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