Microbial Weathering

Question 1

What is physical weathering?

Answer 1

fracturing 
disaggregation (crumbling/breaking rock)
microbes do not have a huge impact ono physical weathering as they are single celled organisms

Question 2

What is chemical weathering?

Answer 2

dissolution of primary and secondary minerals
precipitation of secondary minerals - biomineralisation
direct transformation of minerals

Question 3

Why do minerals chemically weather?

Answer 3

minerals are formed under different pressures and temperatures than the environment where they are - not in thermodynamic equilibrium with the surface environment. further away from equilibrium/saturation, the reaction is likely to occur faster.

Question 4

What do rates of reaction depend on?

Answer 4

• thermodynamic saturation state of the reacting fluid
• mineral composition, structure, surface area and defects
• temperature - higher temp = faster reaction (generally)

Question 5

What is proton promoted dissolution?

Answer 5

Proton promoted dissolution - acid attack of mineral surface, hydrolysis - water breaking apart bounds of the mineral

Question 6

What is ligand promoted dissolution?

Answer 6

• complexation of mineral surface cations by ligands
• precipitation of secondary minerals
• redox - occurs as part of dissolution or precipitation. can be part of an insitu transformation
• ligands donate electrons to the cation during complexation

Question 7

What are chelates?

Answer 7

multi-dentate ligands. chelate can also refer to the cation-ligand complex

Question 8

How are chelates formed?

Answer 8

2 hydroxyl groups approach each other.
H is kicked out.
Direct bonds are formed.
Destabilises the other bonds in the mineral lattice and may alter mineral composition.

Question 9

What are siderophores?

Answer 9

A special kind of organic ligands with very high complex formation constants. 
Are chelates (multi-dentate ligands). 
Produced by microorganisms. Very high affinity of Fe(III).
Energy intensive to produce.
microbes produce siderophores when in Fe-deficient environments.

Question 10

What are ligands?

Answer 10

Ligands are anions (or neutrally charged) molecules that associate with a cation to form a complex. This complexation forms a new mineral.
Ligands donate electrons to the cation during complexation.

Question 11

What are the direct microbial effects on dissolution

Answer 11

Attachment and colonisation
inhibit or enhance dissolution
preferential attachment to nutrient sources. Can provide nutrients and substrates (E donors) and can provide electron acceptors.
Microorganisms often preferentially attach to nutrient/energy sources.