Introductory Soil Science

1
Introductory Soil Science

• Weathering and the formation of clay minerals
• Study Questions

2
Weathering

• Where does weathering occur?
• At or near the surface of Earth
• Requires interaction between the lithosophere ,
  atmosphere and hydrosphere and perhaps biosphere
• Powered by solar energy
• What is regolith?
• It is the weathered outer part of the crust

3
Weathering

• What are the two main mineral weathering products
  that produce clastic sediment and the mineral
  component of soil.
• Quartz and clay
• Quartz is a primary mineral that resists chemical
  weathering so is only affected by mechanical
  processes
• Clay is a secondary mineral that is formed by the
  processes of chemical weathering
• recombination of the alumina and silica sheets
  that result from the hydrolysis, carbonation and
  oxidation of primary silicates form
  anions(micelles) that attract the free cations to
  form clays

4
Weathering

• What is the difference between chemical and
  mechanical/physical weathering?
• Chemical weathering leads to the formation of new
  minerals and soluble products
• Mechanical simply breaks the rock into smaller
  particles with no change in chemical composition

5
Clays

• Briefly describe the 3 different meanings of the
  term clay in soil usage.
• Mineral family name
• Particle size designation
• Textural term

6
Clays

• How do clay minerals form?
• By chemical weathering processes
• Original silicate minerals are chemically broken
  into octahedral alumina tetrahedral silicate
  sheets that recombine to form the micelles
• Micelles are negatively charged (anions)
• Attract cations to form clay minerals

7
Clays

• Describe the difference between the octahedral
  structure of the alumina layer and the
  tetrahedral structure of the silica layer in the
  clay micelle.
• In the octahedral layer the aluminium atom is
  surrounded by 6 oxygen atoms
• In the tetrahedral layer the silicon atom is
  surrounded by 4 oxygen atoms

8
Clays

• What is the difference in the structure of 11
  and 21 clays?
• 11 clays have 1 alumina and 1 silica layer
• 21 clays have 1 alumina and 2 silica layers

9
Clays

• How does this difference explain differences in
  the shrink/swell properties of clays?
• Sheets in 11 clays are strongly held together
  and do not allow water in
• Sheets in 21 clays are not as strongly held
  together and allow water in
• What is cation exchange?
• The exchange of cations between clay minerals and
  the surrounding environments, eg. plant roots