Weathering

WEATHERINGMECHANISMS & PRODUCTS

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 weathering breaks down rocks into smaller fragments

Chemical weathering alters the original material to new products

Physical residue that is partly or wholly chemically altered –”insoluble”

“Soluble” ions released in solution to ground & surface waters (solutes)

RegolithWeathering profile

Freshrock

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

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

Bioturbation in action

Termetaria recycling top soil, quartz gravel and branches

Tree fall moving and breaking down sub surface material

Chemical Weathering/water rock interactionDissolution

• 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• 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

H+ & H2O

Ca2+, Na+, Mg2+ & K+

Released as solutes

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 O2- to 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 micro-macro 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 Saprolite

Soil B horizon Soil B horizon

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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

Pyroxene Wethering

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)

FeS2 + 15/4O2 +7/2H2O > Fe(OH)3 + 4H+ + 2SO42-

14Fe2+ + 3.5O2 14H+ > 14Fe3+ + 7H2OIron oxidation is microbially catalyzed

Acid-producing potential (AP)

CaCO3 + 2H+ > Ca2+ + CO2 + H2OCaAl2S2O8 + 8H+ > Ca2+ + 2Al3+ + 2H4SiO4

Fe(OH)3 + H+ > Fe3+ + H2O

Net Neutralization Potential = NP - AP

Neutralization Potential (NP)

Factors affecting weatheringClimate & 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 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

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

Weathering of Rock Types

Ultramafic – high smectite

Plutonic – quartz + clay

Volcanic - clay