N. Sivakugan

1. Clay Mineralogy N. Sivakugan Duration = 15 mins.

2. 8-35 km crust 82.4% 12500 km dia 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

3. Parent Rock 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.

4. Parent Rock ~ formed by one of these three different processes sedimentary igneous metamorphic formed by gradual deposition, and in layers formed by cooling of molten magma (lava) formed by alteration of igneous & sedimentary rocks by pressure/temperature e.g., limestone, shale e.g., granite e.g., marble

5. Residual Soils Formed by in situ weathering of parent rock

6. Transported Soils Transported by: Special name: • wind “Aeolian” • sea (salt water) “Marine” • lake (fresh water) “Lacustrine” • river “Alluvial” • ice “Glacial”

7. Atomic Structure

8. hydroxyl or oxygen oxygen aluminium or magnesium silicon 0.26 nm 0.29 nm Basic Structural Units Clay minerals are made of two distinct structural units. Silicon tetrahedron Aluminium Octahedron

9. tetrahedron hexagonal hole Tetrahedral Sheet Several tetrahedrons joined together form a tetrahedral sheet.

10. Tetrahedral & Octahedral Sheets For simplicity, let’s represent silica tetrahedral sheet by: Si and alumina octahedral sheet by: Al

11. Different Clay Minerals Different combinations of tetrahedral and octahedral sheets form different clay minerals: 1:1 Clay Mineral (e.g., kaolinite, halloysite):

12. Different Clay Minerals Different combinations of tetrahedral and octahedral sheets form different clay minerals: 2:1 Clay Mineral (e.g., montmorillonite, illite)

13. 0.72 nm joined by strong H-bondno easy separation Al Al Al Al joined by oxygen sharing Si Si Si Si Kaolinite Typically 70-100 layers

14. Kaolinite • used in paints, paper and in pottery and pharmaceutical industries • (OH)8Al4Si4O10 Halloysite • kaolinite family; hydrated and tubular structure • (OH)8Al4Si4O10.4H2O

15. 0.96 nm Si Si Si Al Al Al joined by weakvan der Waal’s bond Si Si Si Montmorillonite • also called smectite; expands on contact with water easily separated by water

16. swells on contact with water Montmorillonite • A highly reactive (expansive) clay • (OH)4Al4Si8O20.nH2O Bentonite high affinity to water • montmorillonite family • used as drilling mud, in slurry trench walls, stopping leaks

17. joined by K+ions 0.96 nm Si Si Si Al Al Al Si Si Si Illite fit into the hexagonal holes in Si-sheet

18. Si Al or Mg Al Others… Chlorite • A 2:1:1 (???) mineral. Vermiculite • montmorillonite family; 2 interlayers of water Attapulgite • chain structure (no sheets); needle-like appearance

19. Plate-like or Flaky Shape A Clay Particle

20. Clay Fabric edge-to-face contact face-to-face contact Dispersed Flocculated

21. Clay Fabric • Electrochemical environment (i.e., pH, acidity, temperature, cations present in the water) during the time of sedimentation influence clay fabric significantly. • Clay particles tend to align perpendicular to the load applied on them.

22. Identifying Clay Minerals

23. plate-like structure Scanning Electron Microscope • common technique to see clay particles • qualitative

24. Others… X-Ray Diffraction (XRD) • to identify the molecular structure and minerals present Differential Thermal Analysis (DTA) • to identify the minerals present

25. montmorillonite illite kaolinite halloysite chlorite Casagrande’s PI-LL Chart

26. Special Terms

27. 1 mm cube 10 mm cube Specific Surface • surface area per unit mass (m2/g) • smaller the grain, higher the specific surface e.g., soil grain with specific gravity of 2.7 spec. surface = 222.2 mm2/g spec. surface = 2222.2 mm2/g

28. positively charged edges + + + + _ + _ negatively charged faces _ _ _ _ + _ _ _ _ _ + _ _ _ _ _ _ _ _ _ _ _ _ Isomorphous Substitution • substitution of Si4+ and Al3+ by other lower valence (e.g., Mg2+) cations • results in charge imbalance (net negative) Clay Particle with Net negative Charge

29. known as exchangeable cations milliequivalents Cation Exchange Capacity (c.e.c) • capacity to attract cations from the water (i.e., measure of the net negative charge of the clay particle) • measured in meq/100g (net negative charge per 100 g of clay) • The replacement power is greater for higher valence and larger cations. Al3+ > Ca2+ > Mg2+ >> NH4+ > K+ > H+ > Na+ > Li+

30. A Comparison

31. + + clay particle + - -- -- -- -- -- -- - + + + cations + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + double layer Cation Concentration in Water • cation concentration drops with distance from clay particle free water

32. - -- -- -- -- -- -- - adsorbed water Adsorbed Water • A thin layer of water tightly held to particle; like a skin • 1-4 molecules of water (1 nm) thick • more viscous than free water

33. adsorbed water - -- -- -- -- -- -- - 1nm 50 nm Clay Particle in Water free water double layerwater

34. Practical Significance

35. Summary - Clays • Clay particles are like plates or needles. They are negatively charged. • Clays are plastic; Silts, sands and gravels are non-plastic. • Clays exhibit high dry strength and slow dilatancy.

36. Summary - Montmorillonite • Montmorillonites have very high specific surface, cation exchange capacity, and affinity to water. They form reactive clays. • Montmorillonites have very high liquid limit (100+), plasticity index and activity (1-7). • Bentonite (a form of Montmorillonite) is frequently used as drilling mud.