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Lecture 4
Soil Mineralogy
and
Chemistry
Phyllosilicate
Minerals
Phyllosilicates, NO layer charge
T:O
trioctahedral
dioctahedral
oct
oct
(class)
octahedra
gibbsite
brucite
1:1
tet
oct
tet
oct
kaolinite
serpentine
2:1
tet
oct
tet
oct
tet
tet
talc
pyrophyllite
Building of Tetrahedral Sheets
* Linked SiO4 tetrahedra
SiO4 Tetrahedron
z
y
x
Si6O18
Linking Rings to Form
Tetrahedral Sheets
Tetrahedral Sheet
x
y
z
Si:O
2:5
Building of Octahedral Sheets
Al(OH)6 or Mg (OH)6 Octahedra
OH
Octahedral Sheet
OH
A Georgian
Ultisol
Octahedral-Tetrahedral
Linkage
Sharing of Apical Oxygens in Tetrahdral Sheet with
Hydroxyls of Octahedral Sheet
Serpentine (1:1 trioctahedral mineral)
tet
oct
Sharing of Apical Oxygens in Tetrahdral Sheet with
Hydroxyls of Two Octahedral Sheets
Talc (2:1 trioctahedral mineral)
tet
oct
tet
Short-hand Symbols
tet
oct
Short hand,
Block notation
Phyllosilicates, NO layer charge
T:O
trioctahedral
dioctahedral
oct
oct
(class)
octahedra
gibbsite
brucite
1:1
tet
oct
tet
oct
kaolinite
serpentine
2:1
tet
oct
tet
oct
tet
tet
pyrophyllite
talc
Charge Development
Isomorphic Substitution
 permanent (layer) charge
Isomorphic Substitution
tet
Si4+
oct
Mg2+
Al3+
Al3+,
Fe3+
NET
Charge
-
-
-
-
-
-
-
++
-
++
-
++
-
++
-
++
-
++
-
++
-
0
Isomorphic substitution
-
-
-
-
-
-
-
++
-
+
-
++
-
++
-
++
-
+
-
++
-
-2
Charge Development
Terminal Bonds
pH-dependent charge
(ionizable functional groups)
Terminal Bonds
terminal
bonds
Terminal Bonds: Ionizable Functional Groups
O
Al
Al-OH2+  Al-OHo + H+  Al-O- + H+
Low pH
High pH
-pH-dependent charge
- edges of phyllosilicates; all surface on Fe- and Al-oxides
Phyllosilicates: 2:1 with layer charge
K+
micas
2:1 clay minerals
tet
oct
tet
oct
tet
tet
K+
K+
tet
oct
tet
1 unit of (-) layer charge
per formula unit
2+ H2O H O
2
H2O Ca
K+
tet
oct
tet
< 1 unit of (-) layer charge
per formula unit
General Classes (layer build-up) of Phyllosilicate Minerals:
Layer Type
Charge†
Trioctahedral
Dioctahedral
1 octahedra
0
brucite, Mg(OH)2
gibbsite, Al(OH)3
1 tet. : 1 oct.
0
serpentine, Mg3Si2O5(OH)4
kaolinite, Al2Si2O5(OH)4
2 tet. : 1 oct.
0
talc, Mg3Si4O10(OH)2
2 tet: 1 oct.
1
phlogopite
KMg3(AlSi3O10)(OH)2
1
biotite
KFe3(AlSi3O10)(OH)2
0.6-0.8
illite (hydrous mica)
K(Na,Ca) Al1.3Fe0.4Mn0.2Si3.4Al0.6O10(OH)2
pyrophyllite, Al2Si4O10(OH)2
muscovite
KAl2(AlSi3O10)(OH)2
0.6-0.9
vermiculite
0.25-0.6
smectite
† The layer charge per formula unit
Swelling Potential?
Interlayer Status: d-spacing
Structure
d-spacing
K+
(and mica)
Properties of Clay Minerals
2
Kaolinite
Smectite
Vermiculite
Illite
Humus
Size (m)
0.1-5.0
<1.0
0.1- 5.0
0.1-2.0
coatings
Surface Area (m /g)
External
Internal
10-50
70-150
500-700
50-100
450-600
50-100
5-100
-
Interlayer
Spacing (nm)
0.7
1.0-2.0
1.0-1.4
1.0
-
CEC
Cation
cmol/Kg
Sorption
5-15
85-110
100-120
15-40
100-300
Accessory
Minerals
Kaolin and Oxide
Rich Soil
Western GA
Ultisol
Smectite Soil
Iron and Al-oxide Rich
?
NE Montana Vertisol
Hawaiian Oxisol
Clay mineralogy reflects weathering processes
Micas  Vermiculite  Smectite  Kaolinite  Al,Fe-Oxides
Young, weakly weathered soils
= fine-grained mica, chlorite, vermiculite
(Entisol, Inceptisol)
Intermediate weathering
= vermiculite, smectite, kaolinite
(Mollisol, Alfisol, Ultisol)
Strong weathering
= kaolinite, hydrous oxides
(Ultisol--> Oxisol)
Mol
E/I
E/I
Alf
Ult
Organic Matter
- reactive functional groups: carboxyl, hydroxyl, phenolic
* Humus, Humic Acid, Fulvic Acid
Flocculation and Aggregation
+
Flocculation (chemical)
Aggregation
(organic gluing)
Organic Matter Promoted Aggregation