Chapter 4 wxingproducts

Chapter 4- Products of WeatheringSeveral things can happen to products 1- removal of materials by leaching e.g., CaCO 3 2- reaction of materials, either in situ or as they are physically

Chapter 4- Products of Weathering

Several things can happen to products

1- removal of materials by leaching

e.g., CaCO 3 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 ) Si 4 O 10 (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)

QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

QuickTime™ and a TIFF (Uncompressed) decompressor

are needed to see this picture.

Silica tetrahedron

Hydroxyl octahedron tetrahedron

Building blocks of clay minerals

Structure of Kaolinite

A 1:1 phyllosilicate clay

mineral

Note the single

tetrahedron attached

to the octahedron

QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

Phyllosilicate mineral structures- including Kaolinite

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

QuickTime™ and a TIFF (Uncompressed) decompressor

QuickTime™ and a TIFF (Uncompressed) decompressor

are needed to see this picture.

QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

2:1 clays structure

Shrink/swell type

e.g., Smectite

Non-shrink/swell

type

e.g., Illite

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

Unusual clay minerals often encountered

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