1

1:1 Clay Minerals1:1 Clay Minerals

Repeat TO layers Repeat TO layers bonded with weak bonded with weak electrostatic bondselectrostatic bonds

2

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

3

2:1 Clays2:1 Clays

General StructureGeneral Structure

TOT StructureDioctahedral

+3 cation-1 Hydroxyl

+4 Silicon-2 Oxygen

c

b

4

2:1 Clay Minerals2:1 Clay Minerals2:1 Phyllosilicate Clay Minerals

5

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)

6

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

7

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

8

Iron Oxide and Hydroxide Iron Oxide and Hydroxide MineralsMinerals

Very common weathering productsVery common weathering products

9

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

10

WeatherWeathered ed

SurfacesSurfaces

Weathering Weathering reactions are reactions are surface surface reactionsreactions

Via growth of Via growth of itch pitsitch pits

11

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--

12

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

13

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)

14

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––

15

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?

16

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

17

18

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

19

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)

20

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

21

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

22

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)

23

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)

24

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

25

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

26

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.

27

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-

28

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.

29

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

30

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

31

32

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

33

34

AMDAMD(Acid Mine Drainage)(Acid Mine Drainage)

(Abandoned Mine (Abandoned Mine Drainage)Drainage)

35

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

36

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

37

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

38

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

39

PA

WV

40