Biogeochem - Silicon cycle and clays Flashcards
(20 cards)
what is the second most abundant element in the earths crust?
Silicon
What does silicate weathering control?
The earths climate and temperature of the earths surface which controls weathering and thus the formation of soils
why can marine and freshwater systems be limited by silicon
because organisms such as diatoms, radiolaria and sponges use silicon for the formation of endo- or exoskeletons
What is the CO2 thermostat feedback loop?
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.
how does silicate weathering link to the CO2 thermostat?
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.
How does the accumulation of clay minerals impede weathering and thus the CO2 thermostat
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.
What is the structure of silicates
silicon tetrahedron
silicate in the centre with 4 oxygen atoms attached to it
Silicate mineral examples
Feldspar
Olivine
Mica
Quartz
Pyroxene
What type of rocks with silica present get weathered
(eg sedimentary… etc)
igneous
What is the structure of clay minerals?
Silicon tetrahedra and Aluminium octahedra
What is isomorphous replacement?
refers to the substitution of one type of cation with another of similar size but different valence (charge) within the mineral’s crystal lattice
Which minerals are isomorphus replacement more likely to occur in
TOT and TOTO minerals
Why does isomorphus replacement not occur in TO (2:1) clay minerals?
In TO minerals, due to the structure and composition of the mineral, they are less prone to substitution and a neutral charge remains
Why is isomorphous replacement more likely to occur in TOT and TOTO clay minerals?
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
In isomorphous replacement, what is substituted / replaced and what happens?
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.
What is the outcome of Isomorphous replacement?
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.
What happens in cation exchange capacity during isomorphous replacement?
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
List the properties of Kaolinite (TO)
1:1 clay –> TO
No isomorphous replacement occurs
Non-swelling
low cation exchange capacity
List the properties of Montmorillonite (TOT)
- 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
List the properties of illite (TOT)
- 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
