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WEATHERING MECHANISMS & PRODUCTS

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Presentation on theme: "WEATHERING MECHANISMS & PRODUCTS"— Presentation transcript:

1 WEATHERING MECHANISMS & PRODUCTS
Mehrooz F Aspandiar CRC LEME WASM, Applied Geology, Curtin University of Technology

2 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

3 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

4 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

5 Chemical weathering products
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

6 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

7 Bioturbation in action
Tree fall moving and breaking down sub surface material Termetaria recycling top soil, quartz gravel and branches

8 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

9 Solubility –Equilibrium based
Solubility of a mineral – amount that dissolves in water to establish equilibrium with the mineral and its ionic components in solution CaCO 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 = = 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

10 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 The general dissolution reaction of silicates in inorganic aqueous systems involves multi-step process of initial rapid exchange of cations (K, Na, and/or Ca) for protons at the mineral surface, followed by a slow, rate determining hydrolysis (formation of activated complex) and subsequent detachment of silica and alumina species from the remaining framework (e.g., Aagaard and Helgeson, 1982). A number of studies have demonstrated that mineral hydrolysis occurs via surface complexes formed by the adsorption and desorption of protons and/or ligands from solution (Blum and Lasaga, 1988; Carroll-Webb and Walther, 1988; Brady and Walther, 1989). Destruction of framework bonds are known to occur through decomposition of these surface complexes when they are in the activated state. Therefore, the overall dissolution rate can be expressed as Rate=k[S], where S is the surface concentration of the reaction precursor (surface species which is in equilibrium with the activated complex) and k is constant. Also the reaction rate depends on pH and temperature of the reactants. The pH-dependency of hydrolysis is thought to be controlled by the acid-base properties and bonding in the metal-oxygen bond, and the mechanism of hydrolysis (e.g., Casey and Bunker, 1990).

11 K-feldspar + H+  kaolinite + K+ + H4SiO4
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

12 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

13 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+  Fe Mn2+  Mn3+ So  S6+ Reduced Fe/Mn/S bearing minerals (olivines, pyroxenes, sulphides) undergo oxidation

14 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

15 Some other processes.. Fire or heat Impacts
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!

16 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)

17 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)

18 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

19 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)

20 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

21 Change of Al coordination on weathering
Change from four fold (tetrahedral) to six-fold (octahedral) on weathering

22 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

23 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!

24 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)

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

26 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)

27 Fresh Granodiorite Saprolite Hb Bt Fld Soil B horizon Soil B horizon

28 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

29 Plagioclase altering to Al-smectite (incongruent)
Ca2Al2Si2O8 + H+ + H2O > Ca2+ + Al2Si2O5(OH)4

30 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)

31 Acid-producing potential (AP)
FeS2 + 15/4O2 +7/2H2O > Fe(OH)3 + 4H+ + 2SO42- 14Fe O2 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

32 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

33 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

34 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

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


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