Aqueous Geochemistry Flashcards
Physical geologic processes
With or without participation of geofluids
Chemical geologic processes
Participated by geofluids
Composition of geofluids
Variable amounts of components derived from dissolution of solid materials and gases
Dissolved species
Materials dissolved in water may be in the forms of molecules or ions
What reaction is always in the background?
H2O = H+ + OH-
Dissolved components
May be in the form of different species.
The total amount of the dissolved component is the sum of all the dissolved species
Avogadro’s number
1 mole of a substance = 6.02 x 10^23 that substance
Molality (m) is = to what?
moles / kg water
Molarity (M) is = to what?
moles / liter solution
What are the two types of geochemical reactions?
1) fluid-fluid reactions
2) fluid-rock reactions
Open system
- Fluid flows through the fluid-rock system
- Composition is controlled externally
- Fluid-rock ratio = high
- Chem eq = not reached
Closed system
- Fluid is stagnant in the fluid-rock system
- Composition is controlled internally
- Fluid-rock ratio = low
- Chem eq = reached
True or False: The geochemical reaction equation is NOT unique for a given pair of reactant & product
True
Chemical reaction equilibrium
Forward reaction rate = Reverse reaction rate
Le Chatelier’s Principle
If equilibrium is disturbed, reaction will proceed in the direction that will counteract the disturbance
Difference between concentration and activity
Concentration: (A)
Activity: [A]
Law of mass action equation
K =[C]^c [D]^d / [A]^a [B]^b
How is equilibrium constant K determined?
- Experiment at specific P-T-X conditions
- Calculations from thermodynamic database
Congruent dissolution
The dissolution of one mineral is not accompanied by precipitation of another mineral
Incongruent dissolution
The dissolution of one mineral is accompanied by the precipitation of another
Solubility of a mineral
The max amount of a compound in a mineral that can be dissolved in solution at a given P-T condition (must be in eq)
Mineral solubility constant (Ksp)
Chemical reaction constant (K) for the mineral dissolution equation
Ion activity product (IAP)
The activity of dissolved ions before chemical equilibrium
What happens when IAP = Ksp?
Saturation
What happens when IAP < Ksp?
Undersaturation
What happens when IAP > Ksp?
Oversaturation
Geochemical divide
When the solution ratio approaches zero. Precipitation stops
Solubility diagram
A diagram that shows the variation in solubility of a mineral and the change of the dominant dissolved species
Mineral stability diagram
A diagram that shows the fields in which different minerals are stable
How are mineral stability diagrams constructed? (4 steps)
1) List all minerals involved
2) Construct the chemical reactions between pairs of minerals
3) Law of mass action to construct activity-activity equations for each pair
4) Plot all activity-activity relations
Redox
Chemical reaction where electrons are transferred from one species to another
Oxidized: _____ electrons
Gives away
Reduced: _____ electrons
Gains
Oxidized –> _____ agent
Reducing
Reduced –> _____ agent
Oxidizing
True or False: Reduction and oxidation always occur together
True
Fugacity
Effective partial pressure of a gas in a system
Oxygen fugacity
Effective partial pressure of O2 in a system
Eh
Redox state of a solution
What is the pH equation?
pH = -log [H+]
What happens when two different geologic fluids encounter each other?
They react with one another
What happens when a geologic fluid is in contact with a rock?
Chemical reaction. Rock will be slightly dissolved
How to predict the direction of geochemical reactions?
Le Chatelier’s Principle
What controls the composition (including Eh and pH) of geologic fluids?
Different locations
Different history
Different rocks reacting with one another
