Understanding Clay Minerals and Their Formation Process

1. Chapter 4- Products of Weathering • Several things can happen to products • 1- removal of materials by leaching • e.g., CaCO3 • 2- reaction of materials, either in situ or as they are physically moved, to create new crystalline structures • Clay minerals • Illite, smectite (montmorillonite) • Hydrous oxides • Fe and Al are most common • Also Mg and Mn too.

2. Clay minerals • Mostly of silicate composition • Hydrated Fe, Al, Mg- silicate structures in discrete layers • These layered silicates are called phyllosilicates • Subdivided into two classes - on structure and charge • Structure • 1:1 clay minerals • 1:1 refers to how the crystal structures are arranged • 1 octahedron attached to a tetrahedron • 2:1 clay minerals • 2:1 refers to how the crystal structures are arranged • 1 octahedron attached to 2 tetrahedrons

3. On charge • Based on the amount of chemical substitution • i.e., which ions are most easily mobilized and exchanged • Example: Montmorillonite • Na 0.33 (Al 1.67 Mg 0.33) Si4 O10 (OH) • Mg substitutes for Al, replacing 1 of every 6 atoms • creates a net charge imbalance that allows X 0.33 to balance the formula • Since a number of ions can meet the charge requirement (in this case its Na) we use X as a generic variable term in the formula

4. Water plays a critical role in many clay minerals • Contains elements that act as bonding agents • keeps the crystalline structure together • Most notable are the H+ and the OH- cations and anions • In many circumstances the water can be driven off or can facilitate ion substitution, especially in 2:1 clays • e.g., smectite clays (montmorillonite)

5. Building blocks of clay minerals Silica tetrahedron Hydroxyl octahedron tetrahedron

6. Structure of Kaolinite A 1:1 phyllosilicate clay mineral Note the single tetrahedron attached to the octahedron

7. Phyllosilicate mineral structures- including Kaolinite

8. Forms of Smectite (a 2:1 clay) • Montmorillonite - Mg form of smectite • Both Al and Mg in the octahedral layer • Beidellite- Al form of smectite • Al in the octahedral sheet and substituting for Si in the tetrahedral sheet • Nontronite - Fe variety of smectite • Fe in the octahedral sheet and Al substituting for Si in the tetrahedral sheet

9. Illite (a 2:1 clay mineral) • Chemical formula is similar to muscovite • Differs slightly because Al and Si substitute for one another allowing K+ to act as an interlayer bonding agent • The K+ location in the structure of the layers is close to the negative charges • Resultant chemical bond is strong enough to inhibit water squeezing in • Limits the shrink swell capacity of the clay

11. 2:1 clays structure • Non-shrink/swell type • e.g., Illite • Shrink/swell type • e.g., Smectite

12. Unusual clay minerals often encountered • Montmorillonite often results from the alteration of volcanic ash • the end product is a bentonite clay • In conditions with adequate drainage • Mg is leached (instead of substituting) forming kaolinite instead of montmorillonite. • Smectites commonly result from the weathering and alteration of basic composition rocks. • Nontronite results from the alteration of basaltic glass.

13. Chemical composition of clay minerals • Tough to get clay chemical compositions • Hard to get pure samples to run • often clays have a tendency to be mixed layer clays • the 2:1 clays mix with octahedral sheets (not part of 2:1 structure) • When they do run chemical analysis… • It’s really variable

14. Chemical composition of clays

15. Non crystalline and Crystalline Al and Fe compounds • Non Xtln • Allophane- common Al compound found in soils • not quite crystalline, but not quite amorphous either • Xtln • Al crystalline minerals • Gibbsite • Boehmite

16. Origins of clays

17. Clay type vs. depth

19. Clay types and soil orders • Only 3 strong relationships between soil order and clay types • Vertisols - montmorillonite • Andisols - allophane • Oxisols - oxides and kaolinite • The remaining 8 orders can and do contain a whole range of clay minerals