ESC307 (Geochem) CH1-4 Questions

Chapter 1: Why do geologists need to study geochemistry?

Geochemistry applies chemistry to understand element behaviour and distribution, Earth's origin and evolution, rock formation, geological processes, resource exploration, past environments, and pollution.

Chapter 1: Is geochemistry like chemistry?

Geochemistry applies chemical principles to processes occurring in nature rather than focusing mainly on chemistry in the laboratory. It uses concepts such as electronic structure, pH, and Eh to understand geological systems.

Chapter 1: What are the two main areas covered in geochemistry?

Principles of geochemistry and applications of geochemistry.

Chapter 1: What topics are included in the principles of geochemistry?

Element geochemistry, crystal geochemistry, aqueous geochemistry, stable isotope geochemistry, and isotopic geochronology.

Chapter 1: What are the applications of geochemistry?

Solving geological problems involving sedimentary, magmatic, metamorphic, and tectonic processes; mineral deposit studies and exploration; and environmental studies.

Chapter 2: Why do element concentrations differ across Earth and among rocks?

Electronic structure controls element size, charge, and bonding character. These properties influence how elements behave and become concentrated in different Earth reservoirs and rock types.

Chapter 2: Why are noble gases enriched in the atmosphere?

Noble gases have stable electronic structures and very low electronegativity, affecting how they interact and bond with other elements.

Chapter 2: Why are certain elements associated with each other?

Elements with similar charge, size, and bonding character tend to behave similarly. Elements in the same periodic table group often behave similarly because they have the same valence.

Chapter 2: What are the four Goldschmidt element classes?

Lithophile: concentrated in silicates; siderophile: concentrated in iron; chalcophile: concentrated in sulfides; atmophile: concentrated in the atmosphere.

Chapter 3: Why can minerals contain many trace elements despite having few major elements?

Trace elements can substitute for major ions in a mineral's crystal lattice. Substitution depends on ionic radius, charge, and bonding character.

Chapter 3: Why is uranium concentrated in granitic rocks while chromium is concentrated in ultramafic rocks?

Uranium is generally incompatible and remains in the melt as crystallization progresses, helping enrich granitic rocks. Chromium is compatible with mafic minerals and tends to concentrate in ultramafic rocks.

Chapter 3: How can trace elements estimate mineral formation temperatures?

Element substitution depends on temperature; higher temperatures generally make substitution easier. For example, sphalerite generally contains more Fe at higher temperatures, allowing its composition to estimate formation temperature.

Chapter 4: What happens when two different geofluids encounter each other?

Fluid-fluid reactions can change fluid composition. For example, CO₂ dissolves in water and forms dissolved carbonic acid species.

Chapter 4: What happens when a geologic fluid contacts rock?

Fluid-rock reactions can change fluid composition and cause minerals to dissolve, precipitate, or be replaced. Closed systems have low fluid/rock ratios and are more likely to reach equilibrium; open systems have high ratios and are less likely to reach equilibrium.

Chapter 4: How do you construct a geochemical reaction equation?

Identify reactants and products, write their formulas, balance all elements, use H₂O and H⁺ to balance oxygen and hydrogen when appropriate, and check electrical neutrality. Equations may differ depending on fluid composition and dissolved species.

Chapter 4: How is the equilibrium constant K determined?

K can be measured experimentally under specified pressure, temperature, and composition conditions or calculated using a thermodynamic database. The law of mass action expresses K using product and reactant activities; concentrations can be used for ideal solutions.

Chapter 4: How do you predict reaction direction using Q and K?

If Q < K, the forward reaction is favoured; if Q > K, the reverse reaction is favoured; if Q = K, the system is at equilibrium. For mineral dissolution, compare IAP with Ksp: below means dissolution, equal means equilibrium, and above means precipitation is favoured.

Chapter 4: What determines how much mineral dissolves in a fluid?

Solubility is the maximum amount that dissolves at a given pressure and temperature at equilibrium. Compare IAP with Ksp: IAP < Ksp means undersaturated and dissolution is favoured; IAP = Ksp means saturated equilibrium; IAP > Ksp means oversaturation and precipitation is favoured.

Chapter 4: What controls the composition of geologic fluids?

Dissolved ions and gases, fluid-fluid and fluid-rock reactions, pressure and temperature, open or closed system conditions, minerals present, and chemical equilibria. pH reflects hydrogen-ion concentration; Eh describes the solution's redox state.