Transcript GEOL568_Lecture_5a

1:1 Clay Minerals

Repeat TO layers
bonded with weak
electrostatic bonds
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Cations with +2 and +3 charge
Gibbsite Layer: Trioctahedral - All
three out of three octahedral sites
are occupied by a divalent ion
Brucite layer
Dioctahedral - Only two out
of three octahedral sites are
occupied by trivalent ions
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2:1 Clays
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General Structure
TOT Structure
Dioctahedral
+3 cation
-1 Hydroxyl
+4 Silicon
-2 Oxygen
c
b
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2:1 Clay Minerals
2:1 Phyllosilicate Clay Minerals
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Smectite Group (e.g., Montmorillonite)
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The charged double layers are held
together by interlayer cations Ca
and Na which are surrounded by
one to two layers of water
molecules.
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Cations exchangeable with those is
water
Because variable amounts of water
can be held between the layers, the
layer spacing can expand and
contract depending on the
hydration.
This causes a great deal of
structural damage to buildings sited
on soils with a high smectite clay
content.
Al2Si4O10(OH)2• nH2O
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2:1 Clay, Illite
tetrahedral
octahedral
tetrahedral
K+ K+ K+ K+
tetrahedral
Interlayer sites filled
with K+. Strongly
bonded, so cations
cannot easily
exchange with K+.
octahedral
tetrahedral
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MAJOR CLAY MINERAL GROUPS
Group
Layer
Type
Layer
Charge (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 with
hydroxide
interlayer
Variable
(Al(OH)2.55)4·[Si6.8Al1.2]Al3.4Mg0.6 O20(OH)4
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Iron Oxide and Hydroxide Minerals
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Very common weathering products
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Mechanisms of silicate weathering
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Grain Surface Features affecting Dissolution
Points of fast weathering
Point Defects
 Dislocations
 Microfractures
 Kinks
 Grain or twin boundaries
 Corners
 Edges and ledges
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Weathered
Surfaces
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Weathering
reactions are
surface
reactions
Via growth of
itch pits
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Weathering reagent and products
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Carbonic acid (H2CO3) is the most common weathering reagent
in natural waters
MgSiO4 (forsterite) + 4H2CO30  2Mg2+ + 4HCO3- + H4SiO40
CaAl2Si2O8(anorthite) + 2H2CO30 + H2O(l)  Ca2+ + 2HCO3- +
Al2Si2O5(OH)4(kaolinite)
2NaAlSi3O8(albite) + 2H2CO30 + 9H2O(l)  2Na+ + 2HCO3- +
Al2Si2O5(OH)4(kaolinite) + 4H4SiO40
2K[Mg2Fe]AlSi3O10(OH)2(biotite) + 10H2CO30 + 0.5O2 + 6H2O
 Al2Si2O5(OH)4(kaolinite) + 4H4SiO40 + 2K+ + 4Mg2+ +
2Fe(OH)3(s) (iron hydroxide) + 10HCO311
Primary Weathering Products
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Soluble constituents removed from the weathering site
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Residual primary minerals little affected by weathering
reactions:
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Na+, Ca2+, K+, Mg2+, H4SiO4, HCO3-, SO42-, Cl-
Quartz, zircon, magnetite, ilmenite, rutile, garnet, titanite,
tourmaline, monazite
New, more stable minerals produced by the reactions
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Kaolinite, smectite, illite, chlorite, gibbsite, amorphous silica,
hematite, goethite, boehemite, diaspore, pyrolusite
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3+
Fe
and
3+
Al
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Ferric iron (Fe3+) and Al3+ have very low
solubilities, so when silicates containing these
metals are weathered, Fe and Al oxides form
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Overall, weathering removes, alkalis and alkaline
earths, but leaves behind Fe and Al in soil (recall
dust derived from soils contain high abundance
of Fe and Al)
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Incongruent and congruent weathering
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Incongruent weathering of silicate mineral
2NaAlSi3O8 + 2H2CO3 + 9H2O = 2Na+ + 2HCO3– + 4H4SiO4 + Al2Si2O5(OH)4
(Albite, Na-feldspar)
(Kaolinite)
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Congruent weathering of calcite
CaCO3 + H2CO3 = Ca2+ + 2HCO3–
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Formation of Al ore deposit
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Incongruent kaolinite weathering:
Al2Si2O5(OH)4 (kaolinite) + 5H2O  Al2O3•3H2O
(gibbsite) + 2 H4SiO4
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Gibbsite (or more often bauxite, a gibbsitelike
mineral) is ore for Al
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What conditions favor the formation of
bauxite?
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Weathering products
vary with varying
rainfall
Increasing Cation concentration
Importance
of Climate
Increasing Si concentration
Use of Stability field diagrams:
Degree of flushing
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 of Climate
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For areas with low rainfall (and a source of Mg2+),
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For areas with moderate rainfall,
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3NaAlSi3O8(albite) + 2H2O + Mg2+ 
2Na0.5Al1.5Mg0.5Si4O10(OH)2 (Smectite) + 2Na+ + H4SiO40
2NaAlSi3O8(albite) + 2H2CO30 + 9H2O(l)  2Na+ + 2HCO3+ Al2Si2O5(OH)4 (kaolinite) + 4H4SiO40
For areas with higher rainfall, silicic acid is removed
efficiently to allow:
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NaAlSi3O8(albite) + H2CO30 + 7H2O(l)  Na+ + HCO3- +
Al(OH)3 (gibbsite) + 3H4SiO40
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Biotite Weathering Reaction: formation of iron oxide
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2K[Mg2Fe][AlSi3]O10(OH)2 (biotite) + 10H+ + 0.5O2 +
6H2O  Al2Si2O5(OH)4 (kaolinite) + 2K+ + 4Mg2+ +
2Fe(OH)30 (amorphous iron oxide) + 4H4SiO40
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Over time, the amorphous iron oxide will convert to
common, stable iron mineral goethite (α-FeOOH)
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Dissolution of quartz
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Quartz:
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Adsorption of H2O molecules on middle Si-O bond
Hydrolysis reaction breaks Si-O bond
Further adsorption and bond breaking
H4SiO4 molecule forms and goes into solution
SiO2 + H2O  H4SiO4
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Quatz and amorphous silica
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At low pH values, the solubility of quartz is ~10 ppm
A ph >9, silicic acid dissociates slightly
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H4SiO4  H+ + SiO4Increases the solubility of quartz
Most dissolved silica comes from other weathering reactions
Determining
biogenic opal in
sediments
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EXPERIMENTAL RATES OF
MINERAL WEATHERING
Mean lifetime of a 1 mm crystal at pH = 5
and 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 weathering rates
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Rainfall, relief
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Mean annual temperature (affect dissolution rate
and microbial activity)
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Vegetation (organic acid production)
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Attack by Organic Acids
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Many weathering reactions in the
subsurface and soils are due to the
presence of organic acids created
by bacterial degredation of organic
material.
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These acids include humic, fulvic
and oxalic, among many others
Organic acid reactions may be
approximated by using carbonic
acid.
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This is because organic acids
rapidly breakdown and are found
in much lower concentration than
carbonic acids in ground and river
waters
Fulvic Acid
Humic Acid
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Attack by Organic Acids
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Reaction of albite and oxalic acid in upper soil
zones
2H2C2O4 (oxalic acid) + 4 H2O + NaAlSi3O8 (albite) 
Al(C2O4)+ + Na+ + C2O42- + 3H4SiO40
 As the products of this reaction pass through the
soil, Al(C2O4)+ and C2O42- are bacterially degraded.
 Al is released and will usually precipitate. Therefore,
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4H2C2O4 (oxalic acid) +2O2 + 7H2O + 2NaAlSi3O8 (albite) 
Al2Si2O5(OH)4 + 2Na+ + 2HCO3- + 4H4SiO40 + 6CO2
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Surface Complexation by Ligands
Ligand* attack is a three-step process:
1) A fast ligand adsorption step
O
L
O
O
H
H
k
1
+
2
+
O
M
M
M
M
2
k
1
O
L
O
O
H
2) A slow detachment process:
O
O
L
H
k
2
+
+
(
+
O
M
M
M
2
2
sl
o
O
O
L
*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 Ligands
3) Fast protonation to restore the initial surface:
O
O
k
3
+
+
M
M
fa
O
O
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In this case, formation of the M-L bonds weakens the M-O
bonds and allows the metal to leave the surface.
Once the metal ion leaves the surface, the surface is now negatively
charged and coordinatively unsatisfied.
It therefore grabs the nearest proton to bond with, and the surface
is reprotonated.
the entire process can now be repeated.
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Surface Complexation by Ligands
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Another example:
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Organic-ligand
forms a complex
with surface
hydroxide and
weakens internal
bonds.
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The effect of complex formation
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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
COO-OOC
C OOMeta l
C OO-
CH2
CH2
N C H2
-OOC
CH2
CH2 N
CH2
C OO-
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Carbonate dissolution and
reprecipitation
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Decomposition of organic matter yields carbonic acid
(H2CO3)
H2CO3 + CaCO3 → Ca2+ + 2HCO3H2CO3 + CaMg(CO3)2 (dolomite) → Ca2+ + Mg2+ + 2HCO3-
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When water degas (loss dissolved CO2), CaCO3
reprecipitate
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Cave deposit (stalactites, stalagmites etc.)
Carbonate nodules in soils
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Weathering and groundwater
composition
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The differences in water composition between groundwater and
rainwater are due to rock weathering and plant uptake
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Mobility of ions into groundwater:
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Ca > Na > Mg > Si > K > Al = Fe
Because the most rapidly weathered silicates are Na-Ca silicates
(plagioclase feldspars), Mg-containing silicates (pyroxenes, amphiboles), K
is contained in less rapidly weathered minerals, e.g., biotite, muscovite, Kfeldspar
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AMD
(Acid Mine Drainage)
(Abandoned Mine Drainage)
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AMD
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What is Acid Mine Drainage (AMD)?
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What is Abandoned Mine Drainage (AMD)?
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Any water discharge from a mine.
Typically high in dissolved metals
Not necessarily acidic
How is AMD formed?
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Drainage flowing from or caused by surface mining, deep mining or
refuse piles that is typically highly acidic with elevated levels of dissolved
metals.
AMD is formed by a series of complex geo-chemical and microbial
reactions that occur when water comes in contact with pyrite (iron
disulfide minerals) in coal, refuse or the overburden of a mine operation.
The resulting water is usually high in acidity and dissolved metals.
The metals stay dissolved in solution until the pH raises to a level where
precipitation occurs.
Where is AMD found?
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Anywhere Coal or metal-bearing rocks have been disturbed by mining or
quarrying
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Pyrite in Coal
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Pyrite (FeS2) is disseminated in coal as fine-grained particles
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generally less than 10 µm
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Oxidative-Reductive Dissolution
(attack by microorganisms)
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Weathering of Pyrite
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4FeS2 (pyrite) + 14H2O + 15O2  4Fe(OH)3 + 16H+ +
8SO42-
Acid Mine Drainage (AMD): more discussions on
mechanisms when we discuss oxidation-reduction
reactions
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Common Sulfide minerals
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Pyrite
FeS2
Fool’s gold
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Galena
PbS
Ore of lead
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Sphalerite
ZnS
Ore of zinc
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Chalcopyrite
CuFeS2
Ore of copper
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PA
WV
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