Chemical Weathering and Soil Development

Chemical Weathering
and
Soil Development
No new
minerals
Yakov Kuzyakov
Ökopedologie
der Gemäßigten Zonen
1. Types of weathering
2. Main chemical weathering processes
3. Factors affecting chemical weathering
4. Mineral stability and weathering
5. Mineral transformation
6. Soil development
Rock Weathering
physical
Clastic solid
particles:
Quarz and rock
fragments
biological
Sediment
Soil
chemical
New
minerals
Chemical new:
Clays, Oxides
Carbonates,
Evaporites
Weathering
Weathering is the breakdown and alteration of rocks and minerals at or
near the Earth's surface into products that are more in equilibrium with
the conditions found in this environment.
Weathering is the first step for a number of other geomorphic and
biogeochemical processes. The products of weathering are a major
source of sediments for erosion and deposition. Many types of
sedimentary rocks are composed of particles that have been weathered,
eroded, transported, and terminally deposited in basins. Weathering also
contributes to the formation of soil by providing mineral particles like
sand, silt, and clay.
Products of Weathering
The process of weathering can result in the following three outcomes on rocks and
minerals:
1. The complete loss of particular atoms or compounds from the weathered
surface
2. The addition of specific atoms or compounds to the weathered surface
3. A breakdown of one mass into two or more masses, with no chemical change
in the mineral or rock (= physical weathering)
The residue of weathering consists of chemically altered and unaltered materials. The
most common unaltered residue is quartz. Many of the chemically altered products
of weathering become very simple small compounds or nutrient ions.
These residues can then be:
1. dissolved or transported by water
2. released to the atmosphere as a gas
3. taken up by plants for nutrition.
Some of the products of weathering, less resistant alumino-silicate minerals, become
clay particles. Other altered materials are reconstituted by sedimentary or
metamorphic processes to become new rocks and minerals

Physical weathering
Types of physical weathering are:
- Frost wedging
- Expansion and contraction due to diurnal temperature changes
- Release of overburden pressure -- (sheeting and exfoliation)
- Biological process -- ie. rooting
insolation (thermal) weathering
expansion and contraction with wetting and drying
the surface temperature of dark colored rock can vary from 0-50 o C between day and
night, since rock (especially jointed rock) has low thermal conductivity
the differential stresses of expansion and contraction of the outer 1-5 cm of rock
causes separation of concentric shallow layers called spalling or spheroidal weathering
in boulders
frost shattering
the force of water in rock fractures as it freezes and expands, or is forced into the rock
by the pressure of freezing water
the most common physical weathering process, given the widespread distribution of
frost (even in the tropic at high elevations)
most effective in coastal arctic and alpine environments where there are hundreds of
frost cycles per year
the specific volume (vol./unit mass) of water increases by 9% upon freezing producing
stress that is greater than the strength of all common rock
Chemical weathering
Alteration to cause chemical or mineralogical
changes: weakens rocks
The reactive components in the Earth's atmosphere
include:
– Oxygen O 2
– Carbon Dioxide - CO 2
–Water -H 2 O
Nitrogen, the most abundant consistent in the atmosphere
has little effect on the chemical weathering of rocks and
minerals.

Main schema of chemical weathering
Hydrolysis
mineral cations (e.g., Ca + , Fe + , Na + , K + , Al + ) are replaced by
hydrogen ions (H + ) from acidic water
Mg 2 SiO 4 (Olivine) + 4 H + 2 Mg 2+ + H 4 SiO 4
KAlSi 3 O 2 (Feldspar) + H + Al 2 Si 2 O 5 (OH) 4 (Kaolinite) + K + + H 4 SiO 4
• the most common weathering process
• pure water is a poor H + donor, however biogenic CO 2
dissolves in water to produce carbonic acid:
• the weathering products are in solution or a residue is clay,
that is, the first stage of soil development
• the soil water solution becomes more basic as H + is
consumed
Hydrolysis is the weathering reaction that occurs when the two
surfaces of water and compound meet. It involves the reaction
between mineral ions and the ions of water (OH - and H + ), and
results in the decomposition of the rock surface by forming new
compounds, and by increasing the pH of the solution involved
through the release of the hydroxide ions.
Hydrolysis is especially effective in the weathering of common silicate
and alumino-silicate minerals because of their electrically charged
crystal surfaces.
Muscovite mica
K 2 Al 6 Si 6 O 20 (OH) 4
↓
Illite clay
KAl 5 Si 7 O 20(OH) 4
↓
Kaolinite clay
Al 4 Si 4 O 10 (OH) 8
↓
Gibbsite
Al(OH) 3
(Bauxite)
Hydrolysis
involves the
progressive loss
of the more
soluble cations
(Na, K, Ca, Mg)
and under
extreme
conditions Si
The most common
chemical weathering processes
• Hydrolysis
• Oxidation
• Reduction
• Hydration
• Carbonation
• Chelation/Complexation
• Solution
Oxidation and Reduction
Oxidation is loss of an electron (e - ) to dissolved oxygen
- Fe is the most commonly oxidized mineral element:
Fe 2+ (ferrous iron) Fe 3+ (ferric iron)
8 FeO + 2 O 2 4 Fe 2 O 3
4 FeSiO 3 + O 2 2Fe 2O 3 (hematite) + 4 SiO 2 (aq)
2 FeS 2 (pyrite) + O 2 Fe 2O 3 (hematite) + 2 S
4FeS 2 (pyrite) + 15 O 2 +14H 2 O->
4Fe(OH) 3 (Limonite) + 8 H 2 SO 4
- other readily oxidized mineral elements include Mg, S, Cr
Oxidation is the reaction that occurs between compounds
and O 2. The net result of this reaction is the removal of one
or more e - from a compound, which causes the structure to
be less rigid and increasingly unstable. The most common
oxides are those of Fe and Al, and their respective red and
yellow staining of soils is quite common in tropical regions
which have high temperatures and precipitation.
Atmospheric O 2 is the most abundand oxidizing agent on
Earth, and has been since about 1.8 billion years.
Reduction is the reverse of oxidation: is thus caused by the
addition of one or more e - resulting in more stable products

Acidic sulfate weathering
in a Sulfudept forming
from pyritic main spoil.
A dark sulfidic horizon
underlines a lighted
colored sulfuric horizon.
Oxidation of sulfides may
produce acidic drainage
waters. (10 cm bars)
Carbonation
dissolving of CaCO 3 (limestone) in acidic groundwater
- similar to hydrolysis but the all the products are ionic, there is
no residue
- bicarbonate (HCO 3 - ) is a product of carbonation and a major
part of the dissolved load of most rivers
- the carbonation of limestone results in karst topography:
caves, sinkholes, etc.
CaCO 3 + H 2CO 3 Ca 2+ + 2 HCO 3 -
Solubility of CaCO 3 = 0.01 g/l (pure H 2O) – 0.3 g/l (H 2O+CO 2 sat)
Carbonation is the reaction of CO 3 2- ) and HCO3 - ions with
minerals. The formation of carbonates usually takes place as
a result of other chemical processes.
Carbonation is especially active when
the reaction environment is
abundant with CO 2. The formation
of carbonic acid (H 2CO 3) is
important in the solution of
carbonates and the decomposition
of mineral surfaces because of its
acidic nature
Hydration
Hydration involves the rigid attachment of H + and OH - ions to a
reacted compound. In many situations the H + and OH - ions
become a structural part of the crystal lattice of the mineral
CaSO 4 + 2 H 2O CaSO 4 ·2 H 2O
Anhydrid Gyps
Dehydration (=reverse process)
2Fe(OH) 3 (Limonite, orange) Fe 2 O 3 (Hematite, red) +3H 2 O
Hydration also allows for the acceleration of other
decompositional reactions by expanding the crystal lattice
offering more surface area for reaction.
Chelation / Complexation
bonding of mineral cations and organic molecules produced by plants
• these chelates are stable at a pH at which the cation would normally
precipitate and thus they are leached in seeping soil water
• H + released during chelation from organic molecules is available for
hydrolysis
• thus plants, such as the lichens on bare rocks, contribute to the
decomposition of soil and rock
K 2 (Si 6 Al 2)Al 4 O 20 (OH) 4 (muscovite) + 6 C 2 O 4 2- (oxalat) + 2 0H -
6[Al(C 2 O 4 )] + (aq) + 6 H 4 SiO 4 + 2 K +

Solution
Water and the ions it carries as it moves
through and around rocks and minerals
can further the weathering process
Quarz (SiO 2 ) + H 2 O Silicic acid (H 2 SiO 3 )
Molecules can mix in solution to form a great variety of basic and
acidic decompositional compounds. The extent, however, of
rock being subjected to solution is determined primarily by
climatic conditions. Solution tends to be most effective in areas
that have humid and hot climates.
CaCO 3 (s) + H 2 O Ca 2+ + HCO 3 - + OH -
MgSiO 4 (s) + 4 CO 2 (aq) + 4 H 2 O 2Mg(HCO 3 ) 2 (aq) + H 4 SiO 4
-O-M + H 2 O -O--M--OH 2 O 2- + M-OH + H +
How would climate
influence weathering
rates at locations on
the map?
Weakest -O-M
bonds will weather
first
Bond BE/kj/mol
Ti - O 674
Al - O 582
Si - O 464
Ca - O 423
Mn - O 389
Fe - O 389
Mg - O 377
1. Tropical rainforest (equatorial regions including South America, Africa,
Indonesia, southeast Asia): chemical weathering rates would be rapid as
these regions have both high temperatures and plenty of rainfall.
2. Hot desert (subtropical regions including North Africa [Sahara], southwest
South America [Atacama], southwest Africa [Namib], Asia [Gobi],
southwestern U.S., central Australia): plenty of heat but insufficient water to
cause significant physical and/or chemical weathering.
3. Temperate mountains (Rocky Mountains, Sierra Nevada Mountains, Alps,
Andes Mountains): insufficient temperatures for rapid chemical weathering
but elevations contribute to freeze-thaw cycles necessary for ice wedging.
4. Polar Regions (Alaska, Antarctica, Siberia): too much cold weather to
permit thawing. Water in solid form (ice) unable to react with rock.
Biological Weathering
Organisms can assist in breaking down rock
into sediment or soil
1. Roots of trees and other plants
2. Lichens, fungi, and other micro-organisms
3. Animals (including humans)
Distribution of clay minerals

Factors affecting weathering
• The most important factor affecting all chemical weathering processes is climate.
Climatic conditions control the rate of weathering that takes place by regulating the
catalysts of moisture and temperature. Tropical weathering rates (temperature and
moisture are at maximum) are 3-4 times higher than rates in temperate environments.
– Arid (and cold) climate: mechanical weathering predominate
– Humid warm climate: chemical weathering predominate
• pH
– Many surface & near surface
terrestrial waters are acidic
• Rain (pH=5.6)
• Acid rain (pH~4.4) & fog
– Ocean water is slightly alkaline
Other factors
•Rock/mineral type & distribution
•Time
•Drainage
•Relief
•Vegetation
Rates of weathering
• Highly variable depending on climate
• Chemical weathering rates doubles with 10 °C
• Mechanical weathering is very slow
• Weathering rates vary through time
(prior to terrestrial plants the wheathering was slower)
• Surface area
Rates of Weathering of Clean Rock Surfaces (micro-meters/1000years)
Rock Type Cold Climate Warm, Humid Climate
Basalt 10 100
Granite 1 10
Marble 20 200
Zusammenhang:
Klima – Vegetation –
Nährstoffakkumulation
24

Limitierende Faktoren:
Temperatur (T), Wasser (W), Nährstoffe (NS)
T, W T, NS T, NS W W, (NS) (W) NS (W)
P, ... N, P, N, P, Ca, Mg, S,
Ca, Mg,
B, Mo
B, Mo
+ P, (N),
MikroNS
Wheathering of minerals
+ P, Ca, Mg, S,
B, Cl, Zn,
Cu, Mo
Mineral and chemical composition are important factors as to the extent to
which the stone will be affected and the type of effects it may display.
Generally...
• Granite-type stones are more resistant to the mechanical processes with
the exception of salt decay and more susceptible to the chemical
processes of hydrolysis and in some cases oxidation
• Limestone and marble are vulnerable to salt decay, dissolution, hydration
and in some cases oxidation
• Sandstone is susceptible to the processes of salt decay, oxidation and if it
is a calcareous variety of sandstone, it is vulnerable to the dissolution
• Clay slates are vulnerable to the chemical processes of hydration,
hydrolysis and some varieties are affected by the oxidation
25
Weathering
events
Periods with elevated
CO 2 should have
evidence for
enhanced
weathering: (e.g.,
Cretaceous, ...)
Change in humidity
(e.g., Miocene ca.
24-5 billion years)
Rise of O 2 above a
threshold that
allowed onset of
global atmospheric
oxidation
(Precambian ca.
1.8 billion years)
Most rocks are composed of minerals (ordered arrangements of atoms)
• There are thousands of minerals, but only a few compose the bulk of most rocks.
• The ultimate source of rocky material is magma (molten rock) from within the
earth.
Eruption of magma on the surface (lava) forms fine-grained extrusive rocks:
• Rhyolites (rich in Si,Al and K)
• Basalts (rich in Mg and Fe)
When deeply-buried magmas solidify slowly, they make coarse-grained plutonic rocks:
• Granite, the coarse-grained equivalent to rhyolite
• Gabbro, the coarse-grained equivalent to basalt
The most common elements in basalts and gabbro (given as oxides):
Oxide
Weight % in Basalt
(or gabbro)
Weight % in Rhyolite
(or granite)
SiO2 50.83 72.66
Al2O3 14.07 13.45
K2O 0.82 5.35
Na2O 2.23 2.99
FeO and Fe2O3 11.93 2.00
CaO 10.42 1.13

Factors affecting stability
of parent material minerals
1. Resistance of ingneous minerals to weathering is
the same as the order of crystalization from
cooling magmas
2. Positions of ions in the structure of feldspars:
tetraedra of Al 3+ > Si 4+ // K + > Na + > Ca 2+
3. Tetraedra linkage to each other
4. Fe 2+ and Mn 2+ content
Differential Weathering
Not all rock types are equally affected by these differing weathering reactions. Instead,
it will depend on the type of minerals in the particular rock. Most of the minerals
(especially the silicates) form at very high temperatures and some of them at very
high pressures. That means that these minerals are stable (happy) at these high
temperatures and pressure. If you bring these minerals to conditions that are quite
different (such as the temperature and pressure conditions at the Earth's surface),
these minerals will not longer be stable. A mineral that is unstable is likely to be
more easily weathered.
The higher the temperature and pressure of formation, the more unstable the mineral
is. That means that the group of minerals we've designated the “high temperature”
minerals will be weathered more quickly than those in the “low temperature”
category. So what does this mean for the rocks that contain these minerals? Well,
the mafic rocks contain the high temperature minerals, and therefore will weather
more quickly than the felsic rocks, which contain the low temperature minerals. So,
if an area of basalt is right next to an area of granite, after years of weathering, the
basalt will weather away more quickly and become a valley, while the granite will be
more resistant to weathering and will look like a hill compared to the basalt right
next to it. If limestone will be added to the area, granite will weather even faster
than basalt, because the rock is actually dissolved away, leaving no solid products.
These differences in weathering rates are referred to as differential weathering.

Example: Feldspar weathering
2 KAlSi 3 O 8 + 2 H 2 CO 3 + H 2 O Al 2 Si 2 O 5 (OH) 4 + 4 SiO 2 (aq) + 2 K + + 2 HCO 3 -
spar + carbonic acid + water clay (with high water content) + dissolved ions
• Start with feldspar end up with clay plus dissolved ions (Framework silicate -> sheet
silicate plus ions)
• Some of the water is absorbed by the clay (hydration)
• Dissolved ions transported away
• The dissolved silica again becomes important as a cement
• Several other minerals weather to form clays such as amphiboles and micas
• Other minerals, such as olivine and pyroxene (top of Bowens' reaction series) are
so unstable they often dissolve completely)
One of the most stable minerals is quartz (bottom of Bowen's reaction Series)
• major component in beach sands - is often the only mineral left
• Resistant to weathering for 2 reasons:
– very stable (Bowen again)
– lacks cations that can be easily substituted by H +

Generilized schema of mineral weathering to clay,
and clay transformations
Primary minerals weather to form clays that weather to solutes and other clays
Chemical
weathering on
granite
Unweathered granite contains these minerals:
• Na Plagioclase feldspar
• K feldspar
• Quartz
• Lesser amounts of biotite, amphibole, or muscovite
1.The feldspars will undergo hydrolysis to form kaolinite
(clay) and Na and K ions
2.The Na and K ions will be removed through leaching
3.The biotite and/or amphibole will undergo hydrolysis to
form clay, and oxidation to form iron oxides
4.The quartz (and muscovite, if present) will remain as
residual minerals because they are very resistant to
weathering

Schematic presentation of basic and acidic zones through soil during development
(this sequence also represents soil profiles from arid to humid to humid tropical regions)
Slightly
weathered
Organic
Neutral to
slightly alkaline
Slightly
alkaline
CaCO 3
accumulation
Soluble salt
accumulation
Parent
material
Weathering and soil development
Organic
Slightly acid
Neutral to
slightly alkaline
Parent
material
Moderately weathered
Organic
Highly acid
Acid
Parent
material
Strongly
weathered
Organic
Neutral to
slightly acid
Highly acid
Parent
material
Sequence of clay mineral distribution with
increasing soil development
Relat. degree of Prominent minerals in soil clay fraction
soil development
1. Gypsum, sulfides, and soluble salts
2. Calcite, dolomite, and apatite
3. Olivine, amphiboles, and pyroxenes
4. Micas and clorite
5. Feldspars
6. Quartz
7. Muscovite
8. Vermiculite and hydrous micas
9. Montmorillonites
10. Kaolinite and halloisite
11. Gibbsite and allophane
12. Goetite, limonite, and hematite
13. Titanium oxides, zircon, and corrundum
Effects of chemical weathering
• Weaken coherence between mineral grains,
so rock crumbles
• Forms solutions, which can be removed.
So rocks crumbles more
• Clay forms, increase volume of weathered material.
So weathered rocks pulls away and exposes fresh rock.
So more weathering occurs

Summary
• Weathering is a prerequisite for soil formation
• Types of weathering
–Physical
– Chemical: Hydrolysis, Oxidation, Reduction, Hydration, Carbonation,
Chelation/Complexation, Solution
– Biological
• Factors
– Climate: Temperature, Precipitation
–pH
– Mineral structure
– Formation conditions
– Chemical composition
• Weathering rates vary between years and billions of years