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Transcript
WEATHERING
MECHANISMS & PRODUCTS
Mehrooz F Aspandiar
CRC LEME
WASM, Applied Geology,
Curtin University of Technology
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
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 Earth’s 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
Weathering processes and products
Physical residue that is partly or wholly
chemically altered –”insoluble”
Regolith
Fresh
rock
Weathering
profile
“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
• Breaks down rocks into smaller
particles which increases surface area
for solution attack
• Opens up fractures, joints and microcracks in rocks due by exerting stress
and facilitate solution access (chemical
weathering)
• Several types : Frost wedging, salt
weathering, unloading, thermal
weathering, bioturbation
Chemical weathering
products
Increasing weathering intensity
Physical weathering
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
Bioturbation in action
Tree fall moving and breaking
down sub surface material
Termetaria recycling top
soil, quartz gravel and
branches
Chemical Weathering/water rock interaction
Dissolution
• 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
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
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 > flowing solutions maintain undersaturtion
– pH > lower pH faster rate
– Temperature > higher temperature, faster rate
Hydrolysis
• Water combines with atmospheric and soil CO2 to form a weak acid carbonic acid> 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
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
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
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
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!
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)
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)
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
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 > kaolinite + solutes
• Change of Al coordination from 4 to 6
(hydrolysis facilitated)
• Oxidation of Fe (oxidation)
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 Ca2+, Na+, Mg2+ & K+
• Olivine (Mg,Fe)2SiO4
Released as solutes
• 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
H+ & H2O
Change of Al coordination on weathering
Change from four fold (tetrahedral) to six-fold (octahedral) on weathering
Oxidation of Fe (& Mn)
• Fe2+ in biotite, pyroxene, olivine,
pyrite
• Oxidation > higher charge Fe3+,
smaller ionic radii
• Fe3+ - combines readily with O2to form oxides and hydroxides >
goethite, hematite, maghemite,
lepidocrocite, ferrihydrite
• Fine grained > reddish-brown
hues
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!
Primary mineral stability - exceptions
• The Goldich’s sequence - connectedness of silicate tetrahedras:
orthosilicates > single chain > double chain > 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)
Silicate mineral weathering pathways
Type of mineral and grain size depends on micromacro hydrology and geochemical conditions
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)
Fresh Granodiorite
Hb
Saprolite
Bt
Fld
Soil B horizon
Soil B horizon
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
Plagioclase altering to Al-smectite
(incongruent)
Ca2Al2Si2O8 + H+ + H2O > Ca2+ + Al2Si2O5(OH)4
Mineral weathering – applications
• Silicate and carbonate weathering
– consumes acid (H+) > buffers acidity
– consumes water (hydrolysis) > extra salt in profile
– releases cations to solutes (groundwater) > 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)
Acid-producing potential (AP)
FeS2 + 15/4O2 +7/2H2O > Fe(OH)3 + 4H+ + 2SO4214Fe2+ + 3.5O2 14H+ > 14Fe3+ + 7H2O
Iron oxidation is microbially catalyzed
Neutralization Potential (NP)
CaCO3 + 2H+ > Ca2+ + CO2 + H2O
CaAl2S2O8 + 8H+ > Ca2+ + 2Al3+ + 2H4SiO4
Fe(OH)3 + H+ > Fe3+ + H2O
Net Neutralization Potential = NP - AP
Factors affecting weathering
Climate & Organisms
The Clorpt model = function (climate, organism, relief, parent material,
time..)
• Climate – precipitation & temperature
– Amount of water > alters minerals, flushes solutes, affects
vegetation > 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 > indirectly affects rate
• Organisms (Biota)
– Higher density > more organics > more carbonic acid > faster
weathering
– Denser vegetation > better soil stability > deeper weathering
– Related to climate
Factors affecting weathering
Lithology & 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
Factors affecting weathering
Landform (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
Weathering of Rock Types
Volcanic - clay
Plutonic – quartz + clay
Ultramafic – high smectite