Clay Chemistry

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Clay Chemistry

• Dr. Bakhtyar Kamal Aziz

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Definition

• There is, as yet, no uniform nomenclature for
  clay and clay material
• In General, clay imply a natural, earthy, fine
  grained material which develops plasticity when
  mixed with a limited amount of water.
• As a particle-size term, the clay fraction is
  that size fraction composed of the smallest
  particles. In soil investigations, the tendency
  is to use 2 µ as the upper limit of the particle
  size grade,
• Clay is composed essentially of silica, alumina
  and water. Lesser quantities of iron, magnesium,
  sodium and potassium.
• There are over 400 mineral and rock names to
  describe clay minerals.

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• Clay is present almost everywhere, flying up in
  the air as dust particles, covering the surface
  of the earth as part of the soils, and below the
  surface as part of sedimentary rocks. Clay is
  mainly formed through the process of weathering
  of primary silicate minerals such as feldspars.
• The characteristics of clay deposits are depended
  on-
• 1- The source rocks
• 2-The weathering processes
• 3- Transportation and the environmental conditions

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Elements of Earth
by weight in crust
O 49.2Si 25.7Al 7.5Fe 4.7Ca
3.4Na 2.6K 2.4Mg 1.9other 2.6
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Soil Formation
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
metamorphic
sedimentary
igneous
formed by gradual deposition of weathered igneous
rocks
formed by geological alteration of igneous
sedimentary rocks by pressure/temperature
formed by cooling and solidification of molten
magma (lava)
e.g., limestone, shale
e.g., granite
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 Special name

• wind Aeolian
• sea (salt water) Marine
• lake (fresh water) Lacustrine
• river Alluvial
• ice Glacial

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PHYLLOSILICATES
Repeat formula (Si2O5)2-n
Basic tetrahedral unit. The Si - O combination
has a radius ratio of 0.30, which means that the
silicon ion fits nicely into a tetrahedral
polyhedron.
Silicon ion shares its charge equally between the
four oxygen ions, leaving each oxygen with an
excess charge of negative one.
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PHYLLOSILICATES
If each of the four oxygen ions bond with two
silicon ions the result is a QUARTZ crystal. In
the phyllosilicates only one plane of oxygen ions
bond with two silicon ions. This bonding is
extended in two directions to form a sheet of
silicon tetrahedrons. This sheet of tetrahedral
units with unbalanced charges on the apical O
ions.
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PHYLLO (layer, sheet) SILICATES
Repeat formula (Si2O5)2-n
The result is the creation of an infinite,
2-dimensional sheet of tetrahedra.
The closest packing arrangement for spheres in
two dimensions is a plane having hexagonal
symmetry. If we examine any one sphere in the
hexagonal closest packing arrangement, we can see
that there are 6 interstitial spaces or holes
contained within each hexagonal ring.
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The basic structure of clay minerals can be
obtained though the stacking of two sheets
tetrahedral sheets and octahedral sheets sometime
separated by an interlayer. Different clay
minerals are formed by (1) different combination
of these two units and the interlayer and (2)
changes in the composition of the sheets The
tetrahedral sheet The basic unit of this layer
is a tetrahedron, which contains normally one
Si4 in the centre with four O2- at the corners.
The tetrahedra are linked to neighboring
tetrahedra by sharing three oxygen atoms each to
form a hexagonal mesh pattern. All the unshared
corners with the apical oxygen atoms point in the
same direction to form part of the adjacent
octahedral sheet
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The octahedral sheet The unit is an octahedron,
contains mainly Al3 or Mg2 surrounded by six
oxygen atoms or hydroxyl groups. When the cations
are trivalent, the sheet contains two cations per
half unit cell and one vacancy, and is known as a
dioctahedral structure. When the cations are
divalent, the sheet contains three cations per
half unit cell and no vacancy, and is known as a
trioctahedral structure. Octahedral sheets can
contain other cations including Li , Fe2 , and
Fe3
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PHYLLOSILICATES

• Aluminum shares 0.5 of its charge with each of
  the surrounding oxygen ions, leaving each oxygen
  ion with a negative 1.5 charge.
• In this case aluminum is slightly less
  electropositive than is silicon and is able to
  approach close enough that corner oxygen ions can
  be shared. In a matrix of these octahedral units
  each oxygen will be bonded to two aluminum ions,
  leaving it with a remaining -1 charge.

The charge can be satisfied by attaching a proton
(hydrogen ion) and when this type of structure is
continued in three dimensions we have the mineral
GIBBSITE
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PHYLLOSILICATES
Remember we left a sheet of octahedral units with
apical oxygen ions still having an unbalanced
charge? Option2 The two sheets can be brought
together with the apical oxygen ions of the
tetrahedral layer also being in the octahedral
layer. As a result, the charge on these oxygen
ions is balanced by bonding to one silicon ion
and two aluminum ions.
Ex. Kaolinite 11 clay nonexpandable
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PHYLLOSILICATES
The Si-O tetrahedral linkage represents the
strongest bonds within the silicate structures
and, therefore, tends to dominate other bonding
linkages with respect to stability and general
properties of the silicate minerals.
How many types of Phyllosilicates exist? How do
they differ?
It is the combination of various tetrahedral and
octahedral sheets which distinguishes the
individual members of the phyllosilicate minerals
and also the presence of various elements in the
structures.
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• The fundamental units of tetrahedral sheets and
  octahedral sheets can combine with the hydroxyl
  group of the tetrahedral layer contributing to
  the octahedral layer.
• Different combinations of these units and
  chemical modification of the basic structure give
  rise to the range of clay minerals with different
  properties.

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Tetrahedral Octahedral Sheets
For simplicity, lets represent silica
tetrahedral sheet by
Si
and alumina octahedral sheet by
Al
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Typically 70-100 layers
Typically 70-100 layers
Typically 70-100 layers
Typically 70-100 layers
Typically 70-100 layers
Typically 70-100 layers
Typically 70-100 layers
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Trioctahedral vs. Dioctahedral Octahedral sheets
may have either divalent cations such as Mg2 and
Fe2, or trivalent cations such as Al3 and Fe3.
Obviously, charge balance must still be
maintained. Therefore, the structure of
octahedral sheets having divalent cations differs
from that of octahedral sheets having trivalent
cations.
In general, Al-OH and Mg-OH give each other
neutrality. Macrocrystals (for ex. in Brucite)
form when van der Waals force links together OH
of adjacent layers.
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Isomorphous Substitution
One ion can substitute one other both in
tetrahedral and in octahedral sheet
What if a 2 substitutes for a 3 in octahedral
sheet?
In dioctahedral phyllosilicates, where 2 charge
cations substitute for 3 cations, thereby
producing a net negative charge on the layer
structure.
What happens in tetrahedral sheets?
(in these structures typically we have Si4).
However, Al3 commonly substitutes for Si in the
tetrahedral sheet, producing a negative charge.
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Isomorphous Substitution
21 phyllosilicate minerals may have charges
derived from substitutions in the tetrahedral
layer, substitutions in the octahedral layer, or
both. The relative amount of tetrahedral and
octahedral charge is the basis for part of the
classification system for smectites
If this type of substitution occurs without
appreciably altering the structure of the
mineral, it is referred to as isomorphous
substitution or atom proxying.
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Isomorphous Substitution
Al3 and Mg2 ions are not the only ones that can
occupy octahedral sheet positions. Fe2, Fe3,
Mn2, Zn2, Cu2, Ti4, Li, Cr3, and numerous
others can substitute in the octahedral sheet.
Substitutions in both the tetrahedral and
octahedral sheets produce permanent charges.
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When do isomorphous substitutions occur?
Typically during the nucleation and growth of
crystals. Rare during weathering and only at the
very surface of the mineral.
Is it easy to move an ion in or out of an intact
silicate structure?
NO! It requires high energy and significant
structural damage would occur. Isomorphous
substitution may involve ions of different
valences. The only valence requirement is that
the electrical neutrality of the structure must
be maintained on a local scale. This does not
imply at an atom by atom scale, but
neutralization must occur within a few Å.
Consequently, the proxying of ions of different
charge leads to the creation of permanent charge
within a structure.
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Variable charges
A second source of charge on the minerals is the
broken bonds found at the mineral edges. The
structure cannot extend infinitely, so at some
point there will be oxygens without all charges
satisfied by associating with cations. In these
cases a hydrogen ion from solution will normally
satisfy the requirement. Whether this can occur
will, however, depend on the solution pH.
Therefore, these charges are called either
pH-dependent charge or variable charge.
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Clay Mineral Groups

• There are over 400 mineral and rock names to
  describe clay minerals. We will restrict our
  attention only to a few minerals that are most
  common and most applicable minerals to petroleum
  technology
• Montmorillonite
• Kaolin
• Micas
• Chlorite

• These clay minerals are built up by different
  ratios of silica layer to octahedral layer.
• The most important groups is
• 21 layer
• 211 layer
• 11 layer

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Clay minerals
211 clay (two tetrahedral sheets for each
octahedral sheet Brucite sheet)
21 clays (two tetrahedral sheets for each
octahedral sheet)
Smectites
Micas
Vermiculites
Chlorites
Montmorillonite,beidellite, saponite, etc.
Illite, muscovite, biotite, etc.
Cookeite, chamosite, etc.
Tri- or di-vermiculite
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11 Clay Minerals

• Like an open face sandwich
• One silica tetrahedron (bread)
• One aluminum octahedron (filling)
• The most common 11 minerals is Kaolinite in
  which one tetrahedral and one octahedral sheet
  are the repeat unit with no layer charge. The
  interlayer is occupied by hydroxyl groups and
  oxygen atoms from the octahedral and tetrahedral
  sheets connected by weak hydrogen bonds. Members
  of this group can be dioctahedral such as
  kaolinite, containing Al and Si with no
  substitutions, or trioctahedral such as
  chrysotite containing Mg and Si

No interlayer cations or layer charge are present
in the kaolinite structure. The layers are
connected by Si-O-Al bonds
Kaolinite