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1:1 Clay Minerals1:1 Clay Minerals
Repeat TO layers Repeat TO layers bonded with weak bonded with weak electrostatic bondselectrostatic bonds
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Cations with +2 and +3 Cations with +2 and +3 chargecharge
Brucite layerDioctahedralDioctahedral - Only - Only two out of three two out of three octahedral sites are octahedral sites are occupied by trivalent occupied by trivalent ionsions
Gibbsite Layer: TrioctahedralTrioctahedral - All three - All three out of three octahedral out of three octahedral sites are occupied by a sites are occupied by a divalent iondivalent ion
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2:1 Clays2:1 Clays
General StructureGeneral Structure
TOT StructureDioctahedral
+3 cation-1 Hydroxyl
+4 Silicon-2 Oxygen
c
b
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2:1 Clay Minerals2:1 Clay Minerals2:1 Phyllosilicate Clay Minerals
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The charged double layers are held The charged double layers are held together by interlayer cations Ca and together by interlayer cations Ca and Na which are surrounded by Na which are surrounded by one to one to two layers of water moleculestwo layers of water molecules. . Cations exchangeable with those is waterCations exchangeable with those is water
Because variable amounts of water Because variable amounts of water can be held between the layers, the can be held between the layers, the layer spacing can expand and layer spacing can expand and contract depending on the hydrationcontract depending on the hydration. .
This causes a great deal of structural This causes a great deal of structural damage to buildings sited on soils damage to buildings sited on soils with a high smectite clay content. with a high smectite clay content.
Al2Si4O10(OH)2• nH2O
Smectite Group (e.g., Smectite Group (e.g., Montmorillonite)Montmorillonite)
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2:1 Clay, Illite2:1 Clay, Illite
K+ K+K+K+
Interlayer sites filled with K+. Strongly bonded, so cations cannot easily exchange with K+.
tetrahedral
tetrahedral
tetrahedral
tetrahedral
octahedral
octahedral
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MAJOR CLAY MINERAL MAJOR CLAY MINERAL GROUPSGROUPS
Group LayerType
LayerCharge (x)
Typical Chemical Formulaa
Kaolinite 1:1 <0.01 [Si4]Al4O10(OH)8·nH2O)
Illite 2:1 1.4-2.2 Mx[Si6.8Al1.2]Al3Fe0.25Mg0.75O20(OH)4
Vermiculite 2:1 1.2-2.0 Mx[Si7Al]Al3Fe0.5Mg0.5O20(OH)4
Smectite 2:1 0.5-1.2 Mx[Si8]Al3.2Fe0.2Mg0.6O20(OH)4
Chlorite 2:1 withhydroxideinterlayer
Variable (Al(OH)2.55)4·[Si6.8Al1.2]Al3.4Mg0.6 O20(OH)4
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Iron Oxide and Hydroxide Iron Oxide and Hydroxide MineralsMinerals
Very common weathering productsVery common weathering products
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Mechanisms of silicate Mechanisms of silicate weatheringweathering
Grain Surface Features affecting Grain Surface Features affecting DissolutionDissolution
Points of fast weatheringPoints of fast weathering Point DefectsPoint Defects DislocationsDislocations MicrofracturesMicrofractures KinksKinks Grain or twin boundariesGrain or twin boundaries CornersCorners Edges and ledgesEdges and ledges
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WeatherWeathered ed
SurfacesSurfaces
Weathering Weathering reactions are reactions are surface surface reactionsreactions
Via growth of Via growth of itch pitsitch pits
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Weathering reagent and Weathering reagent and productsproducts
Carbonic acid (HCarbonic acid (H22COCO33) is the most common weathering ) is the most common weathering reagent in natural watersreagent in natural waters
MgSiOMgSiO44 (forsterite)(forsterite) + 4H + 4H22COCO3300 2Mg 2Mg2+2+ + 4HCO + 4HCO33
-- + H + H44SiOSiO4400
CaAlCaAl22SiSi22OO88(anorthite)(anorthite) + 2H + 2H22COCO3300 + H + H22O(l) O(l) Ca Ca2+2+ + 2HCO + 2HCO33
-- + Al+ Al22SiSi22OO55(OH)(OH)44(kaolinite)(kaolinite)
2NaAlSi2NaAlSi33OO88(albite)(albite) + 2H + 2H22COCO3300 + 9H + 9H22O(l) O(l) 2Na 2Na++ + 2HCO + 2HCO33
-- + Al+ Al22SiSi22OO55(OH)(OH)44(kaolinite)(kaolinite) + 4H + 4H44SiOSiO44
00
2K[Mg2K[Mg22Fe]AlSiFe]AlSi33OO1010(OH)(OH)22(biotite)(biotite) + 10H + 10H22COCO3300 + 0.5O + 0.5O22 + +
6H6H22O O Al Al22SiSi22OO55(OH)(OH)44(kaolinite)(kaolinite) + 4H + 4H44SiOSiO4400 + 2K + 2K++ + 4Mg + 4Mg2+2+
+ 2Fe(OH)+ 2Fe(OH)33(s) (iron hydroxide)(s) (iron hydroxide) + 10HCO + 10HCO33--
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Primary Weathering Primary Weathering ProductsProducts
Soluble constituents removed from the weathering Soluble constituents removed from the weathering sitesite NaNa++, Ca, Ca2+2+, K, K++, Mg, Mg2+2+, H, H44SiOSiO44, HCO, HCO33
--, SO, SO442-2-, Cl, Cl--
Residual primary minerals little affected by Residual primary minerals little affected by weathering reactions:weathering reactions: Quartz, zircon, magnetite, ilmenite, rutile, garnet, Quartz, zircon, magnetite, ilmenite, rutile, garnet,
titanite, tourmaline, monazitetitanite, tourmaline, monazite New, more stable minerals produced by the New, more stable minerals produced by the
reactionsreactions Kaolinite, smectite, illite, chlorite, gibbsite, amorphous Kaolinite, smectite, illite, chlorite, gibbsite, amorphous
silica, hematite, goethite, boehemite, diaspore, pyrolusitesilica, hematite, goethite, boehemite, diaspore, pyrolusite
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FeFe3+3+ and Al and Al3+3+
Ferric iron (FeFerric iron (Fe3+3+) and Al) and Al3+3+ have very low have very low solubilities, so when silicates containing solubilities, so when silicates containing these metals are weathered, Fe and Al these metals are weathered, Fe and Al oxides formoxides form
Overall, weathering removes, alkalis and Overall, weathering removes, alkalis and alkaline earths, but leaves behind Fe and alkaline earths, but leaves behind Fe and Al in soil (recall dust derived from soils Al in soil (recall dust derived from soils contain high abundance of Fe and Al)contain high abundance of Fe and Al)
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Incongruent and congruent Incongruent and congruent weatheringweathering
Incongruent weathering of silicate mineralIncongruent weathering of silicate mineral
2NaAlSi2NaAlSi33OO88 + 2H + 2H22COCO33 + 9H + 9H22O = 2NaO = 2Na++ + 2HCO + 2HCO33–– + 4H + 4H44SiOSiO44 + Al + Al22SiSi22OO55(OH)(OH)44
(Albite, Na-feldspar)(Albite, Na-feldspar) (Kaolinite) (Kaolinite)
Congruent weathering of calcite Congruent weathering of calcite
CaCOCaCO33 + H + H22COCO33 = Ca = Ca2+2+ + 2HCO + 2HCO33––
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Formation of Al ore Formation of Al ore depositdeposit
Incongruent kaolinite weathering:Incongruent kaolinite weathering:
AlAl22SiSi22OO55(OH)(OH)44 (kaolinite) + 5H (kaolinite) + 5H22O O Al Al22OO33•3H•3H22O O (gibbsite) + 2 H(gibbsite) + 2 H44SiOSiO44
Gibbsite (or more often bauxite, a Gibbsite (or more often bauxite, a gibbsitelike mineral) is gibbsitelike mineral) is ore for Alore for Al
What conditions favor the formation What conditions favor the formation of bauxite?of bauxite?
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ImportanImportance of ce of
ClimateClimate
Degree of flushing
Increasing Si concentration
Incr
easi
ng
Cati
on
co
nce
ntr
ati
on
Weathering products vary with varying rainfall
Use of Stability field diagrams:
High rainfall removes Si from the solution, promoting the conversion of Kaolinite to gibbsite. Most tropical and subtropical soils contain Kaolinite as the major clay mineral. In poorly drain soils (e.g., aemiarid climate), however, smectite is the characteristic soil mineral
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Weathering products: Impact Weathering products: Impact of Climateof Climate
For areas with low rainfall (and a source of For areas with low rainfall (and a source of MgMg2+2+),), 3NaAlSi3NaAlSi33OO88(albite)(albite) + 2H + 2H22O + MgO + Mg2+2+
2Na2Na0.50.5AlAl1.51.5MgMg0.50.5SiSi44OO1010(OH)(OH)22 ((SmectiteSmectite)) + 2Na + 2Na++ + H + H44SiOSiO4400
For areas with moderate rainfall,For areas with moderate rainfall, 2NaAlSi2NaAlSi33OO88(albite)(albite) + 2H + 2H22COCO33
00 + 9H + 9H22O(l) O(l) 2Na 2Na++ + + 2HCO2HCO33
-- + Al + Al22SiSi22OO55(OH)(OH)4 4 ((kaolinitekaolinite)) + 4H + 4H44SiOSiO4400
For areas with higher rainfall, silicic acid is For areas with higher rainfall, silicic acid is removed efficiently to allow:removed efficiently to allow: NaAlSiNaAlSi33OO88(albite)(albite) + H + H22COCO33
00 + 7H + 7H22O(l) O(l) Na Na++ + HCO + HCO33--
+ Al(OH)+ Al(OH)3 3 ((gibbsitegibbsite)) + 3H + 3H44SiOSiO4400
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Biotite Weathering Reaction: formation Biotite Weathering Reaction: formation of iron oxideof iron oxide
2K[Mg2K[Mg22Fe][AlSiFe][AlSi33]O]O1010(OH)(OH)22 (biotite) + 10H (biotite) + 10H++ + + 0.5O0.5O22 + 6H + 6H22O O Al Al22SiSi22OO55(OH)(OH)44 (kaolinite) + 2K (kaolinite) + 2K++ + + 4Mg4Mg2+2+ + 2Fe(OH) + 2Fe(OH)33
00 (amorphous iron oxide) + (amorphous iron oxide) + 4H4H44SiOSiO44
00
Over time, the amorphous iron oxide will convert Over time, the amorphous iron oxide will convert to common, stable iron mineral goethite (to common, stable iron mineral goethite (αα--FeOOH)FeOOH)
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Dissolution of quartzDissolution of quartz QuartzQuartz::
Adsorption of HAdsorption of H22O molecules on middle Si-O bondO molecules on middle Si-O bond Hydrolysis reaction breaks Si-O bondHydrolysis reaction breaks Si-O bond Further adsorption and bond breakingFurther adsorption and bond breaking HH44SiOSiO44 molecule forms and goes into solution molecule forms and goes into solution
SiOSiO22 + H + H22O O H H44SiOSiO44
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Quatz and amorphous silicaQuatz and amorphous silica
At low pH values, the solubility of quartz is ~10 ppmAt low pH values, the solubility of quartz is ~10 ppm A ph >9, silicic acid dissociates slightly A ph >9, silicic acid dissociates slightly
HH44SiOSiO44 H H++ + SiO + SiO44--
Increases the solubility of quartzIncreases the solubility of quartz Most dissolved silica comes from other weathering Most dissolved silica comes from other weathering
reactionsreactions
Determining biogenic opal in sediments
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EXPERIMENTAL RATES EXPERIMENTAL RATES OF MINERAL OF MINERAL WEATHERINGWEATHERING
Mean lifetime of a 1 mm crystal at pH = 5and 298 K
Mineral Lifetime Mineral Lifetime
Quartz 34 Ma Enstatite 8.8 ka
Muscovite 2.7 Ma Diopside 6.8 ka
Forsterite 600 ka Nepheline 211 a
K-feldspar 520 ka Anorthite 112 a
Albite 80 ka
Source: Lasaga (1984)
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Factors affecting Factors affecting weathering ratesweathering rates
Rainfall, reliefRainfall, relief
Mean annual temperature (affect Mean annual temperature (affect dissolution rate dissolution rate andand microbial microbial activity)activity)
Vegetation (organic acid production)Vegetation (organic acid production)
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Attack by Organic AcidsAttack by Organic Acids Many weathering reactions Many weathering reactions
in the subsurface and soils in the subsurface and soils are due to the presence of are due to the presence of organic acids created by organic acids created by bacterial degredation of bacterial degredation of organic material.organic material. These acids include humic, These acids include humic,
fulvic and oxalic, among fulvic and oxalic, among many othersmany others
Organic acid reactions may Organic acid reactions may be approximated by using be approximated by using carbonic acid. carbonic acid. This is because organic acids This is because organic acids
rapidly breakdown and are rapidly breakdown and are found in much lower found in much lower concentration than carbonic concentration than carbonic acids in ground and river acids in ground and river waterswaters
Fulvic Acid
Humic Acid
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Attack by Organic AcidsAttack by Organic Acids Reaction of albite and oxalic acid in upper Reaction of albite and oxalic acid in upper
soil zonessoil zones 2H2H22CC22OO44 (oxalic acid)(oxalic acid) + 4 H + 4 H22O + NaAlSiO + NaAlSi33OO88 (albite)(albite)
Al(CAl(C22OO44))++ + Na + Na++ + C + C22OO442-2- + 3H + 3H44SiOSiO44
00
As the products of this reaction pass through As the products of this reaction pass through the soil, Al(Cthe soil, Al(C22OO44))++ and C and C22OO44
2- 2- are bacterially are bacterially degraded.degraded.
Al is released and will usually precipitate. Al is released and will usually precipitate. Therefore, Therefore, 4H4H22CC22OO44 (oxalic acid)(oxalic acid) +2O +2O22 + 7H + 7H22O + 2NaAlSiO + 2NaAlSi33OO88 (albite)(albite)
AlAl22SiSi22OO55(OH)4 + 2Na(OH)4 + 2Na++ + 2HCO + 2HCO33-- + 4H + 4H44SiOSiO44
00 + 6CO + 6CO22
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Surface Complexation by Surface Complexation by LigandsLigands
Ligand* attack is a three-step Ligand* attack is a three-step process:process:
1) A fast ligand adsorption step1) A fast ligand adsorption step
2) A slow detachment process:2) A slow detachment process:M
OH
O
M
L
L
+ nH2Ok2
M
OH
O-
+ ML2+(aq)
slow
M
OH
O
M
OH
OH
+ 2HLk1
k -1M
OH
O
M
L
L
+ 2H2O
*Ligand: A compound with electron donating functional groups (e.g. ethylenediamine [H2NCH2CH2NH2] capable of bonding to a metal cation. In soils these are often derivatives of Oxalic, Humic, and Fulvic acids.
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Surface Complexation by Surface Complexation by LigandsLigands
3) Fast protonation to restore the initial surface:3) Fast protonation to restore the initial surface:
In this case, formation of the M-L bonds weakens the In this case, formation of the M-L bonds weakens the M-O bonds and allows the metal to leave the M-O bonds and allows the metal to leave the surface.surface.
Once the metal ion leaves the surface, the surface is Once the metal ion leaves the surface, the surface is now negatively charged and coordinatively unsatisfied. now negatively charged and coordinatively unsatisfied.
It therefore grabs the nearest proton to bond with, and It therefore grabs the nearest proton to bond with, and the surface is reprotonated. the surface is reprotonated.
the entire process can now be repeated.the entire process can now be repeated.
+ H+k3
M
OH
OHfast
M
OH
O-
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Surface Complexation by Surface Complexation by LigandsLigands
Another Another example:example: Organic-ligand Organic-ligand
forms a forms a complex with complex with surface surface hydroxide and hydroxide and weakens weakens internal bonds.internal bonds.
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The effect of complex formation
Increase the solubility over non-complex systems Some metals are present in natural waters almost
completely complexed. Cu2+, Hg2+, Pb2+, Fe3+, U4+
Adsorption / desorption is greatly affected by complexation, e.g., carbonate, sulfate, floride, phosphate complexes
Toxicity, bioavailability of species. Cu2+ is toxic to fish, but is unavailable when it is complexed. Similarly for other metal cations, Cd2+, Zn2+, Ni2+, Hg2+, Pb2+. In general, the most toxic species is the free ion. Thus, toxicity is reduced due to complexation
C OO-
C OO-Metal
N CH2 CH2 NCH2
CH2
COO-
COO-
CH2
CH2
-OOC
-OOC
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Carbonate dissolution and Carbonate dissolution and reprecipitationreprecipitation
Decomposition of organic matter yields Decomposition of organic matter yields carbonic acid (Hcarbonic acid (H22COCO33))
HH22COCO33 + CaCO + CaCO33 → Ca → Ca2+2+ + 2HCO + 2HCO33--
HH22COCO33 + CaMg(CO + CaMg(CO33))22 (dolomite) → Ca (dolomite) → Ca2+2+ + Mg + Mg2+2+ + + 2HCO2HCO33
--
When water degas (loss dissolved COWhen water degas (loss dissolved CO22), ), CaCOCaCO33 reprecipitate reprecipitate Cave deposit (stalactites, stalagmites etc.)Cave deposit (stalactites, stalagmites etc.) Carbonate nodules in soilsCarbonate nodules in soils
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Weathering and Weathering and groundwater compositiongroundwater composition
The differences in water composition between The differences in water composition between groundwater and rainwater are due to rock groundwater and rainwater are due to rock weathering and plant uptakeweathering and plant uptake
Mobility of ions into groundwater:Mobility of ions into groundwater:
Ca > Na > Mg > Si > K > Al = FeCa > Na > Mg > Si > K > Al = Fe Because the most rapidly weathered silicates are Na-Ca Because the most rapidly weathered silicates are Na-Ca
silicates (plagioclase feldspars), Mg-containing silicates silicates (plagioclase feldspars), Mg-containing silicates (pyroxenes, amphiboles), K is contained in less rapidly (pyroxenes, amphiboles), K is contained in less rapidly weathered minerals, e.g., biotite, muscovite, K-feldsparweathered minerals, e.g., biotite, muscovite, K-feldspar
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AMDAMD(Acid Mine Drainage)(Acid Mine Drainage)
(Abandoned Mine (Abandoned Mine Drainage)Drainage)
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AMDAMD What is Acid Mine Drainage (AMD)?What is Acid Mine Drainage (AMD)?
Drainage flowing from or caused by surface mining, deep Drainage flowing from or caused by surface mining, deep mining or refuse piles that is typically highly acidic with mining or refuse piles that is typically highly acidic with elevated levels of dissolved metals. elevated levels of dissolved metals.
What is Abandoned Mine Drainage (AMD)?What is Abandoned Mine Drainage (AMD)? Any water discharge from a mine. Any water discharge from a mine. Typically high in dissolved metalsTypically high in dissolved metals Not necessarily acidicNot necessarily acidic
How is AMD formed?How is AMD formed? AMD is formed by a series of complex geo-chemical and AMD is formed by a series of complex geo-chemical and
microbial reactions that occur when water comes in microbial reactions that occur when water comes in contact with pyrite (iron disulfide minerals) in coal, refuse contact with pyrite (iron disulfide minerals) in coal, refuse or the overburden of a mine operation.or the overburden of a mine operation.
The resulting water is usually high in acidity and The resulting water is usually high in acidity and dissolved metals. dissolved metals.
The metals stay dissolved in solution until the pH raises The metals stay dissolved in solution until the pH raises to a level where precipitation occurs.to a level where precipitation occurs.
Where is AMD found?Where is AMD found? Anywhere Coal or metal-bearing rocks have been Anywhere Coal or metal-bearing rocks have been
disturbed by mining or quarryingdisturbed by mining or quarrying
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Pyrite in CoalPyrite in Coal
Pyrite (FeSPyrite (FeS22) is disseminated in coal as fine-grained particles) is disseminated in coal as fine-grained particles generally less than 10 µmgenerally less than 10 µm
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Oxidative-Reductive Oxidative-Reductive DissolutionDissolution(attack by (attack by
microorganisms)microorganisms) Weathering of PyriteWeathering of Pyrite
4FeS4FeS22 (pyrite) + 14H (pyrite) + 14H22O + 15OO + 15O22 4Fe(OH) 4Fe(OH)33 + 16H+ 16H++ + 8SO + 8SO44
2-2-
Acid Mine Drainage (AMD): more Acid Mine Drainage (AMD): more discussions on mechanisms when we discussions on mechanisms when we discuss oxidation-reduction reactionsdiscuss oxidation-reduction reactions
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Common Sulfide Common Sulfide mineralsminerals
PyritePyrite FeSFeS22 Fool’s goldFool’s gold
GalenaGalena PbSPbS Ore of leadOre of lead
SphaleriteSphalerite ZnSZnS Ore of zincOre of zinc
ChalcopyriteChalcopyrite CuFeS2 CuFeS2 Ore of copperOre of copper
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PA
WV
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