Week 8 - Chemical Weathering and Soils Flashcards
(18 cards)
What is the difference between physical and chemical weathering?
Physical weathering breaks rocks into smaller pieces without changing their composition, while chemical weathering alters the mineral composition through reactions with water, acids, and gases.
Why is physical weathering important for chemical weathering?
It increases surface area, exposes minerals, and enables access of water and gases.
What drives chemical weathering? What are common products of chemical weathering?
Thermodynamic instability of primary minerals at the surface, reacting with water, acids (like CO₂), and oxygen.
Secondary minerals like clays (e.g., kaolinite) and iron oxides (e.g., goethite).
What are the layers seen by geochemists in soil?
Soil, regolith, saprolite, and parent material (protolith
Soil - biologically active, organic-rich, mobile layer. Zone of active biological cycling and nutrient exchange.
Regolith - includes soil and all unconsolidated material above rock, non-lithified. Includes both biologically active and inactive zones.
Saprolite – weathered (chemically altered) but structurally intact rock. It is immobile, unlike soil, and often rich in secondary minerals like clays and oxides.
Parent material / Bedrock – unweathered rock (source)
How is % change in an element due to weathering calculated?
%ΔX = 100 × [(Csoil - Cparent) / Cparent]
What is τ (tau) in the context of weathering geochemistry and how is it calculated?
Tau (τ) is the mass transfer coefficient used to quantify the gain or loss of an element (j) during chemical weathering, relative to a stable, immobile reference element (i). It accounts for dilution, enrichment, and porosity changes.
Because weathering causes volume changes (like porosity increase), simply comparing concentrations can be misleading.
τ normalises changes in the mobile element relative to an immobile one, correcting for volumetric strain and relative enrichment or depletion.
τj = ( [Cj,w / Ci,w] / [Cj,p /Ci,p]) - 1
where:
Cj,w , Ci,w : concentrations in weathered material
Cj,p , Ci,p : concentrations in the parent material (protolith)
j: mobile element
i: immobile element (e.g., Zr, Ti, or quartz)
Elements or minerals that are chemically resistant to weathering and do not migrate easily are used as the immobile reference in τ calculations
If:
τ = 0 → no gain or loss of element j
τ > 0 → net gain (enrichment) of element j
τ < 0 → net loss (depletion) of element j
τ = –1 → complete depletion of element j
τ > 1 → more than double the original content (external addition likely)
What is the Goldich Stability Series, and what does it represent?
It is a conceptual ranking of silicate minerals based on their relative resistance to chemical weathering. It reflects how thermodynamically unstable a mineral is at Earth’s surface conditions, depending on its formation temperature, crystal structure, and bonding.
Why do minerals like olivine weather faster than quartz?
Olivine forms at high temperatures and has isolated SiO₄ tetrahedra with fewer shared bonds, making it more reactive. Quartz forms at low temperatures, with a fully polymerised framework of strong Si–O bonds, making it chemically stable.
What are the general trends in the Goldich series from least to most stable?
Least stable: Olivine → Pyroxene → Amphibole → Biotite
Intermediate: Plagioclase (Ca-rich to Na-rich)
Most stable: Orthoclase → Muscovite → Quartz
What is polymerisation in the context of silicate minerals?
Polymerisation refers to the linking of SiO₄ tetrahedra (silicon-oxygen units) by sharing oxygen atoms at the corners. The degree of polymerisation describes how many corners each tetrahedron shares with others, forming more complex mineral structures.
Progression of polymerisation in common silicate structures:
Isolated tetrahedra: Olivine (no shared O)
Single chains: Pyroxene (2 shared O per tetrahedron)
Double chains: Amphibole (some share 3 O)
Sheets: Mica, clay minerals (3 shared O)
Frameworks: Feldspar, Quartz (4 shared O)
How does polymerisation affect bond strength and chemical weathering? How does it relate to Gibbs Free Energy (ΔG) and mineral stability?
More polymerised structures have more covalent Si–O bonds and are harder to break apart, so they resist chemical weathering more effectively. These have low driving force for dissolution, more positive ΔG.
Minerals with low polymerisation (e.g., olivine) have fewer strong covalent Si–O bonds, making them more reactive and susceptible to faster weathering.
These have more negative ΔG for dissolution at surface conditions (favourable to dissolve).
The degree of polymerisation influences:
Mineral stability in the Critical Zone
Soil mineralogy and fertility
Weathering rates
Carbon cycling and climate feedbacks
What are primary minerals in chemical weathering processes?
Primary minerals are those that are inherited directly from the parent rock (igneous, sedimentary, or metamorphic) and formed under high-temperature and/or pressure conditions different from Earth’s surface.
Because of this, they are often thermodynamically unstable at surface conditions and prone to chemical weathering.
Common characteristics of primary minerals:
-Form deep in the Earth or in marine settings
-Unstable in the Critical Zone
-Susceptible to dissolution, hydration, hydrolysis, oxidation
-Examples:
-Olivine
-Feldspar
-Pyroxene
-Biotite
-Calcite (in carbonate rocks)
What are secondary minerals in chemical weathering processes?
Secondary minerals form as products of chemical weathering of primary minerals. They develop in situ under surface or near-surface conditions and often reflect local climate, parent rock chemistry, and soil age.
Common types of secondary minerals:
-Clay minerals (e.g. smectite, kaolinite)
-Oxides & hydroxides (e.g. hematite, goethite, gibbsite)
-Carbonates (e.g. calcite)
-Sulfates (e.g. gypsum)
Controls on secondary mineral formation:
-Climate: wetter → more intense leaching, different clays
-Parent material: determines available elements
-Soil age: older soils often have more weathered, stable secondary phases
What is hydrolysis, and how does it relate to chemical weathering?
Hydrolysis is a chemical reaction between water and a mineral in which the mineral breaks down into new substances, often producing clay minerals and dissolved ions.
A reaction of a mineral with water, often involving H⁺ ions from carbonic acid (H₂CO₃) formed when CO₂ dissolves in rain or snow.
→ The reaction replaces metal cations with H⁺ and leads to mineral decomposition.
It is one of the main mechanisms driving chemical weathering, especially in silicate rocks.
E.g
Weathering of potassium feldspar (KAlSi₃O₈):
KAlSi3O8 + H2CO3 + H20 —> Al2Si2O5 (OH)4 + K+ + HCO3- + SiO2
Products:
-Clay mineral (kaolinite, solid)
-Dissolved ions: potassium (K⁺), bicarbonate (HCO₃⁻), silica (SiO₂)
Hydrolysis transforms unstable primary minerals into stable secondary clays, and is a key mechanism by which carbonic acid weathers silicate rocks — linking the carbon cycle, soil formation, and climate regulation.
What is a dissolution reaction, and how does it contribute to chemical weathering?
Dissolution is a chemical weathering process where a mineral dissolves directly into water without forming secondary solids.
It is most common in minerals with dominantly ionic bonds, such as salts, carbonates, and some sulfates.
In water, the attraction between:
H+ H2O and anions (e.g. Cl⁻, SO₄²⁻)
O- in H2O and cations (e.g. Na⁺, Ca²⁺)
is stronger than the ionic bond holding the mineral lattice together.
→ The mineral dissociates into its constituent ions.
-No secondary phase formed
-Fully congruent dissolution
-Reversible depending on water saturation (governed by solubility product Ksp)
What is an oxidation reaction, and how does it contribute to chemical weathering?
Oxidation in weathering refers to a reaction where an atom, ion, or molecule loses electrons, often when exposed to oxygen.
It is especially important for minerals containing reduced elements (like Fe²⁺ or S²⁻) that formed in low-oxygen (reducing) environments but are later exposed to oxygen-rich conditions such as air or soil water.
Ions are most commonly oxidised in soils:
-Fe²⁺ (ferrous iron) → oxidised to Fe³⁺ (ferric)
-S²⁻ (reduced sulfur) → oxidised to SO₄²⁻ (sulfate)
Role of Microbes:
Soil microbes can accelerate oxidation by using electrons released from Fe²⁺ as an energy source
This biological mediation makes Fe oxidation much faster than would occur abiotically
Meanwhile, nitrate (NO₃⁻) may be reduced to N₂ gas, completing redox cycling
Why oxidation matters in soils:
-Drives formation of iron crusts, gossans, acid sulfate soils
-Releases acidity into the environment (H⁺ production)
-Controls nutrient cycling, trace metal mobility, and soil colour (red/yellow)
How does chemical weathering buffer environmental acidity?
Chemical weathering—especially through hydrolysis reactions—serves as a natural buffer system that helps maintain pH balance in soils and waters. It works by consuming hydrogen ions (H⁺) from acidic inputs (like rainfall or acid drainage) and sequestering them into secondary minerals or dissolved products, effectively increasing pH and neutralizing acidity.
Mechanism – Hydrolysis of Silicates
-When acidic water interacts with silicate minerals (e.g. feldspars), hydrolysis reactions occur.
-H⁺ from carbonic acid is consumed
-Clay minerals (e.g. kaolinite) are formed
-Acidity is reduced, pH increases
Acid Input Sources
-Rainwater is naturally acidic due to dissolved CO₂ → carbonic acid (H₂CO₃)
-Acid Rock Drainage (ARD): Natural oxidation of sulfide minerals (e.g. pyrite) produces sulfuric acid and H⁺
-Acid Mine Drainage (AMD): Human activity (mining) exposes more sulfides, accelerating ARD and acidifying nearby ecosystems.
Biological Role
Soil microbes can increase acidity by oxidizing Fe²⁺ to Fe³⁺, but when coupled with nitrate or sulfate reduction, they help consume hydrogen ions, raise pH, and buffer environmental acidity.
Example:
In anaerobic or waterlogged soils, sulfate reduction and denitrification dominate → strong acid buffering.
In well-aerated soils, iron oxidation may dominate → acidity may increase, unless neutralized by weathering or organic matter processes.
Why It Matters
-Buffers the natural acidity of rain and ARD/AMD
-Controls soil and water pH, essential for plant and aquatic life
-Regulates nutrient availability
-Plays a major role in the global carbon cycle via CO₂ drawdown in silicate weathering
What is a hydration reaction, and how does it affect minerals during chemical weathering?
Hydration is a chemical weathering process where water molecules are incorporated directly into the crystal structure of a mineral, forming a new hydrated mineral.
The water is split into H⁺ and OH⁻, which are structurally integrated into the mineral lattice, altering its composition and stability.
Unlike hydrolysis (where minerals break apart), in hydration the original mineral stays largely intact structurally, but adds water and forms a new hydrated phase.
Example: Hematite to Goethite
Hematite (anhydrous) becomes goethite (hydrated iron oxide)
This often occurs under moist, low-temperature conditions
Significance
-Hydration is common in tropical and temperate soils with high water activity
-It leads to the formation of secondary hydrated minerals (e.g. clays, hydroxides)
-Influences soil colour, texture, and mineral stability
