soil mineralogy and chemistry lecture 4. phyllosilicate minerals
Post on 20-Dec-2015
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Soil Mineralogy and
Chemistry
Lecture 4
x y
z
PhyllosilicateMinerals
tetoct
oct
tetoct
tet
tetoct
tetoct
tet
oct
trioctahedral dioctahedral
octahedra
1:1
2:1
T:O(class)
brucite gibbsite
serpentine kaolinite
pyrophyllitetalc
Phyllosilicates, NO layer charge
Building of Tetrahedral Sheets
* Linked SiO4 tetrahedra
SiO4 Tetrahedron
z
x yx y
z
xy
z
xy
z
xy
z
xy
z
xy
z
xy
z
xy
zSi6O18
Linking Rings to FormTetrahedral Sheets
x
y
z
Tetrahedral Sheet
Si:O 2:5
Building of Octahedral Sheets
x
y
z
Al(OH)6 or Mg (OH)6 Octahedra
OH
x
yz
x
yz
x
yz
x
yz
x
yz
x
yz
x
yz
x
yz
x
yz
xy
z
Octahedral Sheet
OH
A Georgian Ultisol
Octahedral-TetrahedralLinkage
x y
z
Sharing of Apical Oxygens in Tetrahdral Sheet withHydroxyls of Octahedral Sheet
Serpentine (1:1 trioctahedral mineral)
tet
oct
x y
z
Sharing of Apical Oxygens in Tetrahdral Sheet withHydroxyls of Two Octahedral Sheets
tet
oct
tet
Talc (2:1 trioctahedral mineral)
x y
z
tet
oct
Short hand,Block notation
Short-hand Symbols
tetoct
oct
tetoct
tet
tetoct
tetoct
tet
oct
trioctahedral dioctahedral
octahedra
1:1
2:1
T:O(class)
brucite gibbsite
serpentine kaolinite
pyrophyllite talc
Phyllosilicates, NO layer charge
Isomorphic Substitution
permanent (layer) charge
Charge Development
x y
z
Al3+,Fe3+
Si4+
tet
octMg2+
Al3+
Isomorphic Substitution
- - - - - - -
- - - - - - -
++ ++ ++ ++ ++ ++ ++
- - - - - - -
- - - - - - -
++ + ++ ++ ++ + ++
Isomorphic substitution
NETCharge
0
-2
Terminal BondspH-dependent charge(ionizable functional groups)
Charge Development
x y
z
terminalbonds
Terminal Bonds
Terminal Bonds: Ionizable Functional Groups
Al-OH2+ Al-OHo + H+ Al-O- + H+
AlO
High pHLow pH
-pH-dependent charge- edges of phyllosilicates; all surface on Fe- and Al-oxides
tetoct
tet
tetoct
tet
K+ K+ K+
Phyllosilicates: 2:1 with layer charge
micas
1 unit of (-) layer chargeper formula unit
tetoct
tet
tetoct
tet
K+H2O Ca2+ H2O H2O
< 1 unit of (-) layer chargeper formula unit
2:1 clay minerals
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 pyrophyllite, Al2Si4O10(OH)2
2 tet: 1 oct. 1 phlogopite muscovite KMg3(AlSi3O10)(OH)2 KAl2(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
0.6-0.9 vermiculite 0.25-0.6 smectite † The layer charge per formula unit
Swelling Potential?
Structure d-spacing
Interlayer Status: d-spacing
(and mica)
K+
Surface Area (m2/g) Interlayer CationSize (m) External Internal Spacing (nm) Sorption
Kaolinite 0.1-5.0 10-50 - 0.7 5-15Smectite <1.0 70-150 500-700 1.0-2.0 85-110Vermiculite 0.1- 5.0 50-100 450-600 1.0-1.4 100-120Illite 0.1-2.0 50-100 5-100 1.0 15-40Humus coatings - - - 100-300
CECcmol/Kg
Properties of Clay Minerals
Western GAUltisol
Kaolin and OxideRich Soil
Accessory Minerals
Smectite Soil
NE Montana VertisolHawaiian Oxisol
Iron and Al-oxide Rich
?
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)
Clay mineralogy reflects weathering processes
Micas Vermiculite Smectite Kaolinite Al,Fe-Oxides
Mol
Alf
Ult
E/IE/I
Organic Matter
- reactive functional groups: carboxyl, hydroxyl, phenolic
* Humus, Humic Acid, Fulvic Acid
+ Flocculation (chemical)
Aggregation
Flocculation and Aggregation
(organic gluing)
Organic Matter Promoted Aggregation