1. Weathering -II

2. 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).

3. 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.

5. 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).

6. 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-.

7. Clay minerals : fine grained (<0
Clay minerals : fine grained (<0.002mm) sheet silicate minerals which form as a result of weathering of other silicates.
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

8. Clay minerals II
Vermiculite (Mg,Ca)0.3 (Al,Mg,Fe2+,Fe3+ )3 (Si,Al)4O10 (OH)2.nH2O
Chlorite (Mg, Fe, Al)3 (Si,Al)4O10 (OH)2.(Mg,Fe,Al)3(OH)6

9. 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.

10. Dioctahedral - Kaolinite Group
1:1 CLAY MINERALS
Trioctahedral – Serpentine Group
Dioctahedral - Kaolinite Group
Kaolinite - Al2Si2O5(OH)4
1) Cations cannot get between layers.
2) Solid solution is limited.
octahedral sheet
covalent bonds
tetrahedral sheet

11. The 1:1 (T-O) layer silicates7Å
e.g. kaolinite (dioctahedral) serpentine (trioctahedral)
Al2Si2O5(OH) Mg3Si2O5(OH)4

12. 2:1 CLAY MINERALS micas, illite, smectite, chlorite
solid solution is quite common in the 2:1 clays.
tetrahedral sheet
octahedral sheet
tetrahedral sheet

13. The 2:1 (T-O-T) layer silicates
interlayer
(ii) with interlayer ions
e.g. muscovite (dioctahedral) phlogopite (trioctahedral)
KAl2(AlSi3O10)(OH) KMg3(AlSi3O10)(OH)2

14. 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.

15. ILLITE STRUCTURE tetrahedral octahedral
Interlayer sites filled with K+. Strongly bonded, so cations cannot easily exchange with K+.
tetrahedral K+ K+ K+ K+
tetrahedral
In illite, although interlayer cations exist, the interlayer bonding is so strong that other cations cannot work their way into the interlayer and exchange with potassium ion. So the ability of illite to adsorb contaminant cations is higher than kaolinite, but not as high as clays in which the interlayer cations can be exchanged.
octahedral
tetrahedral

16. 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).

17. Clay-OH + K+  Clay-OK + H+
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.

18. The 2:1 (T-O-T) layer silicates
interlayer
(iii) with interlayer ions and H2O

19. 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

20. The 2:1:1 (T-O-T-o) layer silicates
interlayer
with an octahedral sheet between the T-O-T layers
e.g. chlorite

21. MAJOR CLAY MINERAL GROUPS
This table outlines the properties of five major clay mineral groups. The column titled “Layer Type” refers to the proportion of octahedral to tetrahedral sites in the structure. In the 1:1 layer type, the basic structural unit is composed of one tetrahedral sheet and one octahedral sheet that are tightly bound to one another. This basic unit is then repeated indefinitely. In the 2:1 layer type, we have one octahedral sheet sandwiched between two tetrahedral sheets as the basic structural unit.
In some clay mineral groups, Al3+ may substitute for Si4+ in the tetrahedral sheets, and some divalent metal ions may substitute for Al3+ in the octahedral sheets. This results in a charge imbalance, with the layers having an excess negative charge. This negative charge can be balanced by interlayer cations such as NH4+, K+ and Na+. The column titled “Layer Charge” gives the amount of excess negative charge per formula unit that must be balanced by interlayer cations.
Finally, the last column gives the chemical formula for each clay type, with some structural information. For example, the tetrahedral cations are given in square brackets, the interlayer cations are given in front of the square brackets, and the octahedral cations after the square brackets.
Chlorite has an additional twist. Together with the tetrahedral-octahedral-tetrahedral sandwich, there is an aluminum hydroxide interlayer.
an = 0 is kaolinite and n = 4 is halloysite; M = monovalent interlayer cation.

22. Clay-OH + K+  Clay-OK + H+
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.