Weathering -II

THE CHEMICAL INDEX OF ALTERATION

It is predominantly feldspars that weather to clays. We can thus base a measure of the degree of weathering on how far the composition is from that of an ideal feldspar.

During weathering, Al and Fe are insoluble as oxides or oxyhydroxides. Other cations and Si are quite soluble.

The concentrations are in molecular proportions. CaO* is CaO in silicates (excluding that in carbonates and phosphates).

100OKONaCaOOAl

OAlCIA

22*

32

32

• CIA values of 100% are typical of heavily leached materials such as topical laterites and bauxites.

• Kaolinite and gibbsite occur in well-drained, heavily leached soils.

• Smectites form in poorly drained soils.

Figure 19.1: A. Variation of the chemical composition of saprolites representing increasing intensity of chemical weathering of granitic gneisses from Minnesota. B. Variation of the measured abundances of minerals in the saprolites shown above.From Faure (1997).

SOLID PRODUCTS OF WEATHERING

The final stable products of weathering consist of quartz and clay minerals.

Clay minerals: Hydrous sheet silicates (phyllosilicates) with a grain size < 4 m.

Clays are constructed of two major structural components:

1) Sheets of SiO44- tetrahedra sharing three

oxygens with neighbors.

2) Sheets of Al, Fe and/or Mg in octahedral coordination with O2- and/or OH-.

Clay minerals : fine grained (<0.002mm) sheet silicate minerals which form as a result of weathering of other silicates.

a

OT

OT

7Å7Å

a

K+

OT

T

OT

TT

10Å

a

Na+. Ca 2+.H2O

OT

T

OT

TT

10Å

Kaolinite Al2Si2O5(OH)4

Illite K0.8Al2(Al0.8Si3.2)(OH)2

Smectite Ca0.17(Al,Mg,Fe)2

(Si,Al)4O10 (OH)2.nH2O

Clay minerals II

a

Na+. Ca 2+.H2O

OT

T

OT

TT

10Å

Vermiculite (Mg,Ca)0.3 (Al,Mg,Fe2+,Fe3+ )3 (Si,Al)4O10 (OH)2.nH2O

a

OT

TT

14Å

OT

T

O

Chlorite (Mg, Fe, Al)3 (Si,Al)4O10 (OH)2.(Mg,Fe,Al)3(OH)6

DIOCTAHEDRAL VS. TRIOCTAHEDRAL

Dioctahedral - Only two out of three octahedral sites are occupied by trivalent ions.

Trioctahedral - All three out of three octahedral sites occupied by a divalent ion.

1:1 CLAY MINERALS

Dioctahedral - Kaolinite Group

Kaolinite - Al2Si2O5(OH)4

1) Cations cannot get between layers.

2) Solid solution is limited.

octahedral sheet

tetrahedral sheetcovalent bonds

Trioctahedral – Serpentine Group

The 1:1 (T-O) layer silicates

aaa

{{

tetrahedralsheet

octahedralsheet

e.g. kaolinite (dioctahedral) serpentine (trioctahedral)

T

O

7Å

e.g. kaolinite (dioctahedral) serpentine (trioctahedral)

Al2Si2O5(OH)4 Mg3Si2O5(OH)4

2:1 CLAY MINERALS

micas, illite, smectite, chlorite

solid solution is quite common in the 2:1 clays.

octahedral sheet

tetrahedral sheet

tetrahedral sheet

The 2:1 (T-O-T) layer silicates

interlayer

(ii) with interlayer ions

aa

(ii) with cations (e.g. K+) ininterlayer sites (c = 10Å):muscovite (dioctahedral)phlogopite (trioctahedral)

T

O

T

{{

tetrahedralsheet

octahedralsheet

{tetrahedralsheet

e.g. muscovite (dioctahedral) phlogopite (trioctahedral)

KAl2(AlSi3O10)(OH)2 KMg3(AlSi3O10)(OH)2

ILLITE

Illite - A general term to describe clay-size, mica-type minerals. Generally the composition is similar to muscovite.

• One out of four Si4+ ions are replaced by Al3+ in the tetrahedral sheet. This leads to a strong net negative charge.

• Some octahedral Al3+ may be replaced by Fe2+ and Mg2+, which also leads to net negative charge.

• The charge is neutralized by large cations, usually K+, in the interlayer spaces.

ILLITE STRUCTURE

K+ K+K+K+

Interlayer sites filled with K+. Strongly bonded, so cations cannot easily exchange with K+.

tetrahedral

tetrahedral

tetrahedral

tetrahedral

octahedral

octahedral

SMECTITE

Smectite - similar structurally to illite. However, the 2:1 units are not as tightly bound. Water can penetrate the interlayer sites, causing them to swell. Cations such as H+, Na+, Ca2+ and Mg2+ also can enter the interlayer sites.

Thus, the weak interlayer bonding makes smectites prone to replacement by other cations. This leads to a high cation exchange capacity (CEC).

ION EXCHANGE

Clay-OH + K+ Clay-OK + H+

Clays (smectites) can hold ions both on their surfaces, on their edges, and in interlayer sites.

Clays can be used as adsorbents, e.g., as backfill in nuclear waste repositories.

Natural clays in groundwater aquifers retard the migration of pollutants by adsorption.

Clay surfaces may act as catalysts.

The 2:1 (T-O-T) layer silicates

interlayer

(iii) with interlayer ions and H2O

aa

(iii) with cations + H 2O ininterlayer sites (c = 15Å):montmorillonite (dioctahedral)vermiculite (trioctahedral)

T

O

T

{{

tetrahedralsheet

octahedralsheet

{tetrahedralsheet

Compositions of 2:1:1 Layer silicates The most common mineral in this group is chlorite which can be thought of in its simplest form as a TOT talc layer with a brucite sheet in between, i.e. Chlorite - trioctahedral - Mg3Si4O10(OH)2•Mg3(OH)6

The 2:1:1 (T-O-T-o) layer silicates

interlayer

with an octahedral sheet between the T-O-T layers

aa

{{

tetrahedralsheet

octahedralsheet

{tetrahedralsheet

{octahedralsheet

T

O

T

e.g. chlorite

MAJOR CLAY MINERAL GROUPS

Group LayerType

LayerCharge (x)

Typical Chemical Formulaa

Kaolinite 1:1 <0.01 [Si4]Al4O10(OH)8·nH2O)

Illite 2:1 1.4-2.2 Mx[Si6.8Al1.2]Al3Fe0.25Mg0.75O20(OH)4

Vermiculite 2:1 1.2-2.0 Mx[Si7Al]Al3Fe0.5Mg0.5O20(OH)4

Smectite 2:1 0.5-1.2 Mx[Si8]Al3.2Fe0.2Mg0.6O20(OH)4

Chlorite 2:1 withhydroxideinterlayer

Variable (Al(OH)2.55)4·[Si6.8Al1.2]Al3.4Mg0.6 O20(OH)4

an = 0 is kaolinite and n = 4 is halloysite; M = monovalent interlayer cation.

ION EXCHANGE

Clay-OH + K+ Clay-OK + H+

Clays (smectites) can hold ions both on their surfaces, on their edges, and in interlayer sites.

Clays can be used as adsorbents, e.g., as backfill in nuclear waste repositories.

Natural clays in groundwater aquifers retard the migration of pollutants by adsorption.

Clay surfaces may act as catalysts.