Soil Mineralogy and Chemistry Lecture 4

Phyllosilicate Minerals

tet oct tet oct tet oct tet oct tet oct trioctahedraldioctahedral octahedra 1:1 2:1 T:O (class) brucite gibbsite serpentine kaolinite pyrophyllite talc Phyllosilicates, NO layer charge

Building of Tetrahedral Sheets * Linked SiO 4 tetrahedra

SiO 4 Tetrahedron

x y z

Si 6 O 18

Linking Rings to Form Tetrahedral Sheets

Tetrahedral Sheet 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 tet oct tet Talc (2:1 trioctahedral mineral)

tet oct Short hand, Block notation Short-hand Symbols

tet oct tet oct tet oct tet oct tet oct trioctahedraldioctahedral 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

Al 3+, Fe 3+ Si 4+ tet oct Mg 2+ Al 3+ Isomorphic Substitution

Isomorphic substitution NET Charge 0 -2

Terminal Bonds  pH-dependent charge (ionizable functional groups) Charge Development

terminal bonds Terminal Bonds

Terminal Bonds: Ionizable Functional Groups Al-OH 2 +  Al-OH o + H +  Al-O - + H + Al O High pH Low pH -pH-dependent charge - edges of phyllosilicates; all surface on Fe- and Al-oxides

tet oct tet oct tet K+K+ K+K+ K+K+ Phyllosilicates: 2:1 with layer charge micas 1 unit of (-) layer charge per formula unit tet oct tet oct tet K+K+ H2OH2O Ca 2+ H2OH2O H2OH2O < 1 unit of (-) layer charge per formula unit 2:1 clay minerals

Swelling Potential?

Structure d-spacing Interlayer Status: d-spacing (and mica) K+K+

CEC cmol/Kg Properties of Clay Minerals

Western GA Ultisol Kaolin and Oxide Rich Soil Accessory Minerals

Smectite Soil NE Montana Vertisol Hawaiian 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/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