Chapter 4- Products of Weathering
Several things can happen to products
1- removal of materials by leaching
e.g., CaCO3
2- reaction of materials, either in situ or as they
are physically moved, to create new crystalline
structures
Clay minerals
Illite, smectite (montmorillonite)
Hydrous oxides
Fe and Al are most common
Also Mg and Mn too.
Clay minerals
Mostly of silicate composition
Hydrated Fe, Al, Mg- silicate structures in discrete
layers
These layered silicates are called phyllosilicates
Subdivided into two classes - on structure and charge
Structure
1:1 clay minerals
1:1 refers to how the crystal structures are arranged
1 octahedron attached to a tetrahedron
2:1 clay minerals
2:1 refers to how the crystal structures are arranged
1 octahedron attached to 2 tetrahedrons
On charge
Based on the amount of chemical substitution
i.e., which ions are most easily mobilized and
exchanged
Example: Montmorillonite
Na 0.33 (Al 1.67 Mg 0.33) Si4 O10 (OH)
Mg substitutes for Al, replacing 1 of every 6 atoms
creates a net charge imbalance that allows X
0.33 to balance the formula
Since a number of ions can meet the charge
requirement (in this case its Na) we use X as a
generic variable term in the formula
Water plays a critical role in many clay minerals
Contains elements that act as bonding agents
keeps the crystalline structure together
Most notable are the H+ and the OH- cations and
anions
In many circumstances the water can be driven off or
can facilitate ion substitution, especially in 2:1 clays
e.g., smectite clays (montmorillonite)
Building blocks of clay minerals
Silica tetrahedron
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Hydroxyl octahedron
tetrahedron
Structure of Kaolinite
A 1:1 phyllosilicate clay
mineral
Note the single
tetrahedron attached
to the octahedron
Phyllosilicate mineral structures- including Kaolinite
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Forms of Smectite (a 2:1 clay)
Montmorillonite - Mg form of smectite
Both Al and Mg in the octahedral layer
Beidellite- Al form of smectite
Al in the octahedral sheet and substituting for Si
in the tetrahedral sheet
Nontronite - Fe variety of smectite
Fe in the octahedral sheet and Al substituting for
Si in the tetrahedral sheet
Illite (a 2:1 clay mineral)
Chemical formula is similar to muscovite
Differs slightly because Al and Si substitute for
one another allowing K+ to act as an interlayer
bonding agent
The K+ location in the structure of the layers is
close to the negative charges
Resultant chemical bond is strong enough to
inhibit water squeezing in
Limits the shrink swell capacity of the clay
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2:1 clays structure
Non-shrink/swell
type
e.g., Illite
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Shrink/swell type
e.g., Smectite
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Unusual clay minerals often encountered
Montmorillonite often results from the alteration of
volcanic ash
the end product is a bentonite clay
In conditions with adequate drainage
Mg is leached (instead of substituting) forming
kaolinite instead of montmorillonite.
Smectites commonly result from the weathering and
alteration of basic composition rocks.
Nontronite results from the alteration of basaltic glass.
Chemical composition of clay minerals
Tough to get clay chemical compositions
Hard to get pure samples to run
often clays have a tendency to be mixed layer
clays
the 2:1 clays mix with octahedral sheets
(not part of 2:1 structure)
When they do run chemical analysis…
It’s really variable
Chemical composition of clays
Non crystalline and Crystalline Al and Fe compounds
Non Xtln
Allophane- common Al compound found in soils
not quite crystalline, but not quite amorphous either
Xtln
Al crystalline minerals
Gibbsite
Boehmite
Origins
of clays
Clay type vs. depth
Clay types and soil orders
Only 3 strong relationships between soil order
and clay types
Vertisols - montmorillonite
Andisols - allophane
Oxisols - oxides and kaolinite
The remaining 8 orders can and do contain a
whole range of clay minerals