Clay minerals exhibit colloidal behaviour. That is, their surface forces have greater influence than the negligible gravitational forces.
Geotechnical engineers are interested mainly in the top 100 metres of the earth crust. As you can see from the table, 82% of the elements are oxygen, silicon and aluminium.
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.
Here is a tetrahedral sheet, formed by connecting several tetrahedons. Note the hexagonal holes in the sheets.
The green and yellow blocks represent the tetrahedra and octahedra sheets respectively. The octahedral sheet containing aluminium is also called gibbsite. Sometimes, Al 3+ ions are substituted by Mg 2+ and the octahedral sheet is called brucite.
All clay mineral are made of different combinations of the above two sheets: tetrahedral sheet and octahedral sheet.
Kaolinite is used for making paper, paint and in pharmaceutical industry. A nanometer is 10 -9 metres.
Attapulgite has no sheets. It has a chain structure, and therefore looks like rods or needles.
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.
Clay particles are smaller than 2 microns. Their shapes can be studied by an electron microscope.
The clay particle derives its net negative charge from the isomorphous substitution and broken bonds at the boundaries.
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 Al 3+ can replace Ca 2+ and Ca 2+ can replace Mg 2+ .
The negatively charged faces of clay particles attract cations in the water. The concentration of the cations decreases exponentially with the increasing distance from the clay particle. The negatively charged clay surface and the positively charged cations near the particle form two distinct layers, known as “electric double layer” or simply “double layer”.
Clay mineralogy sivakugan
N. Sivakugan Clay Mineralogy Duration = 15 mins.
Elements of Earth % by weight in crust O = 49.2 Si = 25.7 Al = 7.5 Fe = 4.7 Ca = 3.4 Na = 2.6 K = 2.4 Mg = 1.9 other = 2.6 12500 km dia 8-35 km crust 82.4%
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. Parent Rock
Parent Rock ~ formed by one of these three different processes igneous sedimentary metamorphic formed by cooling of molten magma (lava) formed by gradual deposition, and in layers formed by alteration of igneous & sedimentary rocks by pressure/temperature e.g., limestone, shale e.g., marble e.g., granite
Residual Soils Formed by in situ weathering of parent rock
Basic Structural Units <ul><li>Clay minerals are made of two distinct structural units. </li></ul>Silicon tetrahedron Aluminium Octahedron 0.26 nm oxygen silicon 0.29 nm aluminium or magnesium hydroxyl or oxygen
Tetrahedral Sheet Several tetrahedrons joined together form a tetrahedral sheet. tetrahedron hexagonal hole
Tetrahedral & Octahedral Sheets For simplicity, let’s represent silica tetrahedral sheet by: Si and alumina octahedral sheet by: Al
Different Clay Minerals Different combinations of tetrahedral and octahedral sheets form different clay minerals: 1:1 Clay Mineral (e.g., kaolinite, halloysite):
Different Clay Minerals Different combinations of tetrahedral and octahedral sheets form different clay minerals: 2:1 Clay Mineral (e.g., montmorillonite, illite)
Kaolinite Typically 70-100 layers Si Al Si Al Si Al Si Al joined by strong H-bond no easy separation 0.72 nm joined by oxygen sharing
Kaolinite <ul><li>used in paints, paper and in pottery and pharmaceutical industries </li></ul>Halloysite <ul><li>kaolinite family; hydrated and tubular structure </li></ul><ul><li>(OH) 8 Al 4 Si 4 O 10 .4H 2 O </li></ul><ul><li>(OH) 8 Al 4 Si 4 O 10 </li></ul>
Montmorillonite easily separated by water <ul><li>also called smectite ; expands on contact with water </li></ul>Si Al Si Si Al Si Si Al Si 0.96 nm joined by weak van der Waal’s bond
Montmorillonite <ul><li>A highly reactive (expansive) clay </li></ul><ul><li>montmorillonite family </li></ul><ul><li>used as drilling mud, in slurry trench walls, stopping leaks </li></ul><ul><li>(OH) 4 Al 4 Si 8 O 20 .nH 2 O </li></ul>high affinity to water Bentonite swells on contact with water
Illite fit into the hexagonal holes in Si-sheet Si Al Si Si Al Si Si Al Si 0.96 nm joined by K + ions
Others… <ul><li>A 2:1:1 (???) mineral. </li></ul><ul><li>montmorillonite family; 2 interlayers of water </li></ul><ul><li>chain structure (no sheets); needle-like appearance </li></ul>Chlorite Vermiculite Attapulgite Si Al Al or Mg
Clay Fabric <ul><li>Electrochemical environment (i.e., pH, acidity, temperature, cations present in the water) during the time of sedimentation influence clay fabric significantly. </li></ul><ul><li>Clay particles tend to align perpendicular to the load applied on them. </li></ul>
Specific Surface <ul><li>surface area per unit mass (m 2 /g) </li></ul><ul><li>smaller the grain, higher the specific surface </li></ul>e.g., soil grain with specific gravity of 2.7 spec. surface = 222.2 mm 2 /g spec. surface = 2222.2 mm 2 /g 10 mm cube 1 mm cube
Isomorphous Substitution <ul><li>substitution of Si 4+ and Al 3+ by other lower valence (e.g., Mg 2+ ) cations </li></ul><ul><li>results in charge imbalance (net negative) </li></ul>Clay Particle with Net negative Charge + + + + + + + _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ positively charged edges negatively charged faces
Cation Exchange Capacity (c.e.c) <ul><li>capacity to attract cations from the water (i.e., measure of the net negative charge of the clay particle) </li></ul><ul><li>measured in meq /100g (net negative charge per 100 g of clay) </li></ul><ul><li>The replacement power is greater for higher valence and larger cations. </li></ul><ul><li>Al 3+ > Ca 2+ > Mg 2+ >> NH 4 + > K + > H + > Na + > Li + </li></ul>milliequivalents known as exchangeable cations
A Comparison 20-30 80 Chlorite 80-120 800 Montmorillonite 20-30 80-100 Illite 3-10 10-20 Kaolinite C.E.C (meq/100g) Specific surface (m 2 /g) Mineral
Adsorbed Water <ul><li>A thin layer of water tightly held to particle; like a skin </li></ul><ul><li>1-4 molecules of water (1 nm) thick </li></ul><ul><li>more viscous than free water </li></ul>- - - - - - - - - - - - - - adsorbed water
Clay Particle in Water free water double layer water - - - - - - - - - - - - - - adsorbed water 50 nm 1nm
Summary - Clays <ul><li>Clay particles are like plates or needles. They are negatively charged. </li></ul><ul><li>Clays are plastic; Silts, sands and gravels are non-plastic. </li></ul><ul><li>Clays exhibit high dry strength and slow dilatancy. </li></ul>
Summary - Montmorillonite <ul><li>Montmorillonites have very high specific surface, cation exchange capacity, and affinity to water. They form reactive clays. </li></ul><ul><li>Bentonite (a form of Montmorillonite) is frequently used as drilling mud. </li></ul><ul><li>Montmorillonites have very high liquid limit (100+), plasticity index and activity (1-7). </li></ul>