Geochemistry on the Earth’s surface
for analytical geochemists
3c. Weathering processes and soils
Tento učební materiál vznikl v rámci projektu Rozvoj doktorského studia chemie
č. CZ.02.2.69/0.0/0.0/16_018/0002593
Outline
• What is weathering?
• Goldich scheme
• Physical weathering
• Chemical weathering
– Oxidation
– Hydrolysis
– Hydration
– Dissolution
• Biological weathering
• Congruent and incongruent weathering
• Weathering products (stability diagrams)
• Soils
WEATHERING
Soil profile over granodiorite
Adapted from Appelo & Postma (2005)
Soil profile over granodiorite
Which minerals they are least stable?
What happened during weathering with the elements present in the primary minerals ?
Adapted from Appelo & Postma (2005)
Weathering
• The breakdown of rocks and
minerals by the action of the
atmosphere, water and biota.
• Types of weathering
– Physical
– Chemical
– Biological
Weathering of a sandstone wall in
Kokořínsk.
Importance of weathering
Importance of weathering
• Source of substances in water
• Source of nutrients for biota
• Source of mineral raw materials (bauxite)
• Modeling of the Earth's surface
• CO2 absorption
• Absorption of H+
• Formation and development of soils
• …
Physical weathering
• Mechanical breaking of
rocks into smaller pieces
• Change in shape and size,
composition does not
change
• Frost weathering
• Plant roots
• Animal digging
• Abrasion, particle
impacts, grinding
• Thermal stress
• etc.
Weathering of sandstones in the
Chemical weathering
• Minerals unstable on the Earth's surface
change into secondary minerals
– Especially clay minerals, hydroxides, oxides,
carbonates and sulfates
– The composition of the resulting minerals is
given by
• Climate
• Mother rock
• Soil evolution
• Biotic factors
• Topography…
• Three essential phenomena of
weathering:
1. Change of oxidation states of metals
2. Entry of H+ , OH− or H2O into the mineral
structure
3. Dissolution and transport of part of the
mineral in solution Weathering of rock-face in the Drahan
Highlands – fresh rocks (dark gray-blue color)
are colored brown by a coating of iron oxides.
Thermodynamics
• Olivine under high pressure and temperature:
• Olivine under low pressure and temperature
• In both cases, a decrease in the overall Gibbs
function
Soil solutions
Goldich scheme
• Sequence of rates of
progressive mineral
weathering
• The opposite of
Bowen's diagram of
mineral crystallization
• Polymerization of SiO 2
tetrahedra
– The proportion of ionic
bonds
• Resistance to H + and H
2 O
Adapted from Ryan (2014)
Chemical weathering
• The presence of water – environment for
reactions
• The fundamental influence of the atmosphere
– Dissolving gases in water (open/closed system)
• Binding on the ground
• The great importance of an acidic
environment
– Especially atmospheric CO 2
– H + is small and easily diffuses into the structure
Congruent dissolution
• Congruent weathering
– Complete dissolution of the mineral into solution
– The composition of the solution fully reflects the
composition of the mineral
• Incongruent weathering
– Secondary minerals are formed during weathering
– Therefore, the composition of the solution does not
reflect the complete composition of the original rock
Fe2SiO4 + 1
2
O2 + 2H2O → Fe2O3 + H4SiO4
Types of chemical weathering
• Oxidation
• Hydrolysis
• Hydration
• Dissolution
Oxidation
• Increase in oxidation number of cations (loss
of e − )
• In minerals, usually the reaction of Fe 2+ or S 2−
with oxygen
Fe2SiO4 + 1
2
O2 + 2H2O → Fe2O3 + H4SiO4
• In some environments, reduction
• 2Fe 2 O 3 (Hematite) - O 2 -> 4FeO (iron oxide)
Acid mine waters
• The oxidation of pyrite is partly anomalous in
terms of weathering processes – it releases H +
and the pH decreases
– Acid mine waters - min. 5% pyrite in the rock
– Carbonates and silicates no longer buffer
– − energy
– Environmental problem
Acid mine waters
Adapted from Ryan (2014)
Hydrolysis
• Cations in the mineral are replaced by H +
– Dissolution or formation of secondary
• Monovalent ions are not replaced
• Smaller ions before larger ones
Hydration
• Involvement of H 2 O molecules in the
structure
– a new structure is created
– and OH − or H +
• Unlike hydrolysis, all water is involved
• Volume increase - rock deformation
• Especially the surface of minerals
• Hydrated minerals dissolve better in water
Dissolution
• Especially minerals with ionic bonding
• Decomposition into ions in the presence of
water
– Covalent bonds in water and in anion and cation
of a mineral are stronger than between anion and
cation
CaCO3(s) = CO3
2−
+ Ca2+
• The CO covalent bond is preserved in solution
Biological weathering
• Physical
– Rock erosion
– Mixing
• Chemical
– Release of organic acids
– Cation exchange
– Catalysis of
decomposition
processes
– Soil moisture From Nigel Chadwick , CC BY-SA 2.0,
https://commons.wikimedia.org/w/index.php?curid=9149572
Weathering products
• The weathering of rockforming
minerals mainly
produces clay minerals
.
– Phyllosilicates
– Alternation of octahedral
and tetrahedral layers
– substitution of Al 3+ for Si
4+ creates a charge
imbalance balanced by
the entry of cationsGroup General formula
Smectite AI2 ( OH ) 2Si40 10 _ _
Illit K 0-2 Al 4 (Si 8-6 Al 0-2 ) O 20
(OH) 4
Mobility of elements
Soils on the river terrace are formed by the weathering of volcanic detritus.
1. Express the change in element and mineral content in %.
2. Based on the change in element content, sort them by mobility.
Adapted from Ryan (2014)
What determines weathering
processes?
What determines whether it weathers to montmorillonite,
kaolinite, or gibbsite ?
AdaptedfromAppelo&Postma(2005)
What determines weathering
processes?
What determines whether it changes to montmorillonite, kaolinite or gibbsite ?
Adapted from Appelo & Postma (2005)
Adapted from Giardino & Hauser (20 1 5)
Chemical change index
• CIA – chemical index change
• An effort to quantify weathering
• A number of other indexes
Mobility of elements
Soils on the river terrace are formed by the weathering of volcanic detritus.
1. Express the change in element and mineral content in %.
2. Based on the change in element content, sort them by mobility.
Adapted from Ryan (2014)
Weathering rate
Adapted from Ryan (2014)
Stability diagram
• For certain conditions,
specific minerals will be
stable
• Thermodynamics matters
• Diagrams help assess the
development of systems
Adapted from Ryan (2014
Adapted from Ryan (2014)
SOIL
One of the products of weathering
Soil
• A mixture of minerals,
organic matter, gases,
liquids and organisms
• Substrate for plant
growth
• Water retention and
purification
• Interaction with the
atmosphere
• Habitat for an organism
Black soil on loess.
FromTheoriginaluploaderwasMikhail'sgrandsonatUkrainianWikipedia-ownthesis,CCBY-SA3.0,
https://commons.wikimedia.org/w/index.php?curid=24550054
Soil-forming influences
• Climate
– Precipitation and temperature – breakdown, transport to
leaching
• Living organisms
– Bacteria obtaining energy by the decomposition of
minerals, plant roots, decomposition of org . mass…
• Relief
– Drainage, sedimentation
• Mother rock
• Time
• Anthropogenic influences
Climate
• Large role, directed by:
– Intensity of weathering
– Retention of weathering products in soil
• Temperature
– Higher temperatures – higher biological activity
• Precipitation
– Environment, material relationship, H + source
– Effective precipitation - soil leaching vs. salting (crust
formation)
• Humid Tropics × Arid Polar Regions
Organism
• Acceleration of reactions by microbial activity
• Nutrient uptake by plant roots – loss of ions in
soil water
• Rhizosphere – surrounds roots, organic acids,
concentration gradient given by nutrient
intake
• Humic and fulvic acids (from the
decomposition of organic matter)
• Mechanical mixing of sediment - aeration
Relief
• Hilltops and hillsides higher erosion
– the water table is deeper and the
water flows faster
• In lower areas, the benefit of ions
in the solution slows down
weathering
• Peaks – oxidizing × base of slopes –
reducing ( gley )
Adapted from Ryan (2014)
Mother rock
• The mineral composition determines the rate
of weathering and the chemical composition
of the soil water
• Basalts – rapid weathering, waters rich in CaMg
cations
• Granites – slower weathering, water enriched
with K-Na
• Tectonic faulting, exposed surfaces,
permeability/impermeability
Time
• Decomposition of
unstable minerals
limited by kinetics!
• Processes outside the
rest
• Irreversible
• Steady state not always
achievable
• Fast start, then they
slow down
Adapted from Ryan (2014)
Anthropogenic factors
• Acid rain (acceleration of decomposition
processes)
– Solubility of Al
• Dry fallout (heavy metals)
• Fertilization
• Water pollution
• Replacement of sorbed nutrient cations
Adapted from Ryan (2014)
Soil structure
• With the month products weathering , org.
substances and residues original ones rocks
and water
• T typical soil : 5% organic substances , 95%
inorganic
• P sequence layer ( soil profile ) depends on _
climate (T, precipitation etc. ), vegetation ,
time and substrate horny
Soils
Soil profile
Soil horizons
Humus – insoluble,
refractory organic
substances
Soil - profiles
Oi – leaves and organic waste, mostly undecomposed
Oa – organic waste, partially decomposed
A – dark colored horizon of a mixture of minerals and
organic substances, high biological activity, organic ( fulvic )
acids dissolve/ complex Fe and Al and I bring below
E – light-colored horizon, loss of clay minerals, organic
substances, oxides (areas of intense leaching)
B – maximum accumulation of clay minerals and oxides,
less organic substances, retention of substances carried
away from A
(K – in arid regions precipitation of calcite, gypsum or halite
– caliche , hardpan )
C – weathered source material, sometimes absent,
weathering dominated by CO 2 produced in the soil
R – source rock
Sometimes horizons are completely missing
Sometimes special – laterites ( latere – brick) – soils
washed out of basic cations, dominated by gibbsite ,
kaolinite, FeOOH .
Soil Geochemistry
• Acid-base and exchange reactions in soils
• Macronutrients
• Micronutrients
• Pesticides and chemical wastes in soils
• Land loss – desertification
Tento učební materiál vznikl v rámci projektu Rozvoj doktorského studia
chemie
č. CZ.02.2.69/0.0/0.0/16_018/0002593
Resources
• Images are from the following books:
– , P. (2014). Environmental and Low Temperature Geochemistry. John
Wiley and Sons . 402 p. ISBN 978-1-4051-8612-4 ( pbk .)
– Appelo , C. A. J., & Postma , D. (2005). Geochemistry, groundwater
and pollution : (2nd ed.). Leiden:
– Giardino , John; Houser, Christ (2015). Principles and dynamics of the
critical zone. Elsevier. pp. 1–4. ISBN 9780444633699.