Lecture12-14(2008)

Lecture12-14(2008) - CLAY MINERALS Clay minerals have a...

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CLAY MINERALS Clay minerals have a layer structure, each layer is composed of silica and alumina sublayers or sheets. Within the Si sheet, each silicon ion is surrounded by four oxygen ions — comprising a tetrahedron (Tetrahedral Sheet).
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The metallic ion in these sheets can vary, i.e., Si 4+ , Al 3+ , Mg 2+ , or Fe 2+ These can be replaced with similar size cations,--That is Al 3+ can replace Si 4+ . This is called isomor phous substitution . This results in a negatively charged clay surface. The surface charge varies depending on the type of clay present in a soil. CLAY MINERALS (cont.) Within the Al sheet, each aluminum ion is surrounded by six hydroxyl (OH) ions comprising an octahedron (Octahedral Sheet).
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Geometry of Clay Sheets
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(1:1 layer silicate) (2:1 layer silicate)
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Crystal Structure of Smectite (illustrating three common types of substitution) ** Negative charge affects cation concentration in solution and retention in soil. ** The negative charge repels anions. ** The negative charge on clay is responsible for cation exchange capacity of a given soil.
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Soil Swelling Wet soil particles-H 2 O causes swelling of soil Soil particles after some time of drying ** Combined negative charge and adsorbed cations result in clay soil swelling upon wetting.
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+ - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + DRY - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + WET - - - - - - - - - - + + + + + + + + + + + + + + + + + diffuse layer
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Lecture12-14(2008) - CLAY MINERALS Clay minerals have a...

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