Weathering Flashcards
Physical weathering definition
Fracturing of rock or minerals causing disaggregation (breaking up)
Chemical weathering definition
Dissolution of primary and secondary minerals.
Can lead to the precipitation of secondary minerals (biomineralisation)
Overview of chemical weathering process
- Minerals are formed under different P and T conditions to the surface, not in thermodynamic equilibrium
- In the presence of water, O2 and other reactants will weather
What do chemical weathering rates of reaction depend on?
- Thermodynamic SI of reacting fluid
- Mineral composition/ structure/ SA
- Temp
What are the two types of chemical weathering promoted dissolution?
- Proton promoted dissolution
2. Ligand promoted dissolution
What is proton promoted dissolution?
Acid (protons, H+) attack of a mineral surface.
Known as Hydrolysis.
What is ligand promoted dissolution?
Complexation of mineral surface cations by ligands.
What are ligands?
Anions or neutral molecules which associate with a cation to form a complex (new molecule).
- Tend to donate e- to cations during complexation
- Cleave cation bonds that hold them to the surface or complex cations in solution by changing the SI
What are the two types of ligands?
- Multi-dentate ligands
- Are chelates (cation-ligand complexes) - Siderophores
- Multi-dentate organic chelator.
- High affinity for Fe(III)
- Develop siderophores when under iron stress
How do microbes effect dissolution?
Indirectly - can speed ip dissolution
Directly - can attach to and colonise minerals
How do microbes attach and colonise minerals?
99% attach to mineral surfaces either enhancing or inhibiting dissolution.
Physical weathering by attachment:
Hyphae - Drill/wedge into minerals, pry layers apart
Endolithic bacteria - live in pore spaces, can dissolve cement matrix
Does microbial enhancement of dissolution require attahcment?
No.
Siderophores and EPS increase dissolution the most
