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Secondary minerals
2014-2
Dörte Lehsten
Weathering of primary minerals results
in secondary minerals
When carbonic acid attacks a silicate rock ...
i.e. weathering of albite
(Na-feldspar)
2NaAlSi3O8 + 2H2CO3 + 9H2O →
2Na+
+
2HCO3 +
4H4SiO4+ Al2Si2O5 (OH) 4
formation of a secondary clay mineral
KAOLINITE
When carbonic acid attacks a silicate rock ...
i.e. weathering of albite
(Na-feldspar)
2NaAlSi3O8 + 2H2CO3 + 9H2O →
2Na+
+
Si:Al=3:1
2HCO3 +
4H4SiO4+ Al2Si2O5 (OH) 4
Dominant
anion in
stream water
Loss of Si
to stream
waters
Si:Al=1:1
Exchange of 2 H+ with 2 Na+
Soil fluid acidity becomes less
Na+ available for uptake by vegetation
Incongruent dissolution of weathering primary
sediments:
Not all constituents are released.
( in Kaolinite Si fixed)
Further weathering is possible
Under tropical humid conditions kaolinite
turns into Gibbsite
Al2Si2O5 (OH) 4 + 5H2O →
2H4SiO4 + Al2O3 *3H2O
Congruent dissolution of weathering primary
sediments:
All constituents are released.
No further weathering (release of Kations) is
possible .
Olivine (FeMgSiO4) undergoes congruent
dissolution in water:
Releasing Fe, Mg and Si.
Mg and Si are lost in run-off water to streams.
Fe reacts with O2 in precipitating iron(III)oxide.
Similar to Olivine, also pyrite undergoes
(FeS2) undergoes congruent dissolution in
water:
+
4 FeS 2 + 8 H 2 O + 15O2 → 2 Fe 2 O3 + 16 H + 8SO
The release of high amount of protons by iron
oxidation is reason for acid run-off by mining
2−
4
Secondary minerals
• By-products of chemical
weathering
• Precipitate near the parent mineral
• In temperate forest soils
dominated by layered (silicate)
clay minerals
Clay minerals
• Less tan 0.002 mm
• Layers held together by shared O2ions
• 2 types of crystalline layer
structure
– Silicon layers
– Layers dominated by Fe, Al, and Mg
Clay minerals
• Moderately weathered soils:
– Ratio of Si to metal dominated layers
2:1
• More strongly weathered soils
– Ratio of Si to metal dominated layers
1:1
Clay minerals
• 2:1 clay minerals:
montmorillonite (Mg instead of Al)
illite (K fixed in clay structure)
Structure of the clay minerals leads to a negative charge on
the surface
Clay minerals
• 1:1 clay minerals:
kaolinite
Si tetrahedron layer
Al octahedron layer
Shared oxygen atom
Clay minerals
• elements in secondary minerals
often fixed to their structure Î not
available for plant uptake
• Mg fixed in crystal of
montmorillonite
• K fixed in illite
Clay minerals
• Weathering of secondary minerals
leads to release of sometimes
large quantities of elements
• E.g. Ammonium (NH4) by
weathering of 2:1 clay minerals, a
vegetation growth limiting mineral
Clay minerals
• Cations are than in solution of soil
water which
– either percolates downwards,
washing plant available nutrients into
stream water
– Will be taken up by vegetation
Clay minerals
• Exception is Al and Fe
– downwards percolation only in form of
chelation (only if fulvic acid occurs,
which is produced be microbes
during litter decomposition)
– Otherwise oxidation of Al and Fe into
insoluble oxides and hydrous oxides,
which results in near complete
weathering of secondary minerals
Iron and alluminium oxides
Iron
Alluminium
occur in well weathered soil (mainly in tropical regions)
Geothite α-FeO(OH)
Gibbsite Al(OH)3
Hematite α-Fe2O3
Boehmite γ-AlO(OH)
Cation exchange capacity
• N+
• P+
• K+
• Na+
• Ca+
• mfl+
Layered 2:1 minerals posses a negative charge due to:
ionic substitution of less positive charged element with higher charged
element within the crystalline structure of the clay mineral (K+ with Al3+)
Cation exchange capacity
pH dependent cation exchange
Layered 2:1 minerals posses a negative charge due to:
exposed hydroxide (-OH) groups at the edges of clay minerals while corresponding
H+ protons are dissociated. Dissociation depends on the pH of soil water. (remember:
pH depends on concentration of H+ in the soil water NOT on the concentration of
other cat ions.) As higher pH as easier are H+ dissociated, leading to a negative
charge of clay minerals. Hence pH depending cation exchange
Cation exchange capacity
• N+
• P+
• K+
• Na+
• Ca+
• mfl+
Layered 2:1 minerals posses a negative charge which:
attracts and holds cat ions dissolved in the soil water solution near the clay surface.
Binding is reversible. Cat ions are available for plant uptake.
Cation exchange capacity
pH dependent cation exchange
For example Ca2+, K+ NH4+
Cation exchange capacity
Total negative charge or cation
exchange capacity (CEC)
Expressed in mEq/100g (mol equivalent) or cmol(+)/kg of soil.
Cation exchange is a chemical mass balance with the soil solution.
Functions and models are given in the literature.
In general cations held on the exchange side and displace one
another in the sequence:
Strong Al3+>H+>Sr2+>Ca2+>Mg2+>Rb+>K+>NH4+>Na+>Li+ weak
With an overload of weak cations, stronger can be released and
washed out.
Cation exchange capacity
Base saturation (BS)
Except for Al and hydrogen cations form bases in solution.
Ratio of total CEC occupied by these base cations is termed Base Saturation.
Under initial soil development CEC and BS increase, followed by a decrease
under further soil development and is higher in temperate soils with 2:1 clay
minerals than those with 1:1 minerals.
What about tropical soils?
CEC and soil organic matter
• Phenolic (-OH) and organic (COOH) acids
are able to dissociate the corresponding
H+ depending on the pH of soil solution.
• Hence, it can be substituted with cations
as well.
• In regions with low amount clay minerals
organic matter accounts for CEC
(percentual)
Soil buffering
• Cation exchange acts to buffer acidity
If H+ is added it exchange for instance for Ca2+as it is stronger than
Ca2+ which subsequently released freely into the soil solution
Hence acid will be buffered regarding the CEC.
Soil buffering
The lime potential
If H+ is added to the soil Ca2+ ions increase in the soil water solution.
Hence acid will be buffered regarding the CEC.
pH – ½ (pCa) = k; (p stands for negative logarithmic ’concentration’ )
K remains constant within the CEC.
Lime potential of a soil is defined as constant ratio between
H+ and Ca2+ concentration in the soil water solution within a
given range of pH (sufficient base saturation of at least
15%)
Soil buffering in acid soils
Low CEC
Buffering by aluminum and
Reactions are reversible
Al3+ + H 2 O ↔ Al(OH) 2+ + H +
Al (OH ) 2 + + H 2O ↔ Al (OH ) 2+ + H +
Al (OH ) 2+ + H 2 O ↔ Al (OH ) 3 + H +
Gibbsite dissolve to Al3+ under acid
conditions, which is toxic for aquatic
organisms like fish and amphibians
Anion adsorption capacity (AAC)
In tropical soils under acid conditions:
Anions become adsorbed by positive charge minerals,
(oxides and hydroxides of Al and Fe). H+ from soil solution
will be associated to the surface hydroxide group, resulting
in positive charge of these minerals.
Increase in pH reduces AAC until a zero point of charge is
reached. At higher pH, these soils posses then CEC (in low
amount)
For gibbsite ZPC is at pH9. =>under acid condition AAC
high and present in most tropical soils.
For soils in temperate climate ZPC is at pH2, practical not
present
Anion adsorption capacity
AAC greater on smaller minerals (poorly crystalline forms of
Al and Fe bonds)
Anion’s strengths follow the sequence:
Strongest PO43- > SO42- > CL- > NO3- weakest
Hence, low availability of phosphorus.
Soils reach in organic matter less efficient in anion
adsorption
Special attention to Phosphorus
limiting factor of plant growth.
Apatite contains phosphorus
(only primary mineral which contains
Phosphorus).
Under carbonation weathering
releasing of P
Ca5 ( PO4 ) 3 OH + 4 H 2 CO3 ↔
5Ca 2 + + 3HPO 42 − + 4 HCO3− + H 2 O
Phosphoric acid:
P in solution
⇒Plant uptake available
Special attention to Phosphorus
limiting factor of plant growth.
Apatite contains phosphorus
(only primary mineral which contains
Phosphorus).
Under carbonation weathering
releasing of P
Ca5 ( PO4 ) 3 OH + 4 H 2 CO3 ↔
5Ca 2 + + 3HPO 42 − + 4 HCO3− + H 2 O
Phosphoric acid:
pH<7 P precipitates with Al or Fe
⇒ not available for Plant
Special attention to Phosphorus
limiting factor of plant growth.
Apatite contains phosphorus
(only primary mineral which contains
Phosphorus).
Under carbonation weathering
releasing of P
Ca5 ( PO4 ) 3 OH + 4 H 2 CO3 ↔
5Ca 2 + + 3HPO 42 − + 4 HCO3− + H 2 O
Phosphoric acid:
pH>7.5 P precipitates with Ca
⇒ not available for Plant
Special attention to Phosphorus
Apatite weathering, P loss and P forms in time during soil development
(example from New Zealand)
Special attention to Phosphorus
Apatite weathers rapidly: decline of total P by stream loss
Phosphorus held on the surface of soil minerals (AAC)
With time P accumulates in
occluded forms (unavailable
for organisms)
Plant available P
Apatite weathering, P loss and P forms in time during soil development (example from New Zealand)
Accelerating processes of
soil development in region
of warm humid climate.
www.komar.de
Vegetation and microbiological
activities in the soil accelerate
chemical weathering by CO2
release and production of
organic acids.
Even though carbonate acid is main driver for
chemical weathering on a global scale:
Vegetation and soil organisms produce and release
a number of other (organic) acids:
Acetic and citric acid from vegetation
Phenolic acid ( e.g. tannins) during
litter decomposition
Fulvic and humic acid from soil
microbes
Oxalic acid by fungies
Often these acids dominate the acidity of the upper soil profile.
Acetic and citric acid from vegetation
Fulvic and humic acid from soil
microbes
Weather biotite mica; releasing
potassion (K).
Biotite mica
K(Mg,Fe)3AlSi3O10(F,OH)2
Fulvic and humic acid from soil
microbes
Weather minerals which contain
elements such as phosphorus (P).
Ca5 (PO4 )3 OH
Apatite
Additionally organic acids speed up weathering reaction in the
soil by combining with some weathering products and
transportation into lower soil layers.
Chelation: no equilibrium between dissolved products and
mineral forms
Example: Fe and Al combine with fulvic acid which make
them mobile. They get transported by percolation.
Fe+ solution percolation and
following precipitation as Fe2O3
visible by brown horizon.
Different impact of organic acid on soil
weathering depending on climate
Cool temperate climate: slow and incomplete decomposition of litter =>
More organic acid occurs which dominates weathering processes
Different impact of organic acid on soil
weathering depending on climate
Warm humid climate: fast and complete decomposition of litter
=>
Organic acid is restricted, carbonate acid dominates the
weathering processes
Weathering produces cations
CaCO3 + H2CO3 → Ca 2+ + 2HCO34 FeS 2 + 8 H 2 O + 15O2 → 2 Fe2 O3 + 16 H + + 8SO42−
2NaAlSi3O8 + 2H2CO3 + 9H2O →
-
2Na+ + 2HCO3 + 4H4SiO4+ Al2Si2O5 (OH) 4
Mainly all cations go over in solution and will be washed out by perculating
soil water through the soil if not used by vegetation.
Exceptions: Al and Fe are insoluble if not in chelation, accumulate as oxide in
the soil.
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