EXCHANGE
CAPACITY
CLAY CATION MINERALS
And clay sized fraction
The very very small
very
Kaolinite a clay mineral
stacks of
hexagonal
sheets shown
here are a
characteristic
of many clay
minerals.
Bar is 50m
1/1,000,000 m 1/1000 mm
http://www.reading.ac.uk/cfam/
imageofthemonth/2008/
February2008.htm
What is 50m
micrometer () = 1/1000,000 of a meter
Clay fraction
Usually clay minerals
- but not all
Includes all minerals
Includes organics
Less than 0.002mm
Clay fraction
1) are small in size
The external surface alone of a unit weight
of a clay soil is at least 1000 times that of
the same weight of coarse sand.
2) remain suspended for long periods.
Days
If never settle colloid
3) clay minerals the dominant part of
the clay-sized fraction >0.002 mm.
4) may be mineralic (the clays) or
organic (humus),
and are often closely intermixed.
5) are flocculated by electrolytes
Flocculation - combination of individual
particles into little clumps called
floccules.
WHAT ARE COLLOIDS??
Some of these particles are so small
they are colloidal
BUT some are larger
ALL are less than 0.002 mm
Colloidal suspended
not dissolved
heterogeneous
material
structures 1m or
less
Mixture of 2 phases
of matter
Foams can be colloidal
Foams
air-in-liquid colloids
shaving cream
beer froth
Aerosols
Ice cream is a foam
with air-bubbles in a
continuous matrix of
sugar and fat.
Emulsions
Emulsions
Liquid -in-liquid
colloids
mayonnaise
milk
cosmetics
Dispersions
Dispersions
solid-in-liquid colloids
ink
paint
Sewage
colloids contain an
overwhelmingly large
number of charged
particles that attract
and repel each other in
very unusual ways.
Colloids
colloids contain an
overwhelmingly large
number of charged
particles that attract
and repel each other in
very unusual ways.
WHY ARE THEY
IMPORTANT Many important soil processes occur on
these tiny particles
because they adsorb so much water and
so many ions (nutrients and other salts)
and organic matter.
Stops them being lost by leaching
Adsorb
to gather (a gas, liquid, or dissolved
substance) on a surface in a condensed
layer
solid surface retains essential ions, atoms,
or molecules that were in solution.
and are easily exchanged as the solution
changes
Readily available for plants
These ions, atoms or molecules are
called
solutes when in solution
adsorbates when adsorbed.
ORGANICS
Humus is often colloidal
Organic molecules, such as pesticides,
may be adsorbed.
Non-organic fraction
CLAY MINERALS
Other minerals very very fine. E.g.
quartz.
Clay minerals are
Product of weathering
Very stable in the soil
We cannot see them.
Clay minerals are
Hydrous sheet silicates (phyllosilicates)
Micas are sheet silicates
Mainly O, Al , Si
Need an x-ray / TEM to identify them.
Clay minerals are silicates
We looked at minerals earlier
A little more
A little deeper
To understand clay minerals.
Basic building block
Silica tetrahedra
1 silicon
4 oxygens
Silica tetrahedra
4 sides to the geometric figure
Minerals classified
By silica tetrahedra
How they hold together
The oxygens get shared
NESOSILICATES basic
greek for island
Single tetrahedra
olivine
SOROSILICATE
sister
hemimorphite
INOSILICATE Greek single
ino for fiber
INOSILICATE - double
INOSILICATE - double
inosilicates
cyclosilicate
CYCLOSILICATE
tourmaline
TECTOSILICATE
3-D complex - quartz
PHYLLOSILICATE
what we are interested in
Phyllo dough
Phyllosilicate
A little more complex
2 layers we need to think about
1) tetrahedral sheet / tetrahedral site
2) octahedral sheet / octahedral site
Two sites
Tetrahedral
Down pat
octahedral
Two sites
Tetrahedral
4-fold
Octahedral
6-fold
Octahedral site
Octahedral site
Tetrahedral site
Tetrahedral sheet
Tetrahedral sheet
Sheets can be in 2 directions
Octahedral
Octahedral site
Octahedral sheet
Not O2 but (OH)
So charge is different
Each oxygen has an attracted H
O2 2 (OH) -
2 octahedral sheets
Trioctahedral
brucite sheet
Dioctahedral
Gibbsite sheet
Has gaps in the sheet.
Why the difference?
Brucite Mg2+ Mg(OH)2
Gibbsite Al3+ Al(OH)3
Charge balanced
Why the difference?
Brucite Mg2+ Mg(OH)2 can balance 2(OH)
Gibbsite Al3+ Al(OH)3 3 (OH)
Only need 2 Al for every 3 Mg
T
O
T
O
T
O
T
This is how we build clay
minerals.
1) Tetrahedral layers and their
orientation
2) Octahedral layers and their type.
Kaolinite 1:1 clay
T
O
T
O
Kaolinite 1:1 clay
gibbsite layer - dioctahedral
T
O
T
O
Kaolinite
Harness = 2
Very small crystals.
Why??
Bonding between sheets
Tetrahedra - octahedra
Tip of tetrahedra is a oxygen
Negative charge
Top of octahedra is an (OH)
Positive charge on the H+ attracts oxygen
Tight bond. HYDROGEN BOND
Kaolinite 1:1 clay
T
O
T
O
Tetrahedra - octahedra
Tip of tetrahedra is a oxygen
Negative charge
Top of octahedra is an (OH)
Positive charge on the H+ attracts oxygen
Tight bond. HYDROGEN BOND
Kaolinite 1:1 clay
T
O
T
O
Kaolinite
fixed
Strong bonds layers do not separate
easily
Small shrink swell
Uses pottery / clay, medicine, coated
paper
Used to be kaolinite + pectin
Kaolinite
Al 2 Si 2 O 5 (OH) 4
Often pure little substitution
Miss-match
Stays small in size
Serpentine
1:1 clay mineral
Octahedral sheet trioctahedral
Serpentine minerals
Lizardite
Chrysotile
Dickite
Antigorite
Nacrire
halloysite
2:1 clay minerals
No interlayer e.g. talc
H =1
Talc
Talc 2:1 clay
Strong
hydrogen
bonds
Weak
oxygen
bonds.
Weak
structure
K+ interlayer
muscovite and biotite
K+ attracts O2-
K+ interlayer
muscovite and biotite
K+ attracts O2 FIXED strong bond
Can get large in size
Cations and water interlayer
NOT FIXED
Montmorillonite
Smectite
Vermicullite
Montmorillonite 2:1 clay
dioctahedral
Exchangeable cations
Montmorillonite
expansive
Montmorillonite
Montmorillonite
Formula never ideal
Extensive substitution of ions
Vermiculite 2:1 clay
trioctahedral
2:1:1 layered silicates
T
O
T
O
T
O
T
chlorite
ISOMORPHIC
SUBSTITUTION
CATION EXCHANGE
CAPACITY
CEC
The quantity of positively charged ions
(cations) that a clay mineral or similar
material can accommodate on its
negatively charged surface is expressed
as milli-ion equivalent per 100 g,
Kaolinite 3-15
Halloysite 2H2O 5-10
Halloysite 4H2O 40-50
Montmorillonite-group 70-100
Illite 10-40
Vermiculite 100-150
Chlorite 10-40
Glauconite 11-20+
Palygorskite-group 20-30
Allophane ~70
CEC