ADVANCED SOIL MECHANICS Clay Mineralogy ADVANCED SOIL MECHANICS Clay minerals exhibit colloidal behaviour. That is, their surface forces have greater influence than the negligible gravitational forces. BY JAYANTI PRABHA BHARTI-2k12/GTE/09 GAURAV GUPTA-2K12/GTE/06

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. Steady water cannot sustain the reaction.

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. Weak plane!

PROPERTIES OF 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.

Basic Structural Units Clay minerals are made of two distinct structural units. 0.29 nm aluminium or magnesium hydroxyl or oxygen oxygen silicon All clay minerals are made of two distinct building blocks: tetrahedrons and octahedrons. The tetrahedron on the left has oxygen atoms at the corners, and there is a silicon in the centre. Octahedron has six oxygen or hydroxyl atoms in the corners, and an aluminium or magnesium ion at the centre. 0.26 nm Silicon tetrahedron Aluminium Octahedron

STRUCTURAL UNITS OF SILICATES LAYER IN MOST CLAY MINERAL STRUCTURES, THE SILICA TETRAHEDRA ASSOCIATE IN A SHEET STRUCTURE. THREE OF THE FOUR OXYGENS OF EACH TETRAHEDRON ARE SHARED TO FORM A HEXAGONAL NET. THE BASES OF THE TETRAHEDRA ARE ALL IN THE SAME PLANE& THE TIPS ALL POINT IN THE SAME DIRECTION .

Tetrahedral Sheet Several tetrahedrons joined together form a tetrahedral sheet. tetrahedron Here is a tetrahedral sheet, formed by connecting several tetrahedons. Note the hexagonal holes in the sheets. hexagonal hole

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)

OCTAHEDRAL SHEET The sheet structure is composed of Mg and Al coordinated octahederally with oxygen. If the octahederally coordinated cation is divalent then all possible cations sites are normally filled and the structure is tri- octahedral. If the cation is trivalent then only 2-3 rd the possible cationic spaces are normally filled and the structure is termed as di octahedral. In clay mineral structures a sheet of magnesium octahedral is termed as bruited sheets.

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

1.2 Basic Unit-Summary Mitchell, 1993

tetrahedron

octahedron

Connected tetrahedra, sharing oxygens Tetrahedral sheets Connected tetrahedra, sharing oxygens

CONNECTED OCTAHEDRA, SHARING OXYGENS OR HYDROXYLS Octahedral sheets CONNECTED OCTAHEDRA, SHARING OXYGENS OR HYDROXYLS

Different Clay Minerals Different combinations of tetrahedral and octahedral sheets form different clay minerals: 1:1 Clay Mineral (e.g., kaolinite, halloysite): All clay mineral are made of different combinations of the above two sheets: tetrahedral sheet and octahedral sheet.

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

Kaolinite 0.72 nm Al Si Typically 70-100 layers Al Si Al Kaolinite is used for making paper, paint and in pharmaceutical industry. A nanometer is 10-9 metres. joined by strong H-bond no easy separation Si Al joined by oxygen sharing Si Al

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

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

Montmorillonite BENTONITE stopping leaks (OH)4Al4Si8O20.nH2O A highly reactive (expansive) clay (OH)4Al4Si8O20.nH2O BENTONITE montmorillonite family used as drilling mud, in slurry trench walls, stopping leaks

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

CHLORITE VERMICULITE ATTAPULGITE Others… A 2:1:1 (???) mineral. Si Al Al or Mg VERMICULITE montmorillonite family; 2 interlayers of water Attapulgite has no sheets. It has a chain structure, and therefore looks like rods or needles. ATTAPULGITE chain structure (no sheets); needle-like appearance

Dispersed Flocculated Clay Fabric edge-to-face contact face-to-face contact The term fabric is used to describe the geometric arrangement of the clay particles. Flocculated and Dispersed are the two extreme cases. Flocculated fabric gives higher strength and stiffness. Dispersed Flocculated

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.

Identifying Clay Minerals

Scanning Electron Microscope common technique to see clay particles qualitative plate-like structure Clay particles are smaller than 2 microns. Their shapes can be studied by an electron microscope.

X-Ray Diffraction (XRD) Others… X-Ray Diffraction (XRD) to identify the molecular structure and minerals present DIFFERENTIAL THERMAL ANALYSIS (DTA) to identify the minerals present

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

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. Electronmicroscopy, both transmission and scanning, can be used to identify clay minerals in a soil sample, but the process is not easy and/or quantitative. Bragg’s law: The two rays will constructively interfere if the extra distance ray I travels is a whole number of wavelengths farther then what ray II travels.

OCCURRENCE OF TWO PHENOMENA ON STRIKING OF HIGH SPEED ELECTRON The high speed electron may strike& displace an electron from an inner shell of one of the atoms of the target material. An electron from one of the outer shells then falls into the vacancy to lower the energy state of the atom. If the high speed electron does not strike an electron in the target material but slows down in the intense electric fields near atomic nuclei then the decrease in energy is converted to heat & to x-ray photons.

phenomena An x-ray of wavelength& intensity characteristics of the target atom & of the particular electronic positions is emitted. X-ray produced in this way are independent of the nature of bomb.

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. All these equipment can be found in the Materials Characterization and Preparation Facility. T Temperature (100 ºC)

Isomorphous Substitution substitution of Si4+ and Al3+ by other lower valence (e.g., Mg2+) cations results in charge imbalance (net negative) positively charged edges + _ The clay particle derives its net negative charge from the isomorphous substitution and broken bonds at the boundaries. negatively charged faces Clay Particle with Net negative Charge

Cation Exchange Capacity (c.e.c) known as exchangeable cations 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) milliequivalents The negatively charged clay particles can attract cations from the water. These cations can be freely exchanged with other cations present in the water. For example Al3+ can replace Ca2+ and Ca2+ can replace Mg2+. The replacement power is greater for higher valence and larger cations. Al3+ > Ca2+ > Mg2+ >> NH4+ > K+ > H+ > Na+ > Li+

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 - - - - - - - - - - - - - - adsorbed water

Clay Particle in Water adsorbed water free water double layer water - - - - - - - - - - - - - - 1nm 50 nm free water double layer water