9: Sources of Charges in Colloids

Question 1

Crystalline clays (phyllosilicates or silicate clays)

Answer 1

Dominant source of charge in most soils
Layered structure of mostly Si, Al, and O atoms
Differ widely

Question 2

Phyllosilicates

Answer 2

A type of silicate clay with a leaflike or planar structure

Question 3

Building blocks for silicate clays

Answer 3

Silica tetrahedrons and alumina octahedrons. These building blocks form sheets, and these sheets form layers.

Question 4

1:1 silicate clays

Answer 4

Clays in which each layer contains one tetrahedral and one octahedral sheet

Variable charge

Question 5

2:1 silicate clays

Answer 5

Clays in which each layer contains one octahedral sheet sandwiched between two tetrahedral sheets

Always have negative net charge

Question 6

What are the 2 types of soil colloids?

Answer 6

The two types of soil colloids are crystalline and non-crystalline

Question 7

What are the 2:1 non expanding silicate clays?

Answer 7

Micas and chlorites

Question 8

What are the 2:1 expanding silicate clays?

Answer 8

Vermiculites and smectites

Question 9

Micas

What layer
What stability
What shrink-swell potential
What level and range of CEC
What level surface area

Answer 9

2:1 layer mineral
Relatively stable
Low shrink-swell potential
Moderate-low CEC 10-40 cmolc/kg
Intermediate surface area 70-100m2/g

K resides in interlayer spaces which strongly bonds adjoining sheets
Isomorphous substitution in tetrahedral sheets of Al3+ for Si4+ in every fourth tetrahedral site

Question 10

Chlorites

What layer
What stability
What shrink-swell potential
What level and range of CEC
What level surface area

Answer 10

2:1 layer mineral
Stable
No shrink-swell
Moderate-low CEC 15-40 cmolc/kg
Surface area is 25-150 m2/g

Question 11

Vermiculites

What layer
What stability
What shrink-swell potential
What level and range of CEC
What level surface area

Answer 11

2:1 layer mineral
Relatively stable
Low shrink-swell potential
High CEC 100-150 cmolc/kg
Surface area is 600-800 m2/g

Potential issue is K fixation
Substitution of Al3+ for Si4+

Question 12

Smectites

What layer
What stability
What shrink-swell potential
What level and range of CEC
What level surface area

Answer 12

2:1 layer mineral
Not stable
High shrink-swell potential
Moderate CEC 60-100 cmolc/kg clay
Surface area is 600-800 m2/g

Question 13

Pros and cons of kaolinite

Answer 13

Not optimal for soil fertility (it has a low charge and CEC) but pretty stable for construction

Question 14

Uses for smectite

Answer 14

Once it is saturated with water, it becomes highly impermeable to water so it’s useful for some types of construction

Question 15

Kaolinite

What layer
What stability
What shrink-swell potential
What level and range of CEC
What level surface area

Answer 15

1:1 layer mineral
Stable
No shrink-swell
Low CEC: 4-10 cmolc/kg
Surface area: 7-30 m2/g

Layers held together by hydrogen bonding, very low isomorphic substitution

Question 16

Crystalline Fe and Al oxides (Gibbsite, Goethite)

Where are they found
What atoms are they made up of
Structure
High or low charge, variable or not

Answer 16

Colloids found in highly weathered soils of warm, humid regions
Made up of Fe or Al and O atoms
Modified octahedral sheets with Al3+ (no tetrahedral sheets)
Low charge (low stickiness and plasticity, low water-holding capacity)
Can have variable charge (+, neutral, or -)

Question 17

Amorphous or non-crystalline silicate clays

What are they a mixture of?
Does it have high/low amounts of positive/negative/both charge and why?
Extra note about nutrients

Answer 17

Allophane-Imogolite mixture common in soils weathered from volcanic ash
High amounts of both positive and negative charge derived from the presence of OH- groups in the molecular structure
Tend to “fix” anions such as phosphates PO42- and sulfate SO42-, fixed nutrients are not plant available

Question 18

Organic matter (humus)

What is it made up of
Is it crystalline or not
How is its plasticity and shrink-swell capabilities?

Answer 18

Highly decomposed organic matter that is made up of large molecules of C chains and rings
Non-crystalline
No plasticity or shrink-swell

Question 19

Patterns of colloid charges and pH

What characteristics of colloids result in them having charges which are more pH-dependent or permanent?

Answer 19

Colloids with isomorphous substitution are not really affected by pH changes (like smectite and vermiculite)
For crystalline silica colloids, simpler structure means lower percentage of permanent charge and higher percentage of pH-dependent charge. More complex crystalline colloids are the opposite

Question 20

Anion exchange

Answer 20

Same as cation adsorption but in positively charged colloid surfaces
Higher anion exchange capacity in acid soils
Contributes to retention of important plant nutrients