Clay Types Study Guide

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 cationinterlayer • fine-grained micas • See lecture 16 slides: review of diff’s in properties of clay types

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

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

5. Isomorphous substitution equal shape/size • 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

6. Si2O4 SiAlO4 neutral Substitution in tetrahedral sheet +4, +3, -8 (-2*4) negative Tetrahedral sheet

7. (OH)2Al2O2 (OH)2AlMgO2 neutral Substitution in octahedral sheet -2, +3, +2, -4 negative 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 Al3+ and Si4+

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

11. Kaolinite under low pH Al—OH + H+ Al—OH2+ 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 Mg2+ for Al3+ 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 O Al 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.

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

16. Vermiculites (2:1) • Alteration product of micas (rock form) • Formed from loss of K+ • Interlayer K+ of mica replaced with Mg2+ • 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) • Al3+ substitution for Si4+ 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 K+ K+ 2. K+ comes into the interlayer space to satisfy the charge and “locks up” the structure Structure of Illite

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 • Iron-rich • “locked” structure • Low nutrient supply capacity Mg-Al hydroxy sheet Mg-Al hydroxy sheet = Al = Fe = Mg

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

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) • Al3+ proxy for Si4+ (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 2:1 1:1 Fe/Al Ox Vertisols Ultisols Oxisols Spodosols Entisols, Inceptisols Fig 8.16

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

28. Where to find different clays – see Table 8.3