this document is essential for for the the engineering and geology students to understand chemical weathering processeses

1. Somali National University
Puntland-Baran Branch
CHEMICAL Weathering

2. WEATHERING
There is a distinction between
weathering and erosion
Most of the earth’s surface is covered by
exposure of sediment or sedimentary rock,
by area. But the sediment layer is thin in
most places, with respect to overall crustal
thickness, so sedimentary rock is a minor
volume fraction of the crust.

5. STABILITY OF MINERALS
AGAINST
WEATHERING
• Chemical weathering is driven by thermodynamic energy
minimization, just like chemical reactions at high temperature.
• The system seeks the most stable assemblage of phases.
• The differences are that
(1) kinetics are slow and metastability is common;
(2) the stable minerals under wet, ambient conditions are
different from those at high T and P;
(3) solubility in water and its dependence on water chemistry
(notably pH) are major determinants in the stability of minerals
in weathering.

6. STABILITY OF MINERALS
AGAINST
WEATHERING
• A fresh rock made of olivine and
pyroxenes will end up as clays and iron
oxides, with other elements in solution
• A fresh rock made of feldspars and quartz
will end up as clays, hydroxides, and
quartz in most waters.

9. TYPES OF WEATHERING REACTIONS

10. TYPES OF WEATHERING REACTIONS
Unweathered (left) and weathered (right) surfaces of the same piece of granitic rock. On the unweathered
surfaces the feldspars are still fresh and glassy-looking. On the weathered surface the feldspar has been altered to
the chalky-looking clay mineral kaolinite.

11. TYPES OF WEATHERING REACTIONS
granitic rock containing biotite and amphibole which have been altered near to the rock’s surface to limonite,
which is a mixture of iron oxide minerals.

12. TYPES OF WEATHERING REACTIONS
Chemical Change Process?
Pyrite to hematite
Calcite to calcium and bicarbonate ions
Feldspar to clay
Olivine to serpentine
Pyroxene to iron oxide
Exercise on Chemical Weathering
Complete the following table by indicating which process is primarily responsible for
each of the described chemical weathering changes:

13. Some minerals are congruently soluble in acidic water, leaving no residue
The most abundant is calcite: CaCO3 + H2CO3 = Ca2+ + 2HCO3– (the Tums reaction)
Effects of dissolution (and precipitation) of calcite can be dramatic.
Sinkhole Speleothems
Karst terrain

15. Consequences of Chemical Weathering:
1. Formation of clay minerals:
2. Soil Formation:
1. Chemical weathering is a primary contributor to soil formation.
2. Weathered minerals and organic matter combine to create fertile soils.
3. Landform Evolution:
1. Chemical weathering contributes to the shaping of landscapes over geological time.
2. Karst landscapes, for example, result from the dissolution of carbonate rocks like limestone.
4. Global Elemental Cycling:
1. Chemical weathering plays a crucial role in the cycling of elements between the atmosphere, oceans,
and Earth's crust.
2. It influences the carbon cycle through the dissolution of carbonate minerals.

16. As already noted, clay minerals typically form from the alteration
(hydrolysis) of pre-existing silicate minerals.
The type of clay mineral that will form in any situation depends partly
on what silicate mineral is being altered, but also on a range of other
variables such as the temperature and pressure, and the chemistry of the
solutions that are passing through or over the rock at the time.
Formation of Clay Minerals

17. The most common alteration product of feldspars is kaolinite, Al2Si2O5(OH)4, which serves as
a model for the formation of clays by weathering generally.
The reactions of feldspars to kaolinite illustrate some of the basic trends:
• K, Na, Ca are highly soluble and readily leached by chemical weathering.
• Excess Si can be removed as silicic acid although quartz is relatively insoluble.
• Al is extremely insoluble, and is essentially conserved as clays.
• Weathering is a hydration process, leaving H2O bound in the altered minerals.
2 KAlSi3O8 + 9 H2O + 2 H+  Al2Si2O5(OH)4 + 2 K+ + 4 H4SiO4
• Note the H+ on the left-hand side…only acidic water can drive this reaction
Formation of Kaolinite

18. Formation of Clay Minerals
Primary Silicates Typical Clay Minerals That Will Form Under Weathering Conditions
Olivine smectite
Amphibole & pyroxene smectite, talc, vermiculite & chlorite
Plagioclase feldspar kaolin (especially halloysite or kaolinite)
Potassium feldspar kaolin (and illite less commonly)
Biotite vermiculite, kaolin
Muscovite Tends to be generally resistant to weathering but can convert to illite
A summary of the clay products of weathering of primary silicate minerals is provided in the Table below
Quartz is not in this list because it isn’t subject to chemical weathering.

19. Formation of Clay Minerals
Some Important Clay Minerals, Their Chemical Formulas and Variations
Clay
Mineral
Typ
e
Typical Chemical Formula Variations and (other names)
Kaolin 1:1 Al2Si2O5(OH)4 kaolinite, dickite, halloysite, nacrite
Serpentine 1:1 Mg3Si2O5(OH)4 antigorite, chrysotile (asbestos), lizardite
Illite 2:1 K0.65Al2.0(Al0.65Si3.35O10)(OH)2
glauconite, (hydromuscovite, K-deficient
muscovite)
Pyrophyllite 2:1 Al2Si4O10(OH)2
Smectite 2:1 (Na, Ca)0.33(Al, Mg)2(Si4O10)(OH)2 ∙ nH2O montmorillonite (bentonite), saponite, nontronite
Vermiculite 2:1 (Mg, Fe2+
, Fe3+
)3 ((Al, Si)4O10)(OH)2 ∙ 4H2O
Talc 2:1 Mg3Si4O10(OH)2
Chlorite 2:1 (Mg, Fe)3(SI, Al)4O10(OH)2 ∙ (Mg, Fe)3(OH)6 clinochlore, pennantite, chamosite, sudoite

20. CRYSTAL CHEMISTRY
OF CLAY MINERALS

21. Geochemistry of clay minerals
Clay minerals can be described as hydrated aluminosilicates having grain size
less than 4 microns. They are composed of two main structural units namely
tetrahedral (silica) and octahedral (alumina)

22. Geochemistry of clay minerals

23. The crystal chemistry of clays controls their physical properties such as swelling,
cationic exchange capacity and stability against burial diagenesis
Schematic illustration of the two-layer
(TO) and three-layer clays (TOT)
Swelling
Schematic illustration of some three-layer
clays (note the interlayer cations)
TO clays such as kaolinite and TOT
clays differ with interlayer cations

24. Clays can have significant chemical substitution, they undergo phase
transitions as diagenesis proceeds
Illite  Smectite + Mg2++ Fe2++SiO2+H2O
Al2Si4O10(OH)2*nH2O + KAlSi3O8  KAl2(AlSi3)O10(OH2) + 4
SiO2(aq) + n H2O

25. Properties of Clay Minerals
 They are soft and weak, primarily because of the weak bonds between sheets and the resulting
tendency for the sheets to slide past each other under stress. Talc is number 1 on the Mohs scale, and
most other clay minerals are similarly soft.
 Most clays are malleable when wet—also because of weak inter-layer bonds—and so can easily be
formed into useful shapes for artistic, domestic, industrial and scientific purposes.
 Clay minerals are crystals like other minerals, but they typically only form as very small crystals, so
clay deposits are almost universally fine-grained. Although a body of clay has significant porosity, the
pores are extremely small and most of the water within them is close enough to a grain boundary to be
held tightly by surface tension, making a clay deposit significantly impermeable.,

26. Clay Minerals
Serpentine-Bearing Rock

27. Clay Minerals
Smectite-Bearing Clay

28. Clay Minerals
Expanded Vermiculite

29. Clay Minerals and Earth Systems
 Clay minerals may have played a role in the initial evolution of life from organic
chemicals because the regular structure of the clays could have acted as a template for
the assembly of organic molecules,
 Conversion of silicate minerals to clay consumes atmospheric CO2 and so has climate
implications,
 Clay minerals accumulate trace elements that later become available to plants and
micro organisms.
 Clay minerals accumulate trace elements that may eventually get concentrated into
mineral deposits,

30. Clay Minerals and Earth Systems
Clay minerals can reduce rock strength and so contribute to erosion and
slope failure,
Clay minerals suspended in water or as clouds of dust can be vehicles for
the transfer of trace and major elements from land into the ocean
Clay minerals are the vehicles for the transfer of water from subducted
oceanic crust into the mantle, leading to magma formation.

31. Many factors affect the rate at which a rock will
weather.
Some of these variables are local (e.g., source rock),
some are global. These include temperature and CO2,
leading to the CO2-weathering feedback cycle.
Major Factors Controlling Rates of Weathering

32. Major Factors Controlling Rates of Weathering

33. Major Factors Controlling Rates of Weathering

34. Sorption:
The attraction between a particular mineral surface and an ion or molecule due to:
(1) Electrostatic interaction (unlike charges attract),
(2) Hydrophobic/hydrophilic interactions, and
(3) Specific bonding reactions at the surface
Definitions:
Sorption: Removal of solutes from solution onto mineral surfaces.
Sorbate: The species removed from solution.
Sorbent: The solid onto which solution species are sorbed.

35. Sorption:
There are three types of sorption:
1. Adsorption - solutes held at the mineral surface as a hydrated species.
2. Absorption - solute incorporated into the mineral structure at the surface.
3. Ion exchange - when an ion becomes sorbed to a surface by changing places
with a similarly charged ion previously residing on the sorbent.

36. Adsorption of metal
What factors determine this selectivity?
1. Ionic potential: defined as the charge over the radius (Z/r).
2. Cations with low Z/r release their waters of hydration more easily
3. In a natural solution, many metal cations compete for the available sorption
sites.
In a natural solution, many metal cations compete for the available
sorption sites. Experiments show some metals have greater
adsorption affinities than others.

37. Adsorption of metal
Many isovalent series cations exhibit decreasing sorption affinity with decreasing
ionic radius:
Cs+ > Rb+ > K+ > Na+ > Li+ > Ba2+ > Sr2+ > Ca2+ > Mg2+ > Hg2+ > Cd2+ > Zn2+
For transition metals, electron configuration becomes more important than ionic
radius:
Cu2+ > Ni2+ > Co2+ > Fe2+ > Mn2+

38. CARBONATES
GEOCHEMISTRY

39. The commonest form of carbonate in nature is CaCO3 which, like other carbonates, dissolves in
acid medium:
CaCO3 ↔ Ca+2 + CO3
–2
Saturation state can be expressed as follows:
Ώ = IAP / K
Where Ώ is the saturation index and IAP is the “ion activity product” (= [Ca+2][CO3
–2]) for the
solution, and with the apparent solubility product for a solution in equilibrium with solid CaCO3,
and K is constant. K depends on mineralogy where calcite < aragonite < Mg-calcite, as well as on
the prevailing T and P where solubility is higher in deeper and colder water.
Carbonate Solubility

40. At equilibrium:
Ώ = IAP / K = [Ca+2][CO3
–2] = 1
When Ώ > 1, solution is super-saturated
When Ώ < 1, solution is under-saturated
Carbonate Solubility