Lecture 8: The Colloidal Fraction

Question 1

Draw the basic structure of 1:1 crystalline silicate clays

Answer 1

“open faced sandwich”
Tetrahedral sheet
Octahedral sheet
—-
Tetrahedral sheet
Octahedral sheet

Question 2

Draw the basic structure of 2:1 crystalline silicate clays

Answer 2

“sandwich”
Tetrahedral sheet
Octahedral sheet
Tetrahedral sheet
——-
Water molecules and ions
——
Tetrahedral sheet
Octahedral sheet
Tetrahedral sheet
——–

Question 3

Explain why kaolinite, smectites, vermiculite, fine-grained micas, and chlorites differ in their permanent charge and expansion capacity

Question 4

Discuss uses of crystalline silicate clays and explain how one can identify swelling clays in the field

Question 5

Explain how aluminum and iron oxides modify the properties of crystalline silicate clays

Question 6

Explain the term colloid and their function in soils

Answer 6

-Colloids are organic or inorganic particles that are suspended (not dissolved) in the soil solution.
- attract hydrated ions dissolved in the soil solution
- increase a soil’s reactive surface area
- accumulate nutrients and water to support plants and microorganisms
-retain nutrients and toxins and facilitate (bio-)geochemical transformations

Question 7

Isomorphic substitution

Question 8

Permanent/ Constant

Question 9

Variable/ pH-dependent charge

Question 10

List the 4 types of soil colloids

Answer 10

-Organic colloids
-Inorganic: -crystalline silicate clays
-noncrystalline (amorphous) silicate clays
-Iron (Fe) and Aluminum (Al) oxides
(sesquioxides)

Question 11

Organic colloids: main properties and names of common minerals + chemical formula

Answer 11

-generally negatively charged (net charge) but carry negative and positive charge
-retains cations and water
-Carbon bonded to hydrogen, oxygen, and nitrogen

Question 12

Organic colloids: which soils they are found in or absent from

Answer 12

important in nearly all soils

Question 13

Crystalline silicate clays: main properties and names of common minerals + chemical formula

Answer 13

-mostly negatively charged
-retain cations and water
-great variation in shape, charge, stickiness, plasticity, and swelling capacity
-Secondary minerals consisting of sheets of silicon, oxygen, and aluminum

Question 14

Crystalline silicate clays: which soils they are found in or absent from

Answer 14

-dominant colloid type in most soils
-not found in Andisols (volcanic ash), Oxisols (tropical), Histosols (organic), and within the mineral horizons

Question 15

Noncrystalline (amorphous) silicate clays: main properties and names of common minerals + chemical formula

Question 16

Noncrystalline (amorphous) silicate clays: which soils they are found in or absent from

Question 17

Iron (Fe) and Aluminum (Al) oxides (sesquioxides):main properties and names of common minerals + chemical formula

Question 18

Iron (Fe) and Aluminum (Al) oxides (sesquioxides): which soils they are found in or absent from

Question 19

What are the 6 most important nutrients for soils coming from the ground?

Answer 19

Ca
Mg
N
P
K
S

Question 20

5 types of phyllosilicates (layer silicates)

Answer 20

-Kaolinite
-Smectites
-Vermiculites
-Micas
-Chlorites