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Weathering -II

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.

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Weathering -II

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  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.

  4. 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).

  5. 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-.

  6. 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

  7. 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

  8. 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.

  9. 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

  10. The 1:1 (T-O) layer silicates 7Å e.g. kaolinite (dioctahedral) serpentine (trioctahedral) Al2Si2O5(OH)4 Mg3Si2O5(OH)4

  11. 2:1 CLAY MINERALS micas, illite, smectite, chlorite solid solution is quite common in the 2:1 clays. tetrahedral sheet octahedral sheet tetrahedral sheet

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

  13. 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.

  14. ILLITE STRUCTURE tetrahedral octahedral Interlayer sites filled with K+. Strongly bonded, so cations cannot easily exchange with K+. tetrahedral K+ K+ K+ K+ tetrahedral octahedral tetrahedral

  15. 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).

  16. 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.

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

  18. 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

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

  20. MAJOR CLAY MINERAL GROUPS an = 0 is kaolinite and n = 4 is halloysite; M = monovalent interlayer cation.

  21. 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.

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