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Earth And Space Sciences › Geochemistry

Weathering and Soil Chemistry

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Aqueous GeochemistryMineral Stability and Phase Diagrams→→Environmental Geochemistry
weathering soil-chemistry chemical-weathering clay-minerals pedogenesis

Core Idea

Chemical weathering is the dissolution and transformation of primary minerals (formed at high T-P) to secondary minerals (stable at surface conditions) through reactions with water, dissolved CO2, organic acids, and oxygen. Silicate weathering -- the dominant long-term process -- consumes atmospheric CO2 (e.g., CaSiO3 + CO2 -> CaCO3 + SiO2), acting as Earth's primary thermostat over million-year timescales. Weathering intensity depends on temperature, precipitation, biological activity, rock type, and topography. The products -- dissolved ions (Ca, Mg, Na, K, Si, HCO3-) and secondary clay minerals (kaolinite, smectite, gibbsite) -- determine soil composition, river chemistry, and ultimately the geochemical inputs to the ocean. Soil profiles develop through these processes, with distinct horizons reflecting progressive leaching and mineral transformation with depth.

Explainer

Weathering is the fundamental process connecting solid Earth geochemistry to surface geochemistry. Every ion in every river, every clay mineral in every soil, and every grain of sand on every beach is a product of weathering. At the planetary scale, silicate weathering regulates atmospheric CO2 and has maintained habitable conditions for most of Earth's history.

The thermodynamic driving force for weathering is the instability of high-temperature minerals at surface conditions. Olivine, pyroxene, feldspar, and mica crystallized at 700-1200 C and pressures of kilobars. At 15 C and 1 atm, they are thermodynamically unstable with respect to clay minerals, oxides, and dissolved ions. The Goldich dissolution series (olivine weathers fastest, quartz slowest) mirrors the reverse of Bowen's reaction series -- minerals that crystallize at the highest temperatures are least stable at the surface. This reflects the greater structural adjustment required for high-T minerals to reach equilibrium with surface conditions.

Carbonic acid weathering dominates globally. Soil CO2 concentrations (10-100x atmospheric) from root respiration and microbial decomposition dissolve in soil water to form H2CO3. This attacks silicate minerals: 2KAlSi3O8 + 2H2CO3 + 9H2O -> Al2Si2O5(OH)4 + 4H4SiO4 + 2K+ + 2HCO3-. The products -- kaolinite (secondary clay), dissolved silica, potassium, and bicarbonate -- are transported by rivers to the ocean. The HCO3- eventually precipitates as marine carbonate (CaCO3), completing the long-term carbon cycle. This reaction consumes atmospheric CO2 only when silicate (not carbonate) minerals weather, because carbonate weathering is reversed by carbonate precipitation in the ocean.

Soil chemistry reflects the progressive stages of weathering with depth. A typical soil profile has organic-rich surface horizons (O, A), a leached eluvial horizon (E), a clay/iron-enriched illuvial horizon (B), and weathered parent material (C) grading into bedrock (R). The clay mineralogy changes systematically with weathering intensity: 2:1 clays (smectite, vermiculite) in moderately weathered soils; 1:1 clays (kaolinite) in more intensely weathered soils; and aluminum and iron oxides/hydroxides (gibbsite, goethite) in the most weathered tropical soils. This sequence records progressive loss of silica and base cations, driven by the thermodynamic imperative to reach surface equilibrium.

Practice Questions 3 questions

Prerequisite Chain

Understanding Zero → The Number Zero → Counting to Five → Counting to 10 → Counting to 20 → Counting a Set of Objects Up to 20 → Cardinality: The Last Number Counted → Matching Numerals to Quantities → Subitizing Small Quantities → Addition Within 10 → Number Bonds to 10 → Addition Within 20 → Doubles and Near Doubles → Doubles Facts Within 10 → Near Doubles Facts Within 20 → Mental Math Strategies for Addition → Mental Math: Adding and Subtracting Tens → Addition Within 100 → Repeated Addition as Multiplication → Multiplication as Equal Groups → Multiplication: Arrays → Basic Multiplication Facts (0s, 1s, 2s, 5s, 10s) → Multiplication Facts Within 100 → Division as Equal Sharing → Division as Grouping (Measurement Division) → Division: Grouping (Repeated Subtraction) Model → Division: Fair Sharing Model → Division as Equal Sharing → Division as Grouping → Basic Division Facts → Division Facts Within 100 → Multiplication and Division Fact Families → Relationship Between Multiplication and Division → Division Facts as Inverse of Multiplication → Remainders and Quotients in Division → Division Word Problems → Multi-Step Word Problems → Solving Multi-Step Word Problems → Multiplication Word Problems → Division Word Problems → Introduction to Long Division → Factors and Multiples → Prime and Composite Numbers → Equivalent Fractions → Relating Fractions and Decimals → Decimal Place Value → Integers and the Number Line → Comparing and Ordering Integers → Absolute Value → Adding Integers → Subtracting Integers → Multiplying Integers → Dividing Integers → Unit Rates → Proportions → Percent Concept → Converting Between Fractions, Decimals, and Percents → Operations with Rational Numbers → Two-Step Equations → Solving Multi-Step Equations → Equations with Variables on Both Sides → Angle Pairs: Complementary, Supplementary, and Vertical → Parallel Lines and Transversals → Corresponding Angles → Alternate Interior Angles → Triangle Angle Sum Theorem → Exterior Angle Theorem → Triangle Inequality Theorem → Similar Triangles: AA Similarity → Similar Triangles: SSS and SAS Similarity → Proportions in Similar Triangles → Right Triangle Trigonometry Introduction → Sine, Cosine, and Tangent Ratios → Trigonometric Ratios Review → Radian Measure → Converting Between Degrees and Radians → The Unit Circle → Graphing Sine and Cosine → Graphing Tangent and Reciprocal Trigonometric Functions → Derivatives of Trigonometric Functions → Antiderivatives → Indefinite Integrals → Basic Integration Rules → Riemann Sums → Definite Integral Definition → Double Integrals: Definition and Setup → Iterated Integrals and Fubini's Theorem → Double Integrals over Rectangular Regions → Double Integrals over General Regions → Applications of Double Integrals: Area, Mass, and Moments → Triple Integrals in Cartesian Coordinates → Triple Integrals in Cylindrical and Spherical Coordinates → Change of Variables and the Jacobian Determinant → Applications of Triple Integrals: Volume and Mass → Vector Fields and Their Representations → Line Integrals of Vector Fields → Work and Circulation → Line Integrals of Scalar and Vector Functions → Fundamental Theorem for Line Integrals → Conservative Vector Fields → Conservative Vector Fields and Potential Functions → Curl and Divergence of Vector Fields → Curl and Divergence → Divergence Theorem → Electric Flux and Divergence Theorem → Gauss's Law: Integral Form and Meaning → Solving Problems with Gauss's Law → Conductors in Electrostatic Equilibrium → Capacitance and Capacitors → Dielectrics → Dielectric Constant and Relative Permittivity → Electric Field Inside Dielectric Materials → Dielectric Materials and Polarization → Dielectric Susceptibility and Permittivity → Energy Density in Electric Fields → Electric Current and Current Density → Electrical Resistance and Resistivity → Ohm's Law and Circuit Elements → Electromotive Force (EMF) and Batteries → Kirchhoff's Circuit Laws: Voltage and Current → DC Circuit Network Analysis Methods → Transient Response in RC Circuits → RC Circuits → LC and RLC Circuits → AC Circuits: Fundamentals → Impedance and Reactance → AC Power and Resonance → Electromagnetic Waves → Postulates of Special Relativity → Time Dilation → Length Contraction → Lorentz Transformation → Relativistic Velocity Addition → Relativistic Momentum and Energy → Mass-Energy Equivalence and E=mc² → Photons as Particles with Energy and Momentum → Planck-Einstein Relation: Energy and Frequency → Photoelectric Effect → The Photon: Light as Quanta → Compton Scattering → Wave-Particle Duality → de Broglie Wavelength → The Schrödinger Equation → State Vectors and Wavefunctions → Quantum Superposition → Quantum Entanglement → Bell Theorem and Bell Inequalities → Postulates of Quantum Mechanics → Observables and Quantum Operators → Commutators and Commutation Relations → Quantum Angular Momentum → Quantum Mechanical Treatment of Hydrogen → Solving the Schrödinger Equation for Hydrogen Atom → Quantum Numbers → Electron Configuration → Periodic Trends → Covalent Bonding → Electronegativity and Bond Polarity → Ionic Bonding → Lewis Structures → VSEPR Theory and Molecular Geometry → Molecular Geometry and Electron Pair Geometry → Molecular Polarity and Dipole Moments → Intermolecular Forces → States of Matter and Phase Changes: Melting, Boiling, and Sublimation → Gas Laws and the Ideal Gas Equation → Gas Stoichiometry and Volume-Volume Calculations → Thermochemistry and Enthalpy → Heat Capacity and Calorimetry → Entropy and Molecular Disorder → Spontaneity and ΔG → Entropy and Gibbs Free Energy → Chemical Equilibrium → Statistical Mechanics: Ensembles and the Boltzmann Distribution → Partition Function: Definition and Properties → The Canonical Partition Function and Thermodynamic Derivation → Free Energy and Thermodynamic Relations from Partition Functions → Legendre Transformations and Thermodynamic Potentials → Chemical Potential and Partial Molar Properties → Phase Equilibrium and Coexistence Conditions → Clausius-Clapeyron Equation → Phase Diagrams and Clausius-Clapeyron Equation → Chemical Potential and Thermodynamic Equilibrium → Geochemical Thermodynamics → Mineral Stability and Phase Diagrams → Weathering and Soil Chemistry

Longest path: 186 steps · 1071 total prerequisite topics

Prerequisites (2)

Aqueous Geochemistryhard Mineral Stability and Phase Diagramssoft

Leads To (1)

Environmental Geochemistrysoft