Chemistry of Weathering Processes discusses the chemical reactions that break down rocks during weathering. Weathering occurs through low-temperature chemical processes involving water, air, acids, and organisms at Earth's surface. The main types of chemical weathering are dissolution, hydration/dehydration, hydrolysis, and oxidation-reduction. Dissolution occurs when minerals dissolve in water. Hydration and dehydration involve minerals gaining or losing water molecules. Hydrolysis replaces cations in minerals with hydrogen ions. Oxidation-reduction involves the gain or loss of electrons during reactions with oxygen. Climate, moisture, acidity, and oxygen availability control chemical weathering rates. Weathering breaks down rocks into sediments and soils.

1. Chemistry of Weathering Processes:
Weathering is the combination of processes by which pre-existing rocks
physically disintegrate and chemically decompose into soil, loose clasts
and dissolved components. Weathering products constitute the raw
materials from which sedimentary rocks are made.
Weathering is the simple consequences of exposing pre-existing rocks to
the conditions at the earth’s surface. Low temperature and pressure ,
organic activity and atmospheric gases. Chemical weathering proceeds
into two distinct ways .
1) Some constituents dissolve completely ,e.g. such minerals as calcite
and halite . The constituents dissolved from such minerals are carried
away by groundwater and runoff and can be precipitated elsewhere with
or without the assistance of organisms.

2. 2. Other constituents, such as feldspar and micas, are altered into new minerals
(especially clay minerals). These new minerals from which selected
components are removed and carried away. Because they typically are fine
grained than the original materials, they are more readily removed from
weathering site.
Chemical weathering of rocks and minerals involves several simultaneous
chemical reactions: 1) Simple solution; 2) Hydration; 3) Hydrolysis and 4)
Oxidation-reduction. These reactions proceed moret easily in the presence of
both water and air.
1) Simple solution: (solid minerals +acid or water) Simple solution (dissolution) is
the chemical reaction of solid rocks and minerals with water or acid . Bonds
between ions in rigid crystalline lattices are broken and the freed ions are
disseminated in solution. Solubility can be partial or complete. For example
the minerals quartz are not very soluble. Less than 6 ppm is dissolved in
normal fresh water. Crystals of quartz exposed in an outcrop of granite
typically show little corrosion because of this minimum solubility. They appear
fresh and unscatched by solution, standing in relief above more easily
decomposed minerals such as feldspar.

3. Calcite is much more soluble than quartz. In natural world , exposure of
limestone become pitted over periods of only days as they react with
rainfall. Most natural rainfall becomes carbonic acid as raindrops fall
through Earth’s atmosphere and absorb small amounts of carbondioxide
gas: H2O+CO2= H2CO2 , Carbonic acid and acids in general contain
abundant hydrogen ions . Due to their valancy and small size, they have a
strong affinity for anions and will displace other cations in minerals
structures . Limestone dissolved as hydrogen ions displace calcium ions ,
generally both dissolved Ca and bicarbonate: Ca CO3+H2CO3= Ca+2HCo3
Hydration and Dehydration: (Solid minerals+Water= New Hydrated minerals;
Dehydrated is the reverse); Some weathering processes involve the
chemical combination of pre-existing minerals with water (hydration) or
removal of water from some pre-existing minerals (dehydration). These
processes produce new minerals in greater equilibrium with
environments. Two common reactions are the dehydration of gypsum to
form anhydrite:
CaSo4.2H2O= CaSo4+2H2O and Hydration of Iron oxide (hematite) to from
limonite(this is essentially corrosive rusting)
Fe2O3+3H2O= 2Fe (OH)3 (limonite; oxidized zone in soil).

4. 3) Hydrolysis: Hydrolysis is defined as the replacement of cations in a
minerals structures by hydrogen ions derived either from water or more likely
from acid . Hydrolysis releases to the solution . The cations replaced in the
mineral structures by hydrogen and either converts the original mineral into a
different minerals or dissolved it completely .
Most silicate minerals weather primarily by a series of hydrolysis reactions, and
silicate minerals such as pyroxene amphiboles, mica feldsaprs along with quartz,
make the bulk of the earth’s primary crust.The specific of hydrolysis process and
the extent to which an original mineral is decomposed depend on the material .
Dark coloured (mafic) minerals such as olivine and pyroxene can dissolved
completely :
Mg2SiO4 (olivine)+4H= 2Mg+H4SiO4 (silicic acid)
2CaMgSi2O6 (Pyroxene)+16H=2Ca+2Mg+4H4SiO2 (dissolved silica).
Light –coloured (felsic ) minerals, especially feldspar such as orthoclase and
plagioclase, dissolved silcia and cations and leaving fine grained , easily
transportable clay minerals:
K AlSi3O8 K or Na or Ca feldspar +H=Al2Si2O5(OH)2 Clay mineral (Kaolinite)+K (ion
in solution)+H4SiO4 (dissolved silica) (silicic acid).

5. 4) Oxidation-reduction:(Atmospheric oxygen gains electorns and is
reduced as mineral constiuents lose electrons and are oxidised, producing
new “rusted minerals”.
Oxidation and reduction are inexorably linked . Oxidation does not occur
without reduction, and vie versa. Oxidation is the processes by which an
atom or ion loses electrons. Reduction is the process by which an atom or
ion gains electrons. The best oxidizing agent is atmosheric oxygen , O2;
non-ionized atom of oxygen (zero valancy) in the atmosphere combine
readily with other existing ions and gain electrons to become anions of
oxygen (O2). As a result, oxygen is reduced , but the ion from which the
oxygen atoms gain electrons is oxidized . The most obvious example of this
process involve the oxidation of ferrous iron (Fe2+) to ferric iron (Fe3+) . In
the natural world, this is the process of rusting, so called because it
changes, dull metalic ferrous iron to reddish –orange ferric iron. For
example:
Fe2Sio6 (pyroxene) +O2 (atmospheric oxygen)+ H2O=4Fe(OH)3 +H4Sio4
(dissolved silica);
2FeS2 (pyrite)+O2(atmospheric oxygen)=Fe2O3 (hematite)+2S (dissolved sulfur)

6. Controls of chemical weathering
• Climate: climate is of paramount importance. Higher temperature promote
chemical weathering because kinetic thermal energy facilitates any reaction.
More extensive in warmer climates (lower latitutes) than in colder climates
(higher latitude/higher elevation).
• Moisture (rainfall) is also important because most chemical weathering requires
water-thus humid climates is most favourable. In desert –temperature but water
for hydrolysis/solution and hydration. Arctic region and higher mountain terrain
–water in the from of snow and ice not liquid water.
• Hydrolysis and simple solution depend upon the ready availability of hydrogen
ion. The abundance of hydrogen ion –more specifically the activity of hydrogen
ions in solution is controlled by the acidity or alkalinity of a solution ; that is, the
pH. The pH is a way to designate the concentration of H ions in solution or (H+) .
The PH of 7 is termed as neutral solution. Solutions with an excess of hydrogen
ions have pH values from 1 to just below 7 are called acids.
• Most natural waters have pH value between 4 and 9.The likelihood of oxidation
depends largely on the availability of free atmospheric oxygen. The term Eh
expresses the potential for either oxidation or reduction. Eh, short for redox
potential is measured using an electolytic cell.

7. Mechanical Weathering
• Frost action
– Mechanic effect of freezing (and expanding)
water on rocks
• Pressure release
– Removal of overlying rock allows expansion
and fracturing
• Plant growth
– Growing roots widen fractures
• Burrowing animals
• Thermal cycling
– Large temperature changes fracture rocks by
repeated expansion and contraction

8. Mechanical Weathering

9. Mechanical weathering.

10. Surface Area & Weathering

11. Frost Wedging

12. CHEMICAL WEATHERING (LARGELY CONTROLLED BY CLIMATE)
*ROLE OF WATER
*DISSOLUTION
1. WATER DISSOLVES HALITE AND GYPSUM
2. CARBONIC ACID DISSOLVES LIMESTONE (CO2+H2O----HCO3)
i. CALCIUM CYCLE
ii. ACID RAIN

13. pH Scale

14. Climate weathering

15. 3.OXIDATION (REACTION OF CERTAIN CHEMICALS WITH O2)
i. IRON OXIDES
ii. COPPER OXIDES
4. HYDROLYSIS (REPLACEMENT OF MAJOR POSITIVE IONS
WITH PROTONS) OF POTASSIUM FELDSPAR INTO
i) CLAY: BECOMES PART OF SOIL
ii) SILICIC ACID: CEMENTS SEDIMENTS OR FORMS
ANIMAL SHELLS AND SKELETONS
iii) POTASSIUM IONS: PROVIDE PLANT NUTRIENTS

16. Spheroidal weathering

17. FACTORS THAT INFLUENCE CHEMICAL WEATHERING
1.CLIMATE
i) MOISTURE
ii) HEAT
iii) VEGETATION
2. LIVING ORGANISMS
3. TIME

18. MINERAL COMPOSITION: RELATIONSHIP BETWEEN A MINERAL’S
TEMPERATURE AND PRESSURE OF CRYSTALLIZATION AND ITS
SUSCEPTIBILITY TO WEATHERING (HIGH TEMP CRYSTALLIZATION-----
LESS STABLE AND EASILY WEATHERED-EXAMPLE: OLIVINE &
PYROXENE)
REGOLITH: LOOSE, FRAGMENTED MATERIAL THAT COVERS MUCH OF
THE EARTH’S SURFACE
SOIL: UPPERMOST ORGANIC-RICH PORTION OF THE REGOLITH
Mineral composition

19. Rounded Boulder

20. D.SOME PRODUCTS OF CHEMICAL WEATHERING:
a) CLAY MINERAL
i) KAOLINITE
ii) SMECTITE
iii) PRACTICAL USES FOR CLAYS
b) METAL ORES
i) FORMATION OF BAUXITE
ii) OTHER ORES

21. Soil
• Soil - a layer of weathered, unconsolidated
material on top of bedrock
– Common soil constituents:
• Clay minerals
• Quartz
• Water
• Organic matter
• Soil horizons
– O horizon - uppermost layer; organic material
– A horizon - dark layer rich in humus, organic acids
– E horizon - zone of leaching; fine-grained components
removed by percolating water
– B horizon - zone of accumulation; clays and iron
oxides leached down from above
– A horizon - partially weathered bedrock

22. Bedrock composition on soil.

23. Soils and Climate
• Soil thickness and composition are
greatly affected by climate
– Wet climates:
• More chemical weathering and thicker soils
• Soils in moderately wet climates tend to have
significant clay-rich layers, which may be solid
enough to form a hardpan
– Arid climates:
• Less chemical weathering and thinner soils
• Subsurface evaporation leads to build-up of salts
• Calcite-rich accumulation zones may form,
cementing soil together into a hardpan
– Extremely wet climates (e.g., tropical rainforest)
• Highly leached and unproductive soils (laterites)
• Most nutrients come from thick O/A horizons

24. Vegetation and soil development

25. Typical Mature soil

26. CLASSIFYING SOILS
1. OLD CLASSIFICATION SYSTEM
•  PEDALFERS
•  PEDOCALS
•  LATERITES
2. MODERN CLASSIFICATION SYSTEM
a) BASED ON MANY PHYSIAL & CHEMICAL CHARACTERISTICS
b) EXAMPLES OF SOIL TYPES
1) ENTISOL
2) VERTISOL
3) OXISOL
4) ULTISOL
3. PALEOSOLS (“OLD SOILS”)

27. Typical Mature Soil