Biogeochem - Silicon cycle and clays

Question 1

what is the second most abundant element in the earths crust?

Answer 1

Silicon

Question 2

What does silicate weathering control?

Answer 2

The earths climate and temperature of the earths surface which controls weathering and thus the formation of soils

Question 3

why can marine and freshwater systems be limited by silicon

Answer 3

because organisms such as diatoms, radiolaria and sponges use silicon for the formation of endo- or exoskeletons

Question 4

What is the CO2 thermostat feedback loop?

Answer 4

CO2 in the atmosphere dissolves in rainwater to form carbonic acid. This reacts with silicate minerals in the rock and weathers the rocks. Carbonate and bicarbonate are formed which flow into the oceans where organisms use them. Eventually this gets buried, removing the CO2 from the atmosphere, cooling the climate.

Cooler temperatures cause slower rates of reaction. This reduces the amount of weathering that occurs, decreasing the CO2 consumption as significantly less burial is occurring. Eventually, more CO2 accumulates in the atmosphere (through processes such as volcanic activity) which causes the global temperatures to warm.

Question 5

how does silicate weathering link to the CO2 thermostat?

Answer 5

the amount of CO2 removed from the atmosphere is directly proportional to the amount of silicate weathering in the rock as CO2 directly reacts with the silicate minerals.

Question 6

How does the accumulation of clay minerals impede weathering and thus the CO2 thermostat

Answer 6

Clay minerals are a product of weathering. They get to a state where the can no longer be weathered and so they accumulate on the rock surface. This prevents the surface rock being weathered, blocking thr weathering of silicates and therefore impacts the CO2 thermostat as the removal rate of CO2 is reduced.

Question 7

What is the structure of silicates

Answer 7

silicon tetrahedron

silicate in the centre with 4 oxygen atoms attached to it

Question 8

Silicate mineral examples

Answer 8

Feldspar
Olivine
Mica
Quartz
Pyroxene

Question 9

What type of rocks with silica present get weathered

(eg sedimentary… etc)

Answer 9

igneous

Question 10

What is the structure of clay minerals?

Answer 10

Silicon tetrahedra and Aluminium octahedra

Question 11

What is isomorphous replacement?

Answer 11

refers to the substitution of one type of cation with another of similar size but different valence (charge) within the mineral’s crystal lattice

Question 12

Which minerals are isomorphus replacement more likely to occur in

Answer 12

TOT and TOTO minerals

Question 13

Why does isomorphus replacement not occur in TO (2:1) clay minerals?

Answer 13

In TO minerals, due to the structure and composition of the mineral, they are less prone to substitution and a neutral charge remains

Question 14

Why is isomorphous replacement more likely to occur in TOT and TOTO clay minerals?

Answer 14

TOT clay minerals feature an Al octrahedron layer between 2 Si tetrahedron layers. This allows more room for cation substitutions between the sheets.

TOTO clay minerals feature an Al octahedron layer between 2 TOT layers, this further increases potential for substitutions to occur

Question 15

In isomorphous replacement, what is substituted / replaced and what happens?

Answer 15

tetra-valent Si4+ get replaced with tri-valent Al3+

tri-valent Al3+ get replaced with di-valent Mg2+

The structure remains the same but it leads to a negative charge in the layer and a cation is lost.

Question 16

What is the outcome of Isomorphous replacement?

Answer 16

These negative charges will bind cations and contribute
to the cation exchange capacity.

Clay minerals with net charged layers can take up water and swell. The differences is their physical and chemical features means that the different clay minerals have different influence on soil chemistry, nutrient retention and hence are of varying values for agriculture.

Question 17

What happens in cation exchange capacity during isomorphous replacement?

Answer 17

The spaces between single silicate sheets normally contains cations like K+, Ca2+ and Na+

If there is a surplus of H+ present, the cation will be exchanged for H+ and the oxygen in the silicates will be protonated

This is a reversible reaction

the retention of cations improves soil fertility

Question 18

List the properties of Kaolinite (TO)

Answer 18

1:1 clay –> TO
No isomorphous replacement occurs
Non-swelling
low cation exchange capacity

Question 19

List the properties of Montmorillonite (TOT)

Answer 19

• 2:1 clay
• High potential for isomorphous replacement (Fe)
• High swelling capacity due to the retention of water and ions between lattice layers
• High cation exchange capacity

Question 20

List the properties of illite (TOT)

Answer 20

• 2:1 clay –> TOT
• Non-swelling as K+ ions block the inter-lattice exchange
• Potential for isomorphous replacement (Mg or Fe)
• Moderate ion exchange capacity