Transcript The Biogeochemistry of Soils: Soils from Stars

The Biogeochemistry of Soils: Soils from Stars
•Composition of soils on earth is arguably unexpected
•Soils, and Earth, not reflective of chemistry of Universe
•Soils reflect chemical fractionation processes since
beginning of universe:
–Big Bang
–Subsequent star formation/collapse
–Chemical differentiation during formation of solar
system
–Chemical differentiation during formation of Earth
–Late cometary additions to Earth
Chemistry of
Solar System
6
H
He
4
O
C
Ne
•Exponential decline in
abundance w/ atomic
number (number of
protons)
•Sawtooth pattern
•Elements from Fe have
passed through stars
log (solar system mass fract ion)
N
Fe
MgSi
Si
Ar
Ca
2
Al
Na
Ni
Cr
Mn
P
ClK
Ti
0
Co
Zn
Cu
F
V
Sc
-2
Li
Ge
SeKr
Ga
Sr
Br Zr
As RbY
TeXeBa
Sn
Mo
RuPdCd
Pb
Pt
CeNd
Os
Ir
Nb
Hg
Ag
CsLa
GdDy
Rh Sb
Sm ErYb
QuTl Bi
HfW
Pr
In
Eu Ho
Tb
Re
TmLu
Ta
B
Be
-4
In
Th
U
•Solar system is
dominantly H and He
-6
0
20
40
60
atomic number
80
10 0
Crust vs. Solar System
6
•Core formation
depleted crust in
siderophile
elements (group
VIIIB..)
•Crust also
reflects late
stage cometary
additions of light
elements, etc.
including water
ac tinides
lanthanides
Li B
F
2
O
Al
Na
Si
P
Ti
Ca
Sc
V
Cl
Mg
0
S
Ga
As
Mn Cu
Z
n
Fe
Br
Ge
Cr Co
Ni
C
Mo
TPb
l
Sn
Sb
In
I
Ag
Cd
Bi
Hg
Re Au
Se
Ru
RPd
h
Te
Pt
Os
N
-2
Nb
Rb
Sr Zr
Y
K
ThU
Ba
Ta
La
CPr
e
Nd
Hf
Sm
Cs
GTdbDy
Eu
HEr
oTY
mL
bu W
cr ust enric he d
Be
4
log (crust /solar syst em)
•Depleted in
volatiles (as
other inner
planets)
•Noble gases
(group VIIIA)
•H, C, N
Ar
Ir
H
Xe
Kr
-4
pe riod 2
pe riod 3
pe riod 4
pe riod 5
pe riod 6
-6
He
Ne
-8
0
20
40
60
atomic number
80
10 0
Soil vs. Crust
2
•Soil enriched in
biochemically
impt elements
(C, N, S, Se)
I
)
Br
1
Se
0.5
Sb
As
S
0
Li B
O
-0 .5
Si
Al P Cl
Cd
Ni
Cr
TV
i Mn
ZnGa
Fe Cu
Zr
Y
Rb
Co
Sr
Sc
KCa
Ge
Ag
Mo
Sn
Tm
Dy Yb
La Nd
Lu
Eu
Cs Pr Sm
Tb
Gd
Hf
Ba
Er Ta
Ce
Ho
W
Pb
Bi
Hg
Th
Nb
Mg
Tl
F
-1
U
Au
Na
Be
soil
de ple te d
/crust
Zr
In
Zr
•Date
normalized to a
relatively
immobile
element (Zr)
C
log(soil
•Soil depleted in
alkali and alkine
earths, Si, ….
N
1.5
-1 .5
0
20
40
60
atomic number
80
10 0
Methods of
(reasons for)
Normalization to
Index Element
Original land surface
2 0 % weat hering
loss
Parent
material
S oi l
100 k g@ t=0
80 k g@ t=X
1) Mineralogical com posi ti on of paren t mate ri al and soi l
Parent material
49 kg quartz (SiO)
50 kg Ca silicate (CaSiO)
1 kg zircon (ZrSiO)
Soil
49 kg quartz
30 kg Ca silicat e
1 kg zircon
2) Mole cular weights of elem ents and mi nerals
Molecular Wts (g/mole)
Si=28.1
O=16.0
Ca=40.1
Zr=91.2
4) C oncentration s in parent mate ri al and soil
Parent material
Soil
Enrich/Deplete
Si = .481
Ca = .238
Zr=.007
.518
.179
.009
enrich
deplete
enrich
Formula Wts
SiO=44.1
CaSiO=84.2
ZrSiO=135.3
3) Su m of el e men tal mass in parent material and soils
5) Elem ental rati os w/ an d w/o normal iz ation
No normalization
Casoil/Caparent material
Sisoil/Siparent material
Ratio
.75
1.08
Zr Normalization
(Ca/Zr)soil/(Ca/Zr)pm
(Si/Zr)soil/(Si/Zr)pm
Ratio
.60
.86
 Siparent material = (31.2)quartz+(16.7)Ca silicate+(0.2)zircon= 48.1
 Sisoil = (31.2)quartz+(10.0)Ca silicate+(0.2)zircon= 41.4
 Caparent material = (23.8)Ca silicate
 Casoill = (14.3)Ca silicate
 Zrparent material/soil = (.7)zirl
Weathering Losses of Elements from Soils
Ar
/crust
Zr
)
5
Cl
4
Br
C
S
I
Ag
3
Se
Zr
•Relative concentration
related to chemical
nature of elements and
their reactivity in water
and type of bonds they
form in crust
6
log(wat er
•As might be
expected, water
enriched relative
to crust via
chemical
reactions
N
B
Cd Sb
Ra
Ca
2
As
Mg
Na
LiBe
Zn
Cu
F
Mo
Sr
Bi
Pb
U
K
1
Cr Ni
Co
MFne
V
SiP
0
Hg
Au
Ba
Rb
Ga
Al
Ti
Sc
Nb
Zr
Cs
La
Ce
Sm
Eu
Tb
W
Lu
Yb
Th
Hf
Ta
Sn
-1
0
20
40
60
atomic number
80
10 0
Plant Composition and Soil Chemistry
6
Zr
)
CN
log(plant Zr /crust
Plants reflect
water chemistry
(with some
selectivity) and
photosynthesis/N
fixation
4
H
Eu
Br
I
SCl
B
P
Mo
KCa
Zn
Mn Cu
Cr Co
2
Mg
FNa Si
Al
Hg
Bi
Pb
Tl
Au
Ba
V
Ti
Be
Cd
Ag
Sn
Ni As Rb
Se Y
Zr
Ga
Fe
Li
0
Sr
La
Cs Sm
Lu
Yb
U
Tb Dy
Sb
Ce
W
Th
-2
Sc
-4
0
20
40
60
atomic number
80
10 0
Soil Biogeochemistry Highlights
N
1.5
C
I
Br
1
log (soil Zr /crust
Zr
)
•Biological
group
•Alkali/alkaline
earths
•Halogens
•Rare earths
•Ti group
•Si, Al, Fe, P
2
0.5
S
Zr
0
Ti
Li
Fe
Si
Al P Cl
-0 .5
Rb
Sr
Tm
Dg
Nd
Yb
Lu
Eu
Tb
Cs Pr Sm
Gd
Hf
Ba
Em
Ce
Ho
Th
U
KCa
Mg
F
-1
Na
Be
-1 .5
0
20
40
60
atomic number
80
10 0
Soil Mineralogy: Primary Minerals
•Minerals are associations of elements
•Mineralogical composition a function of elemental
behavior and abundances
–O 474,000 mg/kg
–Si 277,000
–Al 82,000
–Fe 41,000
–Ca 41,000
–Na 23,000
–Mg 23,000
–K 21,000
•Relative abundance and behavior leads to reality that
soils are dominated by aluminosilicates (O,Si, Al).
Structure of Silicates
•Silica tetrahedron
–Net charge
–Role of Al
•Covalent bonds (Si-O, Al-O) vs. ionic bonds (cations-O)
–Bond type based on electronegativity differences and
tendency to attract electrons
•Big differences lead to ionic bonds
•Similar electronegativities lead to covalent bonds
•Linage of tetrahedra dictate classes of silicates and
their chemical behavior
– Nesosilicates
–Inosilicates
–Phyllosilicates
–Tectosilicates
Electronegativities of the Elements
•Electonegativities dictated by position on table: elements with outer shells almost
filled highly electonegative, those just starting new shell not.
•Si-O form mainly covalent bond
The Silica Tetrahedron
•1 Si, 4 O = -4 net charge
•Tetrahedra can be
linked by sharing O,
thereby reducing net
negative charge.
•Class of silicate is
determined by number
of shared O, and need
for cations to neutralize
net negative charge
Nesosilicates: Singe Tetrahedra Linked with Cations
Foresterite
•Single tetra linked with Mg+2
•Other minerals in group have all
Fe+2
•Highly susceptable to chemical
weathering via ejection of cations
by acid (H+)
•Products then form secondary
silicates and oxides
Inosilicates: Chains
Diopside:
•Single chains
Tremolite:
•Double chains
Phyllosilicates: Sheets
Muscovite
•‘dioctahedral w/ Al+3
Phlogopite
•‘trioctahedral’ w/ Mg+2
•K+ strongly adsorbed in
cavities
Tectosilicates: Framework
Anorthite (Ca)
•50% Al for Si
substition
Albite (Na)
•25% Al substition
Quartz
•No substition/O charge
Primary Silicate Summary
Silicate
Classifiication
Tetrahedron
Arrangement
Examples
Chemi cal Formula of
Speicific Minerals
Nesosilicates
independent
tetrahedra
olivine series
(fo resterite)Mg2SiO4
Inosilicates
single chains
pyroxene
group
amphibole
group
double chains
Phyllosilicates
Tectosilicates
sheets
frame work
mi ca group
plagioclase
group
feldspar group
silica group
(+)
charge
per 100
Oxygen
100
Melting
Temperature
(C)
(fayalite)Fe2SiO4
(augite)
Ca(Mg,Fe,Al)(Al,Si)2O6
(hornblende)
NaCa2(Mg,Fe,Al)5(Si,Al)8
O22(OH)2
(biotite)
(Mg,Fe)3(AlSi3O10)(OH)2
(muscovite)
KAl2(AlSi3O10)(OH)2
(anorthite) CaAl2Si2O8
100
66 (2)
1205 (1)
~1200 (1)
(albite) NaAlSi3O8
(orthoclase) KalSi3O8
(quartz) SiO2
1890 (1)
55(1)
80
~1100 (1)
80
~980(3)
100
1550 (1)
50
50
0
1100 (1)
1150 (1)
867 (1)
(1) Data from W.A. Deer, R.A. Howie, and J. Z ussman, An Introductionto the Rock Forming Minerals,
Longman Group, Ltd., Lo ndon (1966)
(2) Va lue for endmember with no Al substitution for Si. Value will decrease in proportion to added Al.
(3) From ranges reported in: D.S. Fanning, V.A. Kerami das, and M.A. El-Desoky, Micas. Chap. 12 in:
J.B. D ixon and S.B. Weed (eds), Minerals in Soil Environments, 2nd Ed., Soil Science Society of America,
Madison, WI (1989).
Mineralogical Composition of Igneous Rocks
Stability of Primary Minerals in Soils
•Increasing Si/O ratio increases stability
–More covalent bonds
–Fewer ionic bonds
–Less susceptable to acids
•Decreasing Si/Al ratio reduces stability
–Al creates charge imbalance and need for cations
•Presence of Fe+2 reduces stability
–Fe+2 oxidizes to +3
–Size and charge altered and Fe is expelled