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”.
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
Transported Soils Transported by : Special name: