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Earth And Space Sciences › Geochemistry

Weathering and Soil Chemistry

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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

Counting to 10 → Counting to 20 → Understanding Zero → The Number Zero → Counting to Five → One-to-One Correspondence → Combining Small Groups Within 5 → Addition Within 10 → Addition Within 20 → Two-Digit Addition Without Regrouping → Two-Digit Addition with Regrouping → Addition Within 100 → Repeated Addition as Multiplication → 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 → Two-Digit by One-Digit Division → Division with Remainders → Remainders and Quotients in Division → Division Word Problems → Introduction to Long Division → Factors and Multiples → Prime and Composite Numbers → Equivalent Fractions → Relating Fractions and Decimals → Decimal Place Value → Reading and Writing Decimals → Comparing and Ordering Decimals → Adding and Subtracting Decimals → Multiplying Decimals → Dividing Decimals → Dividing Fractions → Mixed Number Arithmetic → Order of Operations → Integer Order of Operations → Variable Expressions → Combining Like Terms → One-Step Equations → 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 → 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 → Iterated Integrals and Fubini's Theorem → Double Integrals in Cartesian Coordinates → Double Integrals over Rectangular Regions → Double Integrals in Polar Coordinates → 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 → Green's Theorem → Surface Integrals and Flux of Vector Fields → Surface Integrals and Flux of Vector Fields → Divergence Theorem: Flux and Outflow → Divergence Theorem → Electric Flux → 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 → The Electromagnetic Spectrum → Blackbody Radiation and Planck's Law → Photoelectric Effect → The Photon: Light as Quanta → Compton Scattering → Wave-Particle Duality → de Broglie Wavelength → Heisenberg Uncertainty Principle → Wavefunction and the Born Rule → The Schrödinger Equation → State Vectors and Wavefunctions → Quantum Superposition → Quantum Entanglement → Bell Theorem and Bell Inequalities → Postulates of Quantum Mechanics → Scattering Theory → Introduction to Scattering Theory → Partial Wave Analysis in Scattering → Spin Angular Momentum → Electron Spin and Intrinsic Magnetic Moment → Stern-Gerlach Experiment: Spin Quantization and Measurement → Electron Diffraction and Matter Wave Properties → Davisson-Germer Experiment: Crystal Diffraction of Electrons → Electron Diffraction and Matter Wave Interference → Wavefunctions and Probability Density Interpretation → Quantum Superposition and Linear Combinations of States → Quantum Operators and Observables → Canonical Commutation Relations and Uncertainty → Heisenberg Uncertainty Principle and Measurement Limits → Time-Independent Schrödinger Equation and Eigenvalues → Hydrogen Atom in Quantum Mechanics → Spectral Lines and Energy Transitions → Selection Rules for Atomic Transitions → LS and jj Coupling Schemes in Multi-Electron Atoms → Pauli Exclusion Principle and Antisymmetric Wavefunctions → Electron Configuration and the Aufbau Principle → The Periodic Table and Atomic Electronic Structure → The Periodic Table → Electron Configuration → Periodic Trends → Ionization Energy → Ionic Bonding → Lewis Structures → Resonance Structures and Delocalized Electrons → Resonance and Formal Charge → 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 → Solubility Equilibria → Phase Diagrams and Clausius-Clapeyron Equation → Chemical Potential and Thermodynamic Equilibrium → Geochemical Thermodynamics → Mineral Stability and Phase Diagrams → Weathering and Soil Chemistry

Longest path: 171 steps · 806 total prerequisite topics

Prerequisites (2)

Aqueous Geochemistryhard Mineral Stability and Phase Diagramssoft

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