The dissolution of calcite in rainwater (CaCO3 + CO2 + H2O -> Ca2+ + 2HCO3-) has a negative delta-G at Earth's surface conditions. What does this predict about limestone weathering?
ACalcite precipitation is favored at Earth's surface
BThe reaction is thermodynamically favorable and will proceed spontaneously, dissolving calcite -- consistent with the observed formation of karst landscapes, caves, and limestone dissolution in acidic groundwater
CThe reaction will not occur because calcite is a stable mineral
DKinetics prevent any dissolution regardless of thermodynamics
A negative delta-G means the reaction is thermodynamically favorable in the forward direction. This correctly predicts that calcite dissolves in CO2-bearing water, forming karst landscapes, cave systems, and contributing calcium and bicarbonate to rivers and groundwater. However, thermodynamics only predicts the direction and equilibrium state -- kinetics determines the rate, which is fast for calcite dissolution.
Question 2 True / False
A mineral assemblage that is stable at high temperature and pressure will remain stable when brought to Earth's surface conditions because minerals do not change once formed.
TTrue
FFalse
Answer: False
Thermodynamic stability depends on the ambient T-P conditions. Minerals stable at depth (high-T, high-P) are often metastable or unstable at surface conditions. Olivine weathers rapidly at the surface; high-pressure polymorphs (coesite, diamond) are metastable at 1 atm. The reason some high-T/P minerals persist at the surface is kinetics -- reaction rates are too slow at low temperature for thermodynamic equilibrium to be achieved in geologic time. Thermodynamics predicts what should happen; kinetics determines whether it actually does.
Question 3 Short Answer
Explain why the equilibrium constant for a geochemical reaction changes with temperature, and what this implies for mineral stability across Earth's temperature gradient.
Think about your answer, then reveal below.
Model answer: The van't Hoff equation relates K to temperature: d(ln K)/dT = delta-H/(RT^2). For endothermic reactions (positive delta-H), K increases with temperature, shifting equilibrium toward products. For exothermic reactions, K decreases with temperature. This means mineral stability fields shift systematically with depth -- minerals stable at surface temperatures may dissolve or transform at depth, and vice versa. This T-dependence drives metamorphic mineral reactions, hydrothermal alteration, and the zonation of mineral assemblages in plutonic systems.
The coupling of K with T through enthalpy is why geologists can use mineral assemblages as geothermometers -- the specific minerals present record the temperature at which the system last equilibrated.