Lecture Notes - Aqueous and Environmental Geochemistry

Environmenal Geochemistry
DM Sherman, University of Bristol
2001/2002
Weathering Reactions
and Clay Mineral Formation
Environmental Geochemistry
DM Sherman, University of Bristol
Primary Chemical Weathering Agents
•Water (H2O) can act as a weak acid or base.
•Oxygen (O2) can oxidize Fe2+ and S2-.
•Carbon dioxide (CO2) is a Lewis Acid:
CO2 + 6H2O = H2 CO3
H2CO3 = HCO3- + H+
•Organic acids (e.g., HCOOH), the conjugate bases
are often strong ligands that complex metals.
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Environmenal Geochemistry
DM Sherman, University of Bristol
2001/2002
Elementary Weathering Reactions
•Dissolution and hydrolysis (acid-base reaction):
2H+ + H2O + CaSiO3 = Ca+2 + H4SiO4(aq)
•Oxidation:
FeS2(s) + (7/2)O2(g) + H2 O →
Fe2+ + 2SO42-(aq) + 2H+(aq)
•Complexation or hydration of cations to leach them
from mineral structures:
Mg2+ + 6H2O = Mg(H2O)62+
Mg2+ + HCOO- = Mg(OOCH) +
Weathering Rates of Crustal Minerals
(-log rate in mole/m2/s at pH 5 and 25 ºC)
23,000 y
Olivine (9.5)
Anorthite (8.55)
Enstatite (10.0)
Tremolite (11.7)
Microcline (12.5)
Albite (12.6)
Muscovite (13.1)
24 my
Quartz (13.4)
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Environmenal Geochemistry
DM Sherman, University of Bristol
2001/2002
Weathering as a sink for CO2
The most important source of acid is CO2:
CO2 + H2O = H2CO3
H2CO3 = H+ + HCO3Hence, weathering reactions consume CO2
2CO2 + 3H2O + CaSiO3 →
Ca+2 + H4SiO4(aq) + 2HCO3However, weathering reactions are too slow to buffer
anthropogenic CO2 inputs.
Dissolution of Primary Minerals
Cations such as Mg2+,
Ca2+, Na+ are strongly
hydrated in solution.
SiO2 is soluble
enough for simple
silicates to dissolve
congruently.
Mg2SiO4 (forsterite) + 4H+ = 2Mg2+ + H4SiO40
MgSiO3 (enstatite) + 2H+ + H2O = Mg2+ + H4 SiO40
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Environmenal Geochemistry
DM Sherman, University of Bristol
2001/2002
Dissolution of Primary Minerals(cont.)
Minerals containing Fe will
weather to yield FeOOH
and Fe2O3 under oxic
conditions.
Fe2SiO4 (fayalite) + 4H+
= 2Fe2+ + H4SiO40
4Fe2+ + O2 + 6H2O
= 4FeOOH(s) + 8H+
Abiotic Formation Pathways of Fe
(hydr)oxides
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Environmenal Geochemistry
DM Sherman, University of Bristol
2001/2002
Authigenic Iron(III) (Hydr)oxides
Goethite (α-FeOOH)
Akaganeite (β-FeOOH)
and Schwertmannite
(Fe8O8(OH)6SO4)
Lepidocrocite (γ-FeOOH)
Often produced by bacteria,
these minerals occur as
colloids and precipitates
with very high surface
areas (35-250 m2/g).
Dissolution of Primary Minerals (cont.)
Minerals containing Al tend to dissolve incongruently
because Al3+ is very insoluble at pH 6-7.
3KAlSi3O8 (K-Feldspar) + 2H+ + 12H2O
→ 2K+ + 6Si(OH)40 + KAl3Si3O10(OH)2 (illite)
2KAlSi3O8 + 2H+ + 10H2O
→ 2K+ + 4Si(OH)40 + Al2Si2O5(OH)4 (kaolinite)
KAlSi3O8 + H+ + 7H2O
→ K+ + 3Si(OH)40 + Al (OH)3 (gibbsite)
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Environmenal Geochemistry
DM Sherman, University of Bristol
2001/2002
Clay Minerals
The aluminosilicate which form by incongruent
dissolution occur as very small particles ( < 0.002 mm)
and are part of a group of phases known as clay
minerals. Aside from quartz, clay minerals are the
dominant phase in soils and sediments.
KAl3Si3O10(OH)2 (illite)
Al2Si2O5(OH)4 (kaolinite)
Mg3(OH)6.
(AlMg2)(AlSi3)O10(OH)2
(chlorite)
Phyllosilicate Building Blocks
Tetrahedral layer
(Si3+xAl1-xO10)5-
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Environmenal Geochemistry
DM Sherman, University of Bristol
2001/2002
Phyllosilicate Building Blocks..
Octahedral Layer
2:1 Phyllosilicate Clay Minerals
Illite
Vermiculite
Smectite
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Environmenal Geochemistry
DM Sherman, University of Bristol
2001/2002
Other phyllosilicate clay minerals
Kaolinite
Chlorite
Halloysite
Clay Mineral Compositions
Composition of di-octahedral 2:1 phyllosilicates
Mineral
muscovite
illite
vermiculite
montmorillonite
(general)
montmorillonite
(Wyoming)
montmorillonite
(Cheto)
beidellite
nontronite
Interlayer
Octahedral Tetrahedral
Cations
Cations
Cations
K2
Al4
Al2Si6
K2-x (x ~.5)
Al4
Al2-xSi6+x
(Mg,Ca)0.6-0.9
(Fe,Al)4
Al1.5Si6.5
smectites
MgyAl4-y
AlxSi8-x
Mn+(x+y)/n
Ca0.5
Layer CEC
Charge mol/kg
-2
0
x-2
0.1-0.4
1.2-1.8
1-1.5
-(x+y)
Al0.5Si7.5
-1.0
Ca0.5
Mg0.5
Fe3+0.5Al3
MgAl3
Si8
-1.0
(Na2,Ca)0.35
(Na2,Ca)0.35
Al4
Fe4
Al0.7Si7.3
Al0.7Si7.3
-0.7
-0.7
0.8-1.2
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Environmenal Geochemistry
DM Sherman, University of Bristol
2001/2002
Clay Mineral Compositions
Composition of tri-octahedral 2:1 phyllosilicates
Mineral
phlogopitebiotite
vermiculite
smectites:
saponite
Interlayer
Cations
K2
Octahedral
Cations
(Fe,Mg)6
Tetrahedral
Cations
Al2Si6
Layer CEC
Charge
-2
0
(Mg,Ca)0.6-0.9
(Fe2+,Mg)6
Al2-xSi6+x
1.2-1.8
(Na2,Ca)0.35
Mg6
Al0.8Si7.2
-0.7
1-1.5
Transformations of Clay Minerals
5Mg2+ + H4SiO4 + 5H2O + Al2Si2O5 (OH)4 (kaolinite) =
Mg3(OH)6.(AlMg2)(AlSi3)O10(OH)2 (chlorite) + 10H+
9Mg2+ + 10H4 SiO4 + Na+ + 0.5Al2Si2O5(OH)4 (kaolinite)
= 3 Na0.33Mg3(Al0.33Si3.67)O10(OH)2 (saponite)
+ 19H+ + 17/2H2O
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Environmenal Geochemistry
DM Sherman, University of Bristol
2001/2002
The System H+-K+-Si(OH)4-Al(OH)3
•This is a useful system from which to model the
formation of clay minerals by the weathering of granitic
rocks as the clays have a definite composition.
•There are four phases to be considered in this system:
K-feldspar, Muscovite (=illite), Kaolinite and gibbsite.
•We need to write equilibrium expressions between each
phase.
The System H+-K+-Si(OH)4-Al(OH)3
a) 3KAlSi3O8 (K-Feldspar) + 2H+ + 12H2O
= 2K+ + 6Si(OH)4 + KAl3Si3O10(OH)2 (illite)
K eq =
!
[K + ]2 [Si(OH) 4 ]2
[H + ]2
logK eq = 2log
[K + ]
+ 6log[Si(OH) 4 ]
[H + ]
!
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Environmenal Geochemistry
DM Sherman, University of Bristol
2001/2002
The System H+-K+-Si(OH)4-Al(OH)3
b) 2KAl3Si3O10(OH)2 (illite) + 3H2O + 2H+
= 2K+ + 3Al2 Si2 O5(OH)4 (kaolinite)
K eq =
!
[K + ]2
[H + ]2
logK eq = 2log
[K + ]
[H + ]
!
The System H+-K+-Si(OH)4-Al(OH)3
c) Al2Si2O5(OH)4 (kaolinite)+ 5H2O =
2Si(OH)4 + 2Al (OH)3 (gibbsite)
K c = [Si(OH) 4 ]2
!
logK c = 2log[Si(OH) 4 ]
!
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Environmenal Geochemistry
DM Sherman, University of Bristol
2001/2002
The System H+-K+-Si(OH)4-Al(OH)3
d) KAlSi3O8 (K-Feldspar) + H+ + 7H2O =
K+ + 3Si(OH)40 + Al (OH)3 (gibbsite)
Kd =
[K + ][Si(OH) 4 ]3
[H + ]
logK d = log
!
[K + ]
+ 3log[Si(OH) 4 ]
[H + ]
!
The System H+-K+-Si(OH)4-Al(OH)3
e) KAl3Si3O10(OH)2 (muscovite)+ H+ + 9H2O =
K+ + 3Si(OH)4 + 3Al (OH)3 (gibbsite)
[K + ][Si(OH) 4 ]3
Ke =
[H + ]
!
[K + ]
logK e = log + + 3log[Si(OH) 4 ]
[H ]
!
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Environmenal Geochemistry
DM Sherman, University of Bristol
2001/2002
Minerals buffer Water Composition
•Phase Rule: f=c-p+2
•c = 4 (H2O, K , Si, Al)
•P and T are fixed.
Soil Formation and Horizons
Soils consists of the
weathering products
of parent rocks
distributed in a set of
horizons that result
from the
translocation of
colloidal particles.
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Environmenal Geochemistry
DM Sherman, University of Bristol
2001/2002
Soil (and Sediment) Texture and
Mineralogy
•Sand Fraction is mostly quartz.
•Clay fraction is phyllosilicate clays
and Fe-Mn oxide hydroxides.
Sand = 0.05 - 2.0 mm
Silt = 0.002 - 0.05 mm
Clay < 0.002 mm
Mineralogy of Soil Clay Fraction
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Environmenal Geochemistry
DM Sherman, University of Bristol
2001/2002
Summary
•Primary crustal minerals are unstable at surface
conditions.
•Minerals containing Al will dissolve incongruently to
give Al-bearing clays + dissolved cations:
Primary
silicate
+ H+
Clay
mineral
“base
(+ H4SiO4) + cations”
•Weathering reactions are a sink for atmospheric
CO2. However, the reaction rates are too slow to
mediate anthropogenic input.
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