Primary and secondary minerals

Question 1

what is the definition of a mineral?

Answer 1

• naturally occuring
• inorganic
• solid at room temperature
• regular crystal structure
• defined chemical composition

Question 2

what are three things that are influenced by soil mineralogy?

Answer 2

1. chemical weathering
2. nutrient supply
3. buffering capacity

Question 3

where do primary minerals come from? and where are they found?

Answer 3

• derived from igneous or metamorphic rocks
• mostly found in sand and silt fraction

Question 4

where do secondary minerals come from and where are they found?

Answer 4

• they are inhereted from parent material or formed in situ at surface of Earth or in soil solution
• mostly found in clay fraction

Question 5

what is the basic structural unit of primary minerals?

Answer 5

silicon tetrahedon [SiO4]^4-

Question 6

what capacity grows when more oxygen atoms are shared by Si?

Answer 6

resistance to chemical weathering

Question 7

from smallest to largest, what are the most resistant to weathering structures of silicate tetrahedron polymers?

Answer 7

isolated > rings > chains > sheets > 3-dimensional network

Question 8

what is isomorphous substitution?

Answer 8

the replacement of central Si atom by Al to make aluminosilicate minerals
–> similar size but different valency because Si is 4+ and Al is 3+

Question 9

what happens after isomorphous substitution?

Answer 9

because the valency is different, a cation must be incorporated into the mineral lattice to provide an extra + charge

Question 10

what is the weakest bonding cation in minerals?

Answer 10

K+ (potassium)

Question 11

what are the 5 most common primary minerals in soils?

Answer 11

1. quartz
2. Feldspar
3. Mica
4. Olivine
5. Haematite

Question 12

what are some characteristics of quartz?

Answer 12

SiO2 –> 3D sharing of Si tetrahedra
- very resistant to weathering

Question 13

what are feldspars?

Answer 13

substitution of ~25% of Si by Al and incorporation of cation into mineral lattice

Question 14

what is mica?

Answer 14

complicated isomorphous substitution

Question 15

where do primary phosphate-containing minerals come from?

Answer 15

• sedimentary and igneous rock

Question 16

how do secondary minerals form?

Answer 16

under conditions close to surface temperature and pressure, they are the result of weathering of primary minerals or formed right away in soil

Question 17

what secondary minerals are most common in soils of temperate regions?

Answer 17

alumino-silicates

Question 18

what secondary minerals are most common in soils of tropical regions?

Answer 18

hydroxides of iron and aluminum

Question 19

how are clay minerals formed?

Answer 19

formed by varying combination of Si and Al sheets and by isomorphous substitution

Question 20

what are 5 characteristics of clay minerals?

Answer 20

1. large surface area
2. expansion and contraction upon wetting and drying
3. control cation and anion exchange capacity
4. have both permanent and pH variable charge capable of attracting and retaining plant nutrients with impacts on soil fertility
5. most chemically active inorganic phases in soil

Question 21

what is the additional building block (other than SiO4) in secondary minerals?

Answer 21

aluminum octahedron

Question 22

what would be left if all minerals were weathered away?

Answer 22

would be left with just quartz

Question 23

where does soil chemistry occur (associated with clay minerals)?

Answer 23

in between clay mineral sheets

Question 24

describe the kaolinite mineral

Answer 24

• ratio of 1:1 of sheets of silicon tetrahedron and aluminum octahedron with shared oxygen atom
• hydrogen bonds between sheets

Question 25

describe smecite

Answer 25

2:1 clay mineral (meaning two tetrahedron and one octahedron) and sheets bonded together with hydrated cation

Question 26

which bond is weaker: cation or hydrogen?

Answer 26

cation exchange bonds are weaker than hydrogen bonds

Question 27

describe mica clay mineral

Answer 27

• 2:1 clay mineral
• a lot of isomorphic substitution
• often balanced with potassium cation that is not hydrated
• has been developed from the mica primary mineral

Question 28

describe the chlorite clay mineral

Answer 28

• 2:1:1
• between the 2:1 sheets, there is an octahedron sheet

Question 29

what happens when water molecules are in clay minerals?

Answer 29

allows for expansion and contraction. –When water becomes gaseous, water evaporates and sheets collapse and minerals shrink -> cracking soils because minerals need much less space