clay chemistry(part 1)

8/12/2019 Clay Chemistry(Part 1)

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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 thesmallest particles. In soil investigations, the tendency is to use 2 as the upper

limit of the particle size grade,

Clay is composed essentially of silica, alumina and water. Lesser quantities ofiron, magnesium, sodium and potassium.

There are over 400 mineral and rock names to describe clay minerals.

Definition


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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

12500 km dia

8-35 km crust% by weight in crust

O = 49.2

Si = 25.7Al = 7.5

Fe = 4.7

Ca = 3.4

Na = 2.6K = 2.4

Mg = 1.9

other = 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 andtransportedfar away

by wind, water and ice.


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Parent Rock

~ formed by one of these threedifferent processes

igneous sedimentary metamorphic

formed by cooling and

solidification of moltenmagma (lava)

formed by gradual

deposition of weatheredigneous rocks

formed by geological

alteration of igneous& sedimentary rocks

by

pressure/temperaturee.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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PHYLLOSILIC TES

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.

Repeat formula: (Si2O5)2-

n


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If each of the four oxygen ions bond with two silicon ions the result is a QUARTZ

crystal.

In the phyllosilicatesonly one plane of oxygen ions bond with two silicon ions.This bonding is extended in two directions to form a sheet of silicontetrahedrons. This sheet of tetrahedral units with unbalanced charges on theapical O ions.

PHYLLOSILICATES


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PHYLLO (layer, sheet) SILICATES

The result is the creation of an infinite, 2-dimensional sheet of tetrahedra.

Repeat formula: (Si2O5)2-

n

The closest packing arrangement for spheres in two dimensions is a planehaving hexagonal symmetry. If we examine any one sphere in thehexagonal closest packing arrangement, we can see that there are 6interstitial 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 twosheets: tetrahedral sheets and octahedral sheets sometime separated by aninterlayer. Different clay minerals are formed by: (1) different combination ofthese two units and the interlayer and (2) changes in the composition of thesheets

The tetrahedral sheetThe basic unit of this layer is a tetrahedron, which contains normally one Si4+inthe centre with four O2-at the corners. The tetrahedra are linked to neighboringtetrahedra by sharing three oxygen atoms each to form a hexagonal meshpattern. All the unshared corners with the apical oxygen atoms point in the

same direction to form part of the adjacent octahedral sheet

0.26 nm

oxygen

silicon

tetrahedr

on


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The octahedral sheetThe unit is an octahedron, contains mainly Al3+or Mg2+surrounded by sixoxygen atoms or hydroxyl groups. When the cations are trivalent, the sheetcontains two cations per half unit cell and one vacancy, and is known as adioctahedral structure. When the cations are divalent, the sheet contains threecations per half unit cell and no vacancy, and is known as a trioctahedralstructure. Octahedral sheets can contain other cations including Li+, Fe2+, andFe3+

0.29 nm

aluminium or

magnesium

hydroxyl or

oxygen


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Aluminum shares +0.5 of its charge with each of the surrounding oxygenions, leaving each oxygen ion with a negative 1.5 charge.

In this case aluminum is slightly less electropositive than is silicon and isable to approach close enough that corner oxygen ions can be shared. In amatrix of these octahedral units each oxygen will be bonded to twoaluminum ions, leaving it with a remaining -1 charge.

PHYLLOSILICATES

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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Remember we left a sheet of octahedral units with apical oxygen ions still having anunbalanced charge?

Option2:

The two sheets can be brought together with the apical oxygen ions of thetetrahedral layer also being in the octahedral layer. As a result, the charge on theseoxygen ions is balanced by bonding to one silicon ion and two aluminum ions.

PHYLLOSILICATES

Ex.Kaolinite

1:1 clay

nonexpandable


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PHYLLOSILICATES

The Si-O tetrahedral linkagerepresents 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.

It is the combination of various tetrahedral and

octahedral sheetswhich distinguishes the individualmembers of the phyllosilicate minerals and also the

presence of various elementsin the structures.

How many types of Phyllosilicates exist?How do they differ?


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Tetrahedron and octahedral

units

Layers

Structure

The fundamental units of tetrahedral sheets and octahedral sheets cancombine with the hydroxyl group of the tetrahedral layer contributing to theoctahedral layer.

Different combinations of these units and chemical modification of the basicstructure give rise to the range of clay minerals with different properties.


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Tetrahedral & Octahedral Sheets

For simplicity, lets represent silica tetrahedral sheetby:

Si

and alumina octahedral sheetby:

Al


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Si

Al

SiAl

Si

Al

Si

Al

joined by strong H-bond

no easy separation

0.72 nm

Typically

70-100

layers

joined by oxyge

sharing


no easy separation

Typically

70-100

layers

Si

Al


no easy separation

Typically

70-100

layers Si

Al

Si

Al


no easy separation

Typically

70-100

layers

SiAl

Si

Al

Si

Al


no easy separation

Typically

70-100

layers

Si

Al

SiAl

Si

Al

Si

Al


no easy separation

Typically

70-100

layers

joined by oxyge

sharing

Si

Al

Si

Al

Si

Al

Si

Al


no easy separation

Typically

70-100

layers0.72 nm

joined by oxyge

sharing

Si

Al

Si

Al

Si

Al

Si

Al


no easy separation

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 inoctahedral sheet

In dioctahedral phyllosilicates, where +2 charge cations substitutefor +3 cations, thereby producing a net negative charge on thelayer structure.

What if a 2+ substitutes for a 3+ in octahedral sheet?

(in these structures typically we have Si4+). However, Al3+commonly substitutes for Si in the tetrahedral sheet, producing a

negative charge.

What happens in tetrahedral sheets?


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2:1 phyllosilicate minerals may have charges derived from

substitutions in the tetrahedrallayer, substitutions in theoctahedrallayer, or both. The relative amount of tetrahedral andoctahedral charge is the basis for part of the classificationsystem for smectites


If this type of substitution occurs withoutappreciably alteringthe structure of the mineral, it is referred to as isomorphoussubstitutionor atom proxying.


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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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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 bemaintained on a local scale. This does not imply at an atom by atom scale, but

neutralization must occur within a few .

Consequently, the proxying of ions of different charge leads to the creation of

permanent chargewithin a structure.

When do isomorphous substitutions occur?

Typically during the nucleation and growth of crystals.

Rare during weathering and only at the very surface of themineral.

Is it easy to move an ion in or out of an intact silicate structure?


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A second source of charge on the minerals is thebroken bonds found at the mineral edges. Thestructure cannot extend infinitely, so at some pointthere will be oxygens without all charges satisfiedby associating with cations.

In these cases a hydrogen ion from solution willnormally satisfy the requirement. Whether this canoccur will, however, depend on the solution pH.

Therefore, these charges are called eitherpH-dependent charge or variable charge.

Variable charges


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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 tooctahedral layer.

The most important groups is:2:1 layer

2:1:1 layer

1:1 layer


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Clay minerals

1:1 clays(one tetrahedralsheet for each

octahedral sheet)

Kaolinite,nacrite, dickite,halloysite, etc.

2:1 clays(two tetrahedralsheets for each

octahedral sheet)

Montmorillonite,beidellite,

saponite, etc.

Illite,muscovite,

biotite, etc.

Tri- or di-vermiculite

Cookeite,chamosite,

etc.

Smectites Micas Vermiculites Chlorites

2:1:1 clay(two tetrahedralsheets for eachoctahedral sheet

+ Brucite sheet)

C


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1:1 Clay Minerals Like an open face sandwich

One silica tetrahedron (bread) One aluminum octahedron (filling)

The most common 1:1 minerals is Kaolinite in which one tetrahedral and oneoctahedral sheet are the repeat unit with no layer charge. The interlayer isoccupied by hydroxyl groups and oxygen atoms from the octahedral andtetrahedral sheets connected by weak hydrogen bonds. Members of thisgroup can be dioctahedral such as kaolinite, containing Al and Si with no

substitutions, or trioctahedral such as chrysotite containing Mg and Si

Kaolinite

No interlayer cations or layer charge are presentin the kaolinite structure. The layers areconnected by Si-O-Al bonds

clay chemistry(part 1)

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