1. Clay Types Study Guide Types of Colloids –crystalline silicate clays (covered by this guide) –non-crystalline silicate clays (p 314) –Fe & Al oxides (p 315, 322ff) –Organic (p 315, 325) Basis for distinguishing silicate clay types Isomorphous substitution Review of clay types Distribution Weathering & generalized distribution in US

2. Basis for distinguishing crystalline silicate clays Based on numbers & combinations of structural units –tetrahedral and octahedral sheets –planes combined  sheets combined  layers  crystals (fig 8.4) Two general categories: 1:1, 2:1 –2:1 types: expanding & nonexpanding –also “2:1:1”  Chlorites Number of cations in octahedral sheet –tri- vs. di-octahedra (fig 8.5) Size and location of layer charge (see also lecture 16 slides) Type of bonding between layers (see also lecture 16 slides) : –Strong: ionic > H-bonding > van der Waals :Weak Absence or presence of a cation interlayer  fine-grained micas See lecture 16 slides: review of diff’s in properties of clay types

3. Clay minerals 1:1 clays (one tetrahedral sheet for each octahedral sheet) Kaolinite, nacrite, dickite, halloysite, etc. 2:1 clays (two tetrahedral sheets for each octahedral sheet) Montmorillonite, beidellite, saponite, etc. Illite, muscovite, biotite, etc. Tri- or di- vermiculite Cookeite, chamosite, etc. ‘Weird’, not truly 2:1 SmectitesMicasVermiculitesChlorites

4. Visual comparison of common silicate clays’ structure illite montmorillonite “2:1:1” more strongly held than in smectite

5. Isomorphous substitution The replacement of one ion for another of similar size within the crystalline structure of the clay Often results in change in net charge takes eons – doesn’t change rapidly equalshape/size

6. Substitution in tetrahedral sheet Si 2 O 4 SiAlO 4 neutral Tetrahedral sheet +4, +3, -8 (-2*4) negative

7. Substitution in octahedral sheet (OH) 2 Al 2 O 2 (OH) 2 AlMgO 2 neutral negative -2, +3, +2, -4 Octahedral sheet

8. 1:1 Silicate Clays Layers composed of one tetrahedral sheet bound to one octahedral sheet Kaolinite: one of the most widespread clay minerals in soils; most abundant in warm moist climates Stable at low pH, the most weathered of the silicate clays Synthesized under equal concentrations of Al 3+ and Si 4+

9. Kaolinite A 1:1 clay Little or no isomorphous substitution “nutrient poor” No shrink-swell (stable ‘cuz of H- bonding between adjacent layers) A product of acid weathering (low pH, common in soils of the SE USA

10. Sheets of silicon tetrahedra and aluminum octahedra linked by shared oxygen atoms. Structure of Kaolinite NO ISOMORPHOUS SUBSTITUTION!!!

11. Kaolinite under low pH Al—OH + H +  Al—OH 2 + No charge positive charge

12. 2:1 Silicate Clays Two silica tetrahedral sheets linked to one aluminum octahedral sheet Three key groups: –Smectites (e.g., montmorillonite) –Vermiculites –Micas (e.g., illite) And one “2-1-1” (chlorites)

13. Montmorillonite (2:1, a Smectite) Layer charge originates from the substitution of Mg 2+ for Al 3+ in the octahedral sheet Unstable (weathers to something else) under low pH and high moisture Most swelling of all clays “Nutrient rich”

14. Structure of montmorillonite (a smectite): it is built of two sheets of silicon tetrahedra and one sheet of aluminum octahedra, linked by shared oxygen atoms. Structure of Montmorillonite Al O

15. Structure of Montmorillonite Isomorphous substitution here, in the octahedral sheet Causes cations to move into the interlayer space, where they can be replaced by other cations = Mg

16. Vermiculites (2:1) Alteration product of micas (rock form) Formed from loss of K + Interlayer K + of mica replaced with Mg 2+ Limited shrink-swell …

17. Vermiculites (cont.) High layer charges: BOTH tetra and octa “nutrient rich!” Stable under moderate to low soil pH, high Mg, Fe Common in midwestern US

18. Structure of Vermiculite Lots of charge imbalance, both sheets: High nutrient supply capacity = Al= Fe = Mg

19. Illite (2:1, a Mica) Al 3+ substitution for Si 4+ on the tetrahedral sheet Strong surface charge “fairly nutrient poor” Non-swelling, only moderately plastic Stable under moderate to low pH, common in midwestern US

20. Structure of Illite

21. 1. Isomorphous substitution is in the tetrahedral sheets 2. K+ comes into the interlayer space to satisfy the charge and “locks up” the structure K+

22. Chlorites (2:1:1) Hydroxy octahedral sheet in the interlayer space Restricted swelling “Nutrient poor” Common in sedimentary rocks and the soils derived from them

23. Structure of Chlorite Mg-Al hydroxy sheet Mg-Al hydroxy sheet 1.Iron-rich 2.“locked” structure 3.Low nutrient supply capacity = Al= Fe= Mg

24. Visual comparison of common silicate clays illite montmorillonite “2:1:1” Strongly held H-H

25. Factors affecting mineral stability Number and type of base cations in the structure (base cations are soluble…) Number of silica tetrahedra that are linked (more sharing of oxygens = more stable) Al 3+ proxy for Si 4+ (more proxy = less stable) Presence of Fe (more Fe = less stable) Kinds of bonds –Ionic are heat tolerant –Covalent generally stronger ‘cuz shared electrons between atoms, but not heat tolerant

26. Weathering pattern of clay formation Entisols, Inceptisols Vertisols 2:11:1 Fe/Al Ox Spodosols Fig 8.16 Oxisols Ultisols

27. CEC and weathering intensity Alfisols, Vertisols, Argiudolls* UltisolsOxisols *remember nomenclature structure = “argi-ud-oll”

28. Where to find different clays – see Table 8.3