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Surface Chemistry

Surface Chemistry. SOIL MINERALS and SOIL STRUCTURE. Outline. Clay Minerals Identification of Clay Minerals Specific Surface (S s ) Cation Exchange Capacity (CEC). 1. Clay Minerals. 1.1 Origin of Clay Minerals.

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Surface Chemistry

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  1. Surface Chemistry SOIL MINERALS and SOIL STRUCTURE

  2. Outline • Clay Minerals • Identification of Clay Minerals • Specific Surface (Ss) • Cation Exchange Capacity (CEC)

  3. 1. Clay Minerals

  4. 1.1 Origin of Clay Minerals • “The contact of rocks and water produces clays, either at or near the surface of the earth” (from Velde, 1995). • Rock +Water  Clay • For example, • The CO2 gas can dissolve in water and form carbonic acid, which will become hydrogen ions H+ and bicarbonate ions, and make water slightly acidic. • CO2+H2O  H2CO3 H+ +HCO3- • The acidic water will react with the rock surfaces and tend to dissolve the K ion and silica from the feldspar. Finally, the feldspar is transformed into kaolinite. • Feldspar + hydrogen ions+water  clay (kaolinite) + cations, dissolved silica • 2KAlSi3O8+2H+ +H2O  Al2Si2O5(OH)4 + 2K+ +4SiO2 • Note that the hydrogen ion displaces the cations.

  5. 1.1 Origin of Clay Minerals (Cont.) • The alternation of feldspar into kaolinite is very common in the decomposed granite. • The clay minerals are common in the filling materials of joints and faults (fault gouge, seam) in the rock mass.

  6. 1.2 Basic Unit-Silica Tetrahedra (Si2O10)-4 1 Si 4 O Replace four Oxygen with hydroxyls or combine with positive union Tetrahedron Plural: Tetrahedra Hexagonal hole (Holtz and Kovacs, 1981)

  7. 1.2 Basic Unit-Octahedral Sheet 1 Cation 6 O or OH Gibbsite sheet: Al3+ Al2(OH)6, 2/3 cationic spaces are filled One OH is surrounded by 2 Al: Dioctahedral sheet Different cations Brucite sheet: Mg2+ Mg3(OH)6, all cationic spaces are filled One OH is surrounded by 3 Mg: Trioctahedral sheet (Holtz and Kovacs, 1981)

  8. 1.2 Basic Unit-Summary Mitchell, 1993

  9. 1.3 Synthesis Mitchell, 1993 Noncrystalline clay -allophane

  10. 1.4 1:1 Minerals-Kaolinite Basal spacing is 7.2 Å layer • Si4Al4O10(OH)8. Platy shape • The bonding between layers are van der Waals forces and hydrogen bonds (strong bonding). • There is no interlayer swelling • Width: 0.1~ 4m, Thickness: 0.05~2 m Trovey, 1971 ( from Mitchell, 1993) 17 m

  11. 1.4 1:1 Minerals-Halloysite • Si4Al4O10(OH)8·4H2O • A single layer of water between unit layers. • The basal spacing is 10.1 Å for hydrated halloysite and 7.2 Å for dehydrated halloysite. • If the temperature is over 50 °C or the relative humidity is lower than 50%, the hydrated halloysite will lose its interlayer water (Irfan, 1966). Note that this process is irreversible and will affect the results of soil classifications (GSD and Atterberg limits) and compaction tests. • There is no interlayer swelling. • Tubular shape while it is hydrated. Trovey, 1971 ( from Mitchell, 1993) 2 m

  12. 1.5 2:1 Minerals-Montmorillonite • Si8Al4O20(OH)4·nH2O (Theoretical unsubstituted). Film-like shape. • There is extensive isomorphous substitution for silicon and aluminum by other cations, which results in charge deficiencies of clay particles. • n·H2O and cations exist between unit layers, and the basal spacing is from 9.6 Å to  (after swelling). • The interlayer bonding is by van der Waals forces and by cations which balance charge deficiencies (weak bonding). • There exists interlayer swelling, which is very important to engineering practice (expansive clay). • Width: 1 or 2 m, Thickness: 10 Å~1/100 width n·H2O+cations 5 m (Holtz and Kovacs, 1981)

  13. 1.5 2:1 Minerals-Illite (mica-like minerals) • Si8(Al,Mg, Fe)4~6O20(OH)4·(K,H2O)2. Flaky shape. • The basic structure is very similar to the mica, so it is sometimes referred to as hydrous mica. Illite is the chief constituent in many shales. • Some of the Si4+ in the tetrahedral sheet are replaced by the Al3+, and some of the Al3+ in the octahedral sheet are substituted by the Mg2+ or Fe3+. Those are the origins of charge deficiencies. • The charge deficiency is balanced by the potassium ion between layers. Note that the potassium atom can exactly fit into the hexagonal hole in the tetrahedral sheet and form a strong interlayer bonding. • The basal spacing is fixed at 10 Å in the presence of polar liquids (no interlayer swelling). • Width: 0.1~ several m, Thickness: ~ 30 Å potassium K Trovey, 1971 ( from Mitchell, 1993) 7.5 m

  14. 1.5 2:1 Minerals-Vermiculite (micalike minerals) • The octahedral sheet is brucite. • The basal spacing is from 10 Å to 14 Å. • It contains exchangeable cations such as Ca2+ and Mg2+ and two layers of water within interlayers. • It can be an excellent insulation material after dehydrated. Illite Vermiculite Mitchell, 1993

  15. 1.6 2:1:1 Minerals-Chlorite • The basal spacing is fixed at 14 Å. Gibbsite or brucite

  16. Visual comparison of common silicate clays Strongly held “2:1:1” illite montmorillonite

  17. H-H = Layer bond type = Location of charge imbalance octa octahedral octahedral & tetrahedral octahedral & tetrahedral tetrahedral H-H more strongly held than in smectite H-H O-O O-Cation tetrahedral Ionic octahedral & tetrahedral NONE octahedral & tetrahedral octahedral illite “2:1:1” montmorillonite

  18. O-O & O-Cation van der Waals (weak) Low High/Moderate Low, none Hydrogen (strong) Potassium ions (strong) Low Moderate High / Highest Edges only – No isomorphic substitution Tetra(~balanced by K+’s) so: Edges Octahedral / Octa+Tetra Properties of common silicate clays

  19. 4.7 m Trovey, 1971 ( from Mitchell, 1993) 1.7 Chain Structure Clay Minerals Attapulgite • They have lathlike or threadlike morphologies. • The particle diameters are from 50 to 100 Å and the length is up to 4 to 5 m. • Attapulgite is useful as a drilling mud in saline environment due to its high stability.

  20. 1.8 Mixed Layer Clays • Different types of clay minerals have similar structures (tetrahedral and octahedral sheets) so that interstratification of layers of different clay minerals can be observed. • In general, the mixed layer clays are composed of interstratification of expanded water-bearing layers and non-water-bearing layers. Montmorillonite-illite is most common, and chlorite-vermiculite and chlorite-montmorillonite are often found. (Mitchell, 1993)

  21. 1.9 Noncrystalline Clay Materials • Allophane • Allophane is X-ray amorphous and has no definite composition or shape. It is composed of hollow, irregular spherical particles with diameters of 3.5 to 5.0 nm.

  22. 2. Identification of Clay Minerals

  23. 2.1 X-ray diffraction Mitchell, 1993 • The distance of atomic planes d can be determined based on the Bragg’s equation. • BC+CD = n, n = 2d·sin, d = n/2 sin • where n is an integer and  is the wavelength. • Different clays minerals have various basal spacing (atomic planes). For example, the basing spacing of kaolinite is 7.2 Å.

  24. 2.2 Differential Thermal Analysis (DTA) • Differential thermal analysis (DTA) consists of simultaneously heating a test sample and a thermally inert substance at constant rate (usually about 10 ºC/min) to over 1000 ºC and continuously measuring differences in temperature and the inert material T. • Endothermic (take up heat) or exothermic (liberate heat) reactions can take place at different heating temperatures. The mineral types can be characterized based on those signatures shown in the left figure. • (from Mitchell, 1993) For example: Quartz changes from the  to  form at 573 ºC and an endothermic peak can be observed. T Temperature (100 ºC)

  25. If the sample is thermally inert, If the phase transition of the sample occurs, 2.2 DTA (Cont.) T T Crystallize Melt Time t Time t Endothermic reactions take up heat from surroundings and therefore the temperature T decreases. Exothermic reactions liberate heat to surroundings and therefore the temperature T increases. T= the temperature of the sample – the temperature of the thermally inert substance.

  26. 2.3 Other Methods • Electron microscopy • Specific surface (Ss) • Cation exchange capacity (cec) • Plasticity chart

  27. 2.3 Other Methods (Cont.) • 5. Potassium determination • Well-organized 10Å illite layers contain 9% ~ 10 % K2O. • 6. Thermogravimetric analysis It is based on changes in weight caused by loss of water or CO2 or gain in oxygen. Sometimes, you cannot identify clay minerals only based on one method.

  28. 3. Specific Surface (Ss)

  29. 3.1 Definition Preferred Surface related forces: van der Waals forces, capillary forces, etc. Example: Ss is inversely proportional to the particle size

  30. 3.2 Typical Values 50-120 m2/gm (external surface) 700-840 m2/gm (including the interlayer surface) Montmorillonite Interlayer surface Illite 65-100 m2/gm Kaolinite 10-20 m2/gm

  31. 12. References • Main References: • Holtz, R.D. and Kovacs, W.D. (1981). An Introduction to Geotechnical Engineering, Prentice Hall. (Chapter 4) • Mitchell, J.K. (1993). Fundamentals of Soil Behavior, 2nd edition, John Wiley & Sons (Chapter 3). • Others: • Israelachvili, J. (1991). Intermolecular and Surface Forces, 2nd edition, Academic Press. • Kramer, S.L. (1996). Geotechnical Earthquake Engineering, Prentice Hall. • Lambe, T.W. and Whitman, R.V. (1979). Soil Mechanics, SI Version, John Wiley & Sons. • Santamarina, J.C., Klein, K.A. and Fam, M.A. (2001). Soils and Waves, John Wiley & Sons. • Van Olphen, H. (1991). An Introduction to Clay Colloid Chemistry. Reprint edition, Krieger Publishing Company. • Velde, B. (1995). Origin and Mineralogy of Clays. Springer. • Xanthakos, P.P. (1991). Surry Walls as Structural Systems, 2nd Edition, McGraw-Hill, Inc.

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