lecture 3

Question 1

What is a mineral?

Answer 1

1. Naturally occurring
2. Inorganic
3. Solid at room temperature
4. Regular crystal structure
5. Defined chemical composition

Question 2

What does soil mineralogy influence?

Answer 2

Influences chemical weathering, nutrient supply, and buffering capacity

Question 3

What are primary minerals?

Answer 3

Derived from igneous or metamorphic rocks; mostly found in sand and silt fraction

Question 4

What are secondary minerals?

Answer 4

Inherited from parent material or formed in situ; mostly found in the clay fraction

They can form from chemical reactions or can be a transformation of primary minerals

Question 5

What is the basic structural unit of primary minerals? How does it connect to others atoms?

Answer 5

Silicon tetrahedron, (SiO4)4-

Oxygen’s negative charge shared with adjacent Si atoms or with cations (e.g. Fe3+ or Mg2+) in the mineral lattice

Question 6

What are the different types of polymers created from silicate tetrahedron? What are the characteristics of the formation of polymers of silicate tetrahedron?

Answer 6

Pairs, rings, single chains, double chains, sheets, 3D network.

The more oxygen atoms are shared by Si, the higher the resistance to chemical weathering.

Question 7

What is isomorphous? Describe isomorphous substitution within silicate tetrahedron.

Answer 7

Isomorphous: no change in structure by loss of one positive charge for each atoms.

In silicate tetrahedron, it is the replacement of a central Si by Al, which results in aluminosilicate.
- Similar in size but different valency
- Loss of one positive charge: Si4+ –> Al3+

It requires the incorporation of a cation (e.g. K+, Na+, Ca2+) into the mineral lattice to provide extra positive charge.

Question 8

What does the relative size of the major ions determine? What is the weakest and strongest?

Answer 8

The ratio of charge to relative size determines bonding strength in minerals. As the energies of the formation of cations increase, the bonding strength increases.

The weakest is K+ and the strongest is Si4+. K+ has a lower energy than Si4+.

Question 9

What are common primary minerals in soils from most to least common? Why is the most common one the most common?

Answer 9

1. Quartz (SiO2) - it is not easily weathered away
2. Feldspar
3. Mica

Question 10

What are the characteristics of quartz?

Answer 10

1. SiO2
2. 3D sharing of Si tetrahedron
3. Very resistant to weathering

Question 11

What is the formation process of Feldspars?

Answer 11

Substitution of ca. 25% of Si4+ by Al3+ and incorporation of cation (positive charge) into the mineral lattice

Question 12

What is the formation process of Mica?

Answer 12

Complicated isomorphous substitution, in particular hydroxides

Question 13

What is granite composed of?

Answer 13

Consists of quartz, feldspar, and mica.

Question 14

What are the characteristics of secondary minerals?

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Answer 14

• Mostly found in the clay size fraction, therefore often called clay minerals
• Formed under conditions close to surface temperature and pressure
  Resulting from weathering of primary minerals or formed in situ in soil (in the place/location)
• In soils of temperate regions alumino-silicates are most prevalent
• In soils of tropical regions hydroxides of iron and aluminum are more prevalent

Question 15

What are the 3 types of phyllosilicates?

Answer 15

1. Kaolinite
2. Illite
3. Smectite

Question 16

What are the characteristics of clay minerals?

Answer 16

• Different clay minerals are formed by varying combinations of Si and Al sheets and by isomorphous substitution
• Large surface area
• Expansion and contraction upon wetting and drying
• Control cation and anion exchange capacity
• Have both permanent and pH variable charge capable of attracting and retaining plant nutrients with direct and indirect impacts on the soil fertility
• Clay minerals are the most chemically active inorganic phases in soils

Question 17

What are the chemical proportions of Kaolinite and its characteristics ?

Answer 17

Kaolinite - 1:1 clay mineral
- 1 sheet of silicone tetrahedron, 1 sheet of aluminum octahedron and they share oxygen
- isomorphic substitution w/ Al
- non expanding (no swelling)

Question 18

What do electrostatic charges on clay surfaces attract? What does this influence

Answer 18

Water molecules. Influences the expanding vs nonexpanding minerals
- if the water between sheets becomes gaseous and escapes, it makes the sheets collapse and creates shrinkage of the soil

Question 19

Where do we find an abundance for phyllosilicates in Canada?

Answer 19

Typically found in the clay-sized fractions of soils in Canada

Question 20

Describe the process of the formation of secondary phosphate minerals

Answer 20

Phosphate in soil reacts readily with Ca2+, Al3+, and Fe2+ and Fe3+ forming a variety of secondary precipitates

Question 21

What are sand, silt, and clay minerals mainly composed of?

Answer 21

Sand (large mineral): quartz, a primary mineral

Silt: diverse: primary and secondary minerals

Clay (smallest mineral): consists largely of secondary minerals

Question 22

Why are quartz feldspars most resistant to chemical weathering?

Answer 22

It is a 3D atom structure, where all 4 oxygen atoms of the tetrahedron are shared - more oxygen atoms shared = more resistant to chemical weathering

Question 23

Why are clay minerals the most important mineral in soils?

Answer 23

• highly reactive
• exchange of nutrients
• exchange and absorption/desorption of contaminants in the soil
• plant nutrition

Question 24

What are the chemical proportions of Smectite and its characteristics ? Why is smectite the least stable clay mineral?

Answer 24

Smectite - 2:1 clay mineral
- 2 silicone tetrahedron sheets sharing oxygen with an Al octahedron sheet
- contracts and expands based on water availability

It has water between its bonds (hydrated cations), so there is a larger distance between the bonds. If there is more water, the soil expands, if the water evaporates and escapes then the sheet collapses

A soil that is rich in smectite - during the dry seasons there is shrinkage of the soil. Causes damage to infrastructure/farms built on smectite soils.

Question 25

What are the chemical proportions of Mica and its characteristics ?

Answer 25

Mica - 2:1 clay mineral
- 2 silicone tetrahedron sheets sharing oxygen with an Al octahedron sheet
- Lots of isomorphic substitution, with K
- Not hydrated
- non-expanding

Question 26

What are the chemical proportions of Chlorite and its characteristics ?

Answer 26

Chlorite - 2:1:1 clay mineral
- An additional hydroxide sheet in between the sheet bonds
- non expanding

Question 27

What is the general sequence of formation of clay minerals? What does this sequence represents and why is does it occur in this sequence?

Answer 27

Micas/Chlorite –> Smectite –> Kaolinite –> Oxides of Fe and Al

This sequence represents the weathering process and it depends on the bonding strength of the original primary substrate, the temperature, the moisture

• K, Mg are weathered away before Si due to bonding strength
• not a unidirectional process, the addition of fertilizer can reverse the process

Question 28

Why do tropical soils weather so quickly? What is the chemical process that is occuring?

Answer 28

Tropical soils are located in hot and wet climates, which leads to more weathering. Lots of rain that washes away the minerals

Si has the highest bonding strength so usually it is weathered away last BUT in tropical climates, Si will be lost/washed away and Fe and Al oxides are left behind