Transcript Clay Types Study Guide - University of Colorado Boulder

Clay Types Study Guide
• Types of Colloids
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crystalline silicate clays (covered by this guide)
non-crystalline silicate clays (p 314)
Fe & Al oxides (p 315, 322ff)
Organic (p 315, 325)
Basis for distinguishing silicate clay types
Isomorphous substitution
Review of clay types
Distribution
Weathering & generalized distribution in US
Basis for distinguishing crystalline silicate clays
• Based on numbers & combinations of structural units
– tetrahedral and octahedral sheets
– planes combined  sheets combined  layers  crystals (fig 8.4)
• Two general categories: 1:1, 2:1
– 2:1 types: expanding & nonexpanding
– also “2:1:1”  Chlorites
• Number of cations in octahedral sheet
– tri- vs. di-octahedra (fig 8.5)
• Size and location of layer charge (see also lecture 16 slides)
• Type of bonding between layers (see also lecture 16 slides):
– Strong: ionic > H-bonding > van der Waals :Weak
• Absence or presence of a cation interlayer
fine-grained micas
• See lecture 16 slides: review of diff’s in properties of clay types
Clay minerals
1:1 clays
2:1 clays
(one tetrahedral
sheet for each
octahedral sheet)
(two tetrahedral
sheets for each
octahedral sheet)
Kaolinite,
nacrite, dickite,
halloysite, etc.
Smectites
Montmorillonite,
beidellite,
saponite, etc.
Micas Vermiculites
Illite,
muscovite,
biotite, etc.
Tri- or divermiculite
‘Weird’, not
truly 2:1
Chlorites
Cookeite,
chamosite,
etc.
Visual comparison of common silicate clays’ structure
more strongly held
than in smectite
montmorillonite
illite
“2:1:1”
Isomorphous substitution
equal
shape/size
• The replacement of one ion for another of
similar size within the crystalline structure of
the clay
• Often results in change in net charge
takes eons –
doesn’t change rapidly
Substitution in tetrahedral sheet
Si2O4
SiAlO4
+4, +3, -8 (-2*4)
neutral
negative
Tetrahedral sheet
Substitution in octahedral sheet
(OH)2Al2O2
(OH)2AlMgO2
-2, +3, +2, -4
neutral
negative
Octahedral sheet
1:1 Silicate Clays
• Layers composed of one tetrahedral
sheet bound to one octahedral sheet
• Kaolinite: one of the most widespread
clay minerals in soils; most abundant in
warm moist climates
• Stable at low pH, the most weathered of
the silicate clays
• Synthesized under equal concentrations
of Al3+ and Si4+
Kaolinite
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A 1:1 clay
Little or no isomorphous substitution
“nutrient poor”
No shrink-swell (stable ‘cuz of Hbonding between adjacent layers)
• A product of acid weathering (low pH,
common in soils of the SE USA
Structure of Kaolinite
NO ISOMORPHOUS SUBSTITUTION!!!
Sheets of silicon tetrahedra and aluminum octahedra
linked by shared oxygen atoms.
Kaolinite under low pH
Al—OH + H+
No charge

Al—OH2+
positive charge
2:1 Silicate Clays
• Two silica tetrahedral sheets linked to
one aluminum octahedral sheet
• Three key groups:
– Smectites (e.g., montmorillonite)
– Vermiculites
– Micas (e.g., illite)
• And one “2-1-1” (chlorites)
Montmorillonite (2:1, a Smectite)
• Layer charge originates from the
substitution of Mg2+ for Al3+ in the
octahedral sheet
• Unstable (weathers to something else)
under low pH and high moisture
• Most swelling of all clays
• “Nutrient rich”
Structure of Montmorillonite
O
Al
Structure of montmorillonite (a smectite): it is built of two sheets of silicon
tetrahedra and one sheet of aluminum octahedra, linked by shared
oxygen atoms.
Structure of Montmorillonite
Causes cations to
move into the
interlayer space,
where they can
be replaced by
other cations
Isomorphous
substitution here, in
the octahedral sheet
= Mg
Vermiculites (2:1)
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Alteration product of micas (rock form)
Formed from loss of K+
Interlayer K+ of mica replaced with Mg2+
Limited shrink-swell …
Vermiculites (cont.)
• High layer charges: BOTH tetra and
octa
• “nutrient rich!”
• Stable under moderate to low soil pH,
high Mg, Fe
• Common in midwestern US
Structure of Vermiculite
Lots of
charge
imbalance,
both sheets:
High
nutrient
supply
capacity
= Al
= Fe
= Mg
Illite (2:1, a Mica)
• Al3+ substitution for Si4+ on the tetrahedral
sheet
• Strong surface charge
• “fairly nutrient poor”
• Non-swelling, only moderately plastic
• Stable under moderate to low pH, common
in midwestern US
Structure of Illite
Structure of Illite
K+
K+
1. Isomorphous
substitution is in
the tetrahedral
sheets
2. K+ comes into
the interlayer
space to satisfy the
charge and “locks
up” the structure
Chlorites (2:1:1)
• Hydroxy octahedral sheet in the
interlayer space
• Restricted swelling
• “Nutrient poor”
• Common in sedimentary rocks and the
soils derived from them
Structure of Chlorite
1. Iron-rich
Mg-Al
hydroxy sheet
Mg-Al
hydroxy
sheet
= Al
= Fe
= Mg
2. “locked”
structure
3. Low
nutrient
supply
capacity
Visual comparison of common silicate clays
H-H
Strongly held
montmorillonite
illite
“2:1:1”
Factors affecting mineral stability
• Number and type of base cations in the
structure (base cations are soluble…)
• Number of silica tetrahedra that are linked
(more sharing of oxygens = more stable)
• Al3+ proxy for Si4+ (more proxy = less stable)
• Presence of Fe (more Fe = less stable)
• Kinds of bonds
– Ionic are heat tolerant
– Covalent generally stronger ‘cuz shared electrons
between atoms, but not heat tolerant
Weathering pattern of clay formation
2:1
1:1
Vertisols
Fe/Al Ox
Ultisols
Oxisols
Spodosols
Entisols,
Inceptisols
Fig 8.16
CEC and weathering intensity
Alfisols,
Vertisols,
Argiudolls*
Ultisols
*remember nomenclature structure = “argi-ud-oll”
Oxisols
Where to find different clays
– see Table 8.3