montmorillonite
2 : 1 (expanding)
very weak bonding, great expansion 80 - 150 high 700 - 800 0.98 - 1.8
+
Vermiculite 2 : 1 (expanding)
weak bonding, great expansion 100 -150 high 500 - 700 1.0 - 1.5 +
Hydrous Mica
2 : 1 (non-expanding)
partial loss of K, strong bonding 10 - 40 low 50 - 200 1.0
Chlorite 2 : 1 : 1 (non-expanding)
moderate to strong bonding, non-expanding
10 - 40 none 1.4
Allophane - - 10 - 50 - -
Chain Structure Clay MineralsThey are formed from bands of silica
tetrahedra.
Example: attapulgite, sepiolite and palygorcite.
They have lathelike shape with particle diameter of 50-100 A° and lengths of 4-5 µm.
Quite rarely found.
Mixed Layer Clays• Interstratification of 2 or more different
layer types often occurs within a single particle.
• Interstratification may be regular or random.
The most abundant mixed-layer material is composed of expanded water bearing layers and contracted nonwater bearing layer.
• Montmorillonite-----Illite is the most common.• Chlorite------Vermiculate (often encountered)
• Chlorite------Montmorillonite (often encountered)
Non‐Crystalline Clay Materials• Allophane : noncrystalline silicate clay
materials (amorphous to X-rays).
• They have no definite composition or shape.
• They have a wide range of physical properties.
• They are particularly common in soils derived from volcanic ash.
Oxides • The oxides and hydroxides of Al, Fe and Si are
the ones most frequently encountered.
• They may occur as gels or precipitates and coat mineral particles or they may cement the particles together.
They may also occur as crystalline units as: • gibbsite, • hematite and magnetite.
Limonite [Fe(OH)2] and Bauxite Al(OH3) → amorphous mixture of Fe and Al hydroxides.
