Redox

Question 1

Hydrothermal Fluids and Redox reaction

Answer 1

Hydrothermal fluids undergo redox state changes when heated and interacting with rocks.
Aid in transport of ore-forming elements; redox reactions occur in the solid Earth.
Fe and O are prominent in redox reactions, buffering the mantle’s redox state across temperatures.
Redox reactions vary with depth; most reduced species near the core-mantle boundary.
Deep Earth oxidation occurs at upper mantle subduction zones by breaking down subducted water.

Question 2

Example - Redox reaction in Olivine

Answer 2

Olivine (Fe2SiO4) is a common mantle mineral; fayalite is its Fe form.
Magnetite less common but found in granite; example of redox reaction.
Balancing the equation involves assigning valence numbers to elements.
SiO2 molecules canceled out; focus on species undergoing changes in oxidation state.
Formation of FMQ buffer (fayalite-magnetite-quartz) through redox reactions.
Oxygen fugacity (fO2) measures oxygen concentration; FMQ buffer a redox buffer.

Question 3

Balancing Redox reaction - Olivine

Answer 3

1. Assign valence numbers: O is -2, Si is +4, Fe2+ is +2, Fe3+ is used for simplicity.
2. SiO2 molecules canceled; focus on species undergoing oxidation state changes.
3. Balancing electrons in half reactions; Fe half reaction multiplied by four.
4. Combined half reactions and addition of Fe2+ species on the left side.
5. Addition of SiO44- anions to balance; remaking olivine and quartz.
6. Oxygen fugacity affects redox reactions; adding water increases fO2 at subduction zones.
7. Subduction zones generate magnetite-bearing granite, increasing oxygen fugacity.
8. Drop in fO2 buffered by reactions like FMQ; magnetite and quartz react to form fayalite.

Question 4

Redox reactions in Ingeous Rocks

Answer 4

Variety of redox reactions act as oxygen fugacity buffers in igneous rocks.
FMQ buffer simplified; actual reaction involves fayalite, oxygen, magnesiowustite, magnetite, and majorite.
Majorite, a garnet-like structure, stable at high pressure but not observed at Earth’s surface.
Mg phases excluded from redox equations if they don’t play a role.
Oxygen fugacity lowest at core-mantle boundary, increases toward the surface.

Question 5

Valance numbers in redox reactions

Answer 5

Valence number: charge an element would have if it formed an ion in a solution.
Group 1 Alkali elements: Form +1 cations, valence no. = +1. Exception: H = 0 in H2.
Group 2 Alkali earth elements: Form +2 cations, valence no. = +2.
Group 17 Halogens: Largely form -1 anions, valence no. = -1. Exception: I = +5 in IO3̶.
Valence of elements in pure form is 0.
Sum of valence numbers in a species equals the total charge (FeCl+, FeCl2+, MnCl4).
Hydrogen’s valence: +1, except in metal hybrids (-1), and in H2 (0) (CH4, H2O).
Oxygen’s valence: -2, except in peroxides (-1), and in O2 (0) (MnO, CaSO4, H2S, IO3-, CaCO3, UO2, UO2(CO3)22-).

Question 6

Redox reactions basics

Answer 6

Redox reactions involve electron transfer and change in valence number.
Oxidation: Loss of electrons.
Reduction: Gain of electrons.
Example: 2Fe + O2 + H2O → Fe2O3 + H2.
Oxidation of iron (Fe0) to rust (Fe2O3; Fe3+); Fe0 loses electrons (oxidized), and O2 and H2 gain electrons (reduced).
Involves electron donors and acceptors.

Question 7

Balancing Redox equations steps

Answer 7

1. Identify oxidized and reduced species.
2. Write two half reactions.
3. Balance oxygen with water.
4. Balance hydrogen with protons.
5. Balance charge with electrons.
6. Ensure the same number of electrons in each half reaction.
7. Add two half reactions.
8. Cancel electrons.
9. Compare with the original and add species equally to both sides.
10. Cancel species equally on both sides.
11. Check final reaction for charge and species balance.