WEATHERING MECHANISMS

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WEATHERINGMECHANISMS PRODUCTS

• Mehrooz F Aspandiar
• CRC LEME
• WASM, Applied Geology,
• Curtin University of Technology

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Weathering why bother?

• Primary mechanism by which regolith is produced
  from saprolite to soil
• Influences geochemistry of regolith, ground and
  surface waters
• Main control over geochemical dispersion helps
  exploration environmental management
• Affects salt generation and movement in the
  regolith
• Affects acid generation in the regolith

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Why do rocks weather?

• Most rocks (and minerals) form at high
  temperatures and pressures and are therefore at
  equilibrium with the high T P environments
• When rocks are exposed to Earths surface, their
  equilibrium is disturbed, and their minerals
  react and experience transformation so as to
  adjust to low temperature, pressure and water
  conditions
• Three types of weathering
• Physical Mechanical breakdown of rock and
  regolith
• Chemical Chemical decomposition of rock by
  solutions (alters composition and mineralogy of
  rocks) - sometimes referred to as low
  temperature water-rock interactions
• Biological enhancement of chemical (biochemical)
  and physical weathering (biomechanical) -
  combined under physical and chemical weathering

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Weathering processes and products
Physical residue that is partly or wholly
chemically altered insoluble
Regolith Weathering profile
Fresh rock
Soluble ions released in solution to ground
surface waters (solutes)

• Physical weathering breaks down rocks into
  smaller fragments
• Chemical weathering alters the original material
  to new products

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Physical weathering

• Breaks down rocks into smaller particles which
  increases surface area for solution attack
• Opens up fractures, joints and micro-cracks in
  rocks due by exerting stress and facilitate
  solution access (chemical weathering)
• Several types Frost wedging, salt weathering,
  unloading, thermal weathering, bioturbation

Increasing weathering intensity
Chemical weathering products
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Bioturbation Biomechanical Processes

• Burrowing invertebrates - earthworms, ants,
  termites and vertebrates (mammals)
• turn over huge amounts of regolith material
  which via attrition reduces particle size
• Roots
• penetrate rocks and weathered mantle and force
  apart material water access
• Tree fall
• Transfer subsurface rock and regolith to surface
• mixing and breakdown of material at surface

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Bioturbation in action
Tree fall moving and breaking down sub surface
material
Termetaria recycling top soil, quartz gravel and
branches
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Chemical Weathering/water rock interactionDissolu
tion

• Simplest chemical weathering reaction is
  dissolution of easily soluble minerals
  (especially soluble salts)
• CaSO4 ? Ca2 SO42-
• Water causes ionic bonds of mineral to dissociate
  into free ions
• Water unaffected

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Solubility Equilibrium based

• Solubility of a mineral amount that dissolves
  in water to establish equilibrium with the
  mineral and its ionic components in solution
• CaCO3 Ca2 CO3-
• Depends on the conditions - pH, temperature,
  surface area in contact with fluid, other or
  competing ions in solution (kinetics)
• Solubility for a mineral provided by equilibrium
  constant K, or solubility product Ksp
  experimentally determined value for the
  dissociation reaction Ksp calcite aCa2 aCO3
  10-8.4 3.36 x 10-9 resulting in Ca2
  concentration of 2.4 ppm
• Solutions with lower values than the Ksp will
  cause calcite to dissolve into its component ions
  
• pH is critical for some minerals quartz only
  dissolves at high pH

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Rate of weathering - kinetics

• Rate of reactions as important as thermodynamic
  equilibrium between solutions and reacting
  minerals
• e.g. sulphide exposed to air does not always
  oxidize rapidly?
• Varies on type of sulphide (crystal structure,
  grain size, amount of O2)
• CW reactions are multi-step processes
  elementary reactions
• Overall reaction rate is a function of
• surface area flow rate gt flowing solutions
  maintain undersaturtion
• pH gt lower pH faster rate
• Temperature gt higher temperature, faster rate

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Hydrolysis

• Water combines with atmospheric and soil CO2 to
  form a weak acid - carbonic acidgt H2O CO2 ??
  H2CO3 H2CO3 ? H HCO3-
• Metals in minerals are replaced or exchanged by
  H with cation release as metal cation (K, Ca2,
  Na etc) and potential formation of a new clay
  mineral (kaolinite, smectite etc) from retained
  ions (Al3, O2-, Si4)
• K-feldspar H ? kaolinite K
  H4SiO4
• Ligand exchange is another variant, where ligand
  (oxalate) enhances break up the Metal (M) O
  bond and facilitates replacement of M cation by
  H and OH-
• Ligand exchange via oxalates and other organic
  acids enables dissolution of the insoluble Fe-Al
  oxides and hydroxides

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Crystal-chemical details in feldspar altering to
clay
At the molecular level, it is about mineral
structures, bond breakage between atoms, ionic
transport from reaction sites reaction rates or
kinetics, and not purely thermodynamic equilibrium
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Oxidation

• Oxidation reduction accomplished by electron
  transfer
• Oxidation - loss of electrons
• Reduction -gain of electrons of ions
• Oxidation causes change in ionic radii
  facilitates bond breakage
• Commonly oxidized elements and visible in the
  regolith are
• Fe2 ? Fe3 Mn2 ? Mn3 So ? S6
• Reduced Fe/Mn/S bearing minerals (olivines,
  pyroxenes, sulphides) undergo oxidation

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Biochemical weathering

• Microbes vegetation (rhizosphere) release
  organic acids - facilitate hydrolysis of minerals
  complex ions within the mineral and help their
  release
• e.g. K release from biotite is faster
• Microbes and vegetation change solution pH that
  strongly affects silicate carbonate weathering
  by
• Microbial metabolism enhances regolith
  (especially soil) CO2 levels carbonic acid
• Produce acid and alkaline compounds that affect
  solution pH
• Catalyze oxidation-reduction reactions of metals

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Some other processes..

• Fire or heat
• Forest fires new minerals and transform soil
  minerals
• Goethite organic matter heat maghemite
• Calcium oxalate calcite in plants
• Impacts
• Impacts vapourize and reduce size of rock and
  surface materials
• Change the composition of material
• Regolith on the moon is mostly produced by
  impacts!

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What changes accompany rock weathering?

• Colour - from rock colour to grey, red or yellow
  hues due to oxidation of iron (Fe2 to Fe3)
• Density - removal (decrease) or addition
  (increases) of material collapse (decrease) or
  dilation (increase) of original materia
• Composition- mineralogical and chemical change
  towards more stable forms - solubility of
  elements, mineral susceptibility and secondary
  mineral types
• Fabric or texture - change from rock fabric to
  soil fabric (development of new structures)

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Primary minerals

• Most rocks are composed of minerals that weather
  to a degree. Most common are
• Silicates
• Neosilicate (olivine) (Fe-Mg)2SiO4
• Cyclosilicate (beryl, tourmaline)
• Chain/Iono (pyroxene amphibole) (CaMg)2Si2O6
• Sheet/Phyllo (mica, kaolin, talc, chlorite)
  KFeAlSi3O10(OH)
• Framework/Tecto (quartz feldspar) K-Na-CaAlSi3O
• Glass (unstructured)
• Sulphides (pyrite, galena etc)
• Oxides (magnetite, rutile, spinel)

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Types of regolith minerals

• Phyllosilicates or clay minerals
• Smectites, kaolinite, illite, vermiculite
  interstratified varieties of these
• Silicates Opal A opal-CT, quartz
• Oxides hydroxides Fe, Mn, Al Ti
• Geothite, hematite, maghemite, gibbsite,
  lithiophorite, pyrolusite
• Sulphates - Gypsum, jarosite, alunite
• Carbonates Calcite, dolomite, magnesite,
  siderite
• Chlorides - Halite
• Phosphates Crandalite, florencite

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Mineral weathering what does it involve?

• The main processes achieved via mechanisms such
  as hydrolysis, ion exchange, oxidation
• Replacement of more soluble ions by protons
  (hydrolysis)
• K-feldspar water gt kaolinite solutes
• Change of Al coordination from 4 to 6 (hydrolysis
  facilitated)
• Oxidation of Fe (oxidation)

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Replacement of soluble ions by protons (H)

• Primary
• Feldspar (K,Na,Ca)AlSi3O8
• Pyroxene (Mg,Ca,Fe)SiO3
• Amphibole (Ca,Mg,Fe)Si8O22(OH)2
• Olivine (Mg,Fe)2SiO4
• Mica (K,Fe)Al3Si3O10(OH)2
• Secondary
• Kaolinite Al2Si2O5(OH)
• Smectite (Ca,Mg,Fe)AlSi3O10(OH)2.H2O
• Illite KAl3Si3O10(OH)2
• Goethite FeOOH
• Hematite Fe2O3

Ca2, Na, Mg2 K Released as solutes
H H2O
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Change of Al coordination on weathering
Change from four fold (tetrahedral) to six-fold
(octahedral) on weathering
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Oxidation of Fe ( Mn)

• Fe2 in biotite, pyroxene, olivine, pyrite
• Oxidation gt higher charge Fe3, smaller ionic
  radii
• Fe3 - combines readily with O2- to form oxides
  and hydroxides gt goethite, hematite, maghemite,
  lepidocrocite, ferrihydrite
• Fine grained gt reddish-brown hues

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Mineral stability to weathering
A Related to connectedness of tetrahedras B
Does not always follow the above rule - unusual
geochemical conditions can reverse the trends!
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Primary mineral stability - exceptions

• The Goldichs sequence - connectedness of
  silicate tetrahedras orthosilicates gt single
  chain gt double chain gt framework
• Then why is zircon very resistant but olivine
  least? Both are orthosilicates!
• Weathering sequences are affected by
• Bond strengths Zr-O strong (zircon), Mg-O weak
  (olivine)
• Surface or clay coatings on mineral
• Microbes (in some environments, feldspars weather
  faster than olivine because specific bacteria
  catalyze reactions by attacking nutrient rich Ca
  plagioclase first)

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Silicate mineral weathering pathways
Type of mineral and grain size depends on
micro-macro hydrology and geochemical conditions
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Other mineral weathering pathways

• Ions in solutes
• Combine to form new minerals in the profile (Al,
  Si, Fe, K, Mg)
• Combine to form new minerals elsewhere in
  landscape (valleys floors) groundwater (CO3,
  SO4, Fe, U, S)
• Transported to rivers and oceans (Ca, Na, K, Mg)

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Fresh Granodiorite
Saprolite
Bt
Hb
Fld
Soil B horizon
Soil B horizon
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Pyroxene Wethering
Pyroxenes weather to smectite goethite Space is
created, some Ca-Mg lost, some Ca,Mg,Al,Si in
smectite, Fe in geothite
Secondary mineral assemblages along cleavages
dissolution leaves behind space boxwork fabric
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Plagioclase altering to Al-smectite (incongruent)
Ca2Al2Si2O8 H H2O gt Ca2 Al2Si2O5(OH)4
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Mineral weathering applications

• Silicate and carbonate weathering
• consumes acid (H) gt buffers acidity
• consumes water (hydrolysis) gt extra salt in
  profile
• releases cations to solutes (groundwater) gt
  changes composition of groundwater along flow
  path and vertically
• Sulphide weathering secondary iron oxide
  formation
• Generates acid within mine waste piles, tailings,
  underground open cut mines
• Results in formation of gossans (indicators of
  massive sulphides)
• Solutes can accumulate in lower parts of
  landscape salts (halite), oxides (ferricrete),
  silicates (smectite) carbonates (calcrete)

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Acid-producing potential (AP)
FeS2 15/4O2 7/2H2O gt Fe(OH)3 4H 2SO42-
14Fe2 3.5O2 14H gt 14Fe3 7H2O
Iron oxidation is microbially catalyzed
Neutralization Potential (NP)
CaCO3 2H gt Ca2 CO2 H2O
CaAl2S2O8 8H gt Ca2 2Al3 2H4SiO4
Fe(OH)3 H gt Fe3 H2O
Net Neutralization Potential NP - AP
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Factors affecting weatheringClimate Organisms

• The Clorpt model function (climate, organism,
  relief, parent material, time..)
• Climate precipitation temperature
• Amount of water gt alters minerals, flushes
  solutes, affects vegetation gt generally increases
  rate
• Seasonality of precipitation affects rate to a
  degree
• Higher temperatures increase mineral weathering
  rate but only up to a degree and depth
• Controls vegetation gt indirectly affects rate
• Organisms (Biota)
• Higher density gt more organics gt more carbonic
  acid gt faster weathering
• Denser vegetation gt better soil stability gt
  deeper weathering
• Related to climate

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Factors affecting weatheringLithology Structure

• Parent Material (Lithology)
• Mineralogy easily weathered vs resistant
• Olivine, glass pyroxene fast volcanics fast
• Quartz K-feldspar slow plutonics
  quartzite slow
• Porosity high vs low
• Porous sediments better circulation faster
• Impermeable no circulation slower
• Faults and shears
• Enhance weathering rate better water
  circulation
• Sheared regions deeply weathered

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Factors affecting weatheringLandform (relief)
and Time

• Relief (Landform and Tectonics)
• Hill tops better drained faster weathering
• Slopes faster weathering but faster erosion
• Valleys slower weathering, solute precipitation
• Local and regional tectonics
• Mountain ranges faster erosion, more solutes
  (higher Ca, Na, Mg)
• Basins Deeper weathering, retention of products,
  less solutes
• Time
• Affects all the above
• Inheritance of weathering products from one
  climate and landform situation to another is
  critical in evaluating individual factors

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Weathering of Rock Types
Volcanic - clay
Plutonic quartz clay
Ultramafic high smectite