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Clay Minerals and Soil

Clay Minerals and Soil. Products of Weathering. Clays account for ~50% (by volume) of sedimentary rocks Common in shales (mudrocks) and as coatings on minerals being weathered Clay = Tiny size (<1 μ m) Complex chemistry 15-20 wt% water Layered with weak bonding between layers

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Clay Minerals and Soil

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  1. Clay Mineralsand Soil Products of Weathering

  2. Clays account for ~50% (by volume) of sedimentary rocks Common in shales (mudrocks) and as coatings on minerals being weathered Clay = Tiny size (<1 μm) Complex chemistry 15-20 wt% water Layered with weak bonding between layers Composition depends on source of sediment Clay Minerals: Occurrence

  3. Mica and clay minerals are Phyllosilicates Sheet or layered silicates Two dimensional polymerization of silica tetrahedra Common structure is a Si205 layer Sheet Silicates: the Micas and Clay Minerals Phyllosilicates Si2O5 sheets of silica tetrahedra

  4. Two layers make up structure Tetrahedral layer Si2O5 sheets Some Al3+ may substitute for Si4+ Octahedral layer Mg2+ or Fe2+ (brucite layer) or Al3+ (gibbsite layer) Structure of Phyllosilicates Kaolinite: Al2Si2O5(OH)4 1:1 tetrahedral – octahedral sheets

  5. Kaolinite group: Most clay minerals 1:1 TO Mica (illite) group: “True” micas and clays 2:1 TOT Expandible clays: Smectite - montmorillonite complex 2:1 clay minerals Chlorite group: 2:2 TOT with O interlayer Fe- and Mg-rich The Major Clay Mineral Groups

  6. Muscovite and Biotite: Macroscopic model for clay minerals TOT sandwich with a K+ between layers Electrostatically neutral, stable 2:1 TOT Phyllosilicates: “True” Mica

  7. Clay Mineral Properties • Fine particle size (<2-4µm) • High surface area to volume • Electrostatic charge • Interlayer (between “T” and “O” layers) • Exterior surfaces due to broken bonds • Charge deficiency (ex: illite) • Result in: • Flocculation: attract other ions and polar molecules • Electrostatic forces may dominate (relative to gravity) the physical properties of clay mineral-rich systems • Cation exchange capacity (CEC)

  8. Clay Mineral Properties • Water sorption (electrostatic attachment to clay size particles) • Engineering properties of clay-rich Earth materials • Plasticity • Expansion/contraction with changes in humidity • Chemical properties of clay-rich Earth materials • Exchange of metal cations (and nitrogen compounds NH4+) with natural waters (ground water, etc); • CEC

  9. Surface “blanket” of Earth Materials capable of supporting rooted plants. Soil development most strongly influenced by Climate (temperature and precipitation on an annual cycle) Topography Precursor material Determine the type and time frame (hundreds to thousands of years) for soil formation Soils

  10. Soils • Horizons (layers) develop from weathering and leaching of mineral & organic components • Soil profile affected by climate • Warm and humid: deep weathering and thick soil • Cold and arid: shallow weathering and thin soil

  11. Soils • Basic classification on the basis of climate Dry Pedocal Wet Laterite Temperate Pedalfer

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