Clay Mineralogy 101
Peter Ryan, Geology Dept, Middlebury College, June 1, 2012
Definition of “Clay”
• Grain size (e.g. < 2 mm) – could be any mineral;
• Mineral group consisting of sheets of silica tetrahedra
and Al/Mg/Fe octahedra. Known as “clay minerals”,
“phyllosilicates” and “silicate clays”.
Pedogenic smectite: ~ (Ca,Mg)0.3(Al1.2Fe0.7Mg0.15)(Si3.4Al0.6)O10(OH)2
Pedogenic halloysite: Al2Si2O5(OH)4
Silica (SiO4-4) tetrahedron
Basal plane of hexagonally-arranged silica
tetrahedra. = “tetrahedral sheet”.
An individual tetrahedron = SiO4-4, but sharing of
edges in extensive sheet like this results in Si2O5
Octahedrally-coordinated metal cation
(e.g Al+3, Mg+2, Fe+2, Fe+3)
An individual octahedron is Al(OH)6+3 but extensive
sharing of edges makes Al2(OH)6 … or Mg3(OH)6
Molecular-scale structure of clay minerals
(example here is K-mica, a “2:1 clay”)
Interlayer
K Al2 Si3Al O10(OH)2
2:1 Clays with zero layer charge
Al2Si4O10(OH)2
Mg3Si4O10(OH)2
Examples of 2:1 clays that differ based on
layer charge
LC = -1
LC = 0
LC = 0
LC = -1
K Al2 Si3Al O10(OH)2
Al2 Si4 O10(OH)2
K Mg3 Si3Al O10(OH)2
Mg3 Si4 O10(OH)2 (TALC)
Smectite (layer charge ~ -0.3)
Hydrated Na+ or Ca+2
2:1:1 clay (chlorite)
1:1 clay (e.g.kaolinite)
~interlayer
Mg2Al(OH)6.Mg3Si3AlO10(OH)2
Al2Si2O5(OH)4 (kaolinite)
Mg3Si2O5(OH)4 (serpentine)
Serpentine Group Minerals
Mg3Si2O5(OH)4
Chrysotile
Antigorite
Kaolinite [Al2Si2O5(OH)4]
Cation exchange capacity
(1) Layer charge, (2) Interlayer space, (3) surface area : volume
~5 cmolc/kg
~100 cmolc/kg
Cation exchange capacity
Unit of measure = cmolc/kg
Quartz ……………
1 cmolc/kg (low surface area, low charge)
Kaolinite clay ….
5 cmolc/kg (medium surface area, low charge)
Chlorite, mica
10-20 cmolc/kg (low surface area, high charge)
Smectite clay ….
100 cmolc/kg (high surface area, medium chg)
Vermiculite
150 cmolc/kg (~high surface area, ~high charge)
Humus…………….
150 cmolc/kg (high surface area, high chg, edges)
Iron hydroxide…
(anion exchange, pH-dependent)
Nutrient retention, contaminant mitigation
Champlain Valley Soil Clay Mineralogy
Illite > chlorite > smectite (quartz is ubiquitous)
Glacial erosion of
chlorite and mica
from schists,
phyllites and other
mtamorphic rocks
in source areas of
Glacial Lake
Vermont &
Champlain Sea
Taconic slates and phyllites
Additional source
of clay minerals in
Ch V soils =
chemical
weathering of
phyllosilicates
derived from
metamorphic
rocks (in soils…?
In lake
sediments…?),
e.g. biotite →
vermiculite or
smectite
LC ~ -0.5
LC ~ -0.5
Smectite is likely pedogenic clay in Champlain Valley
g = gypsum
c = calcite
Sherman, 1952
Shrink-swell clays aka expandable clays …
correlated with smectite content
Not all clays are shrink-swell
vs
Kaolinite
vs
smectite
< 2 mm fraction of clay soils in Addison
County contain ~10 % smectite
Significance of dehydration on aeration & vertical permeability?
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