1. Chapter 2 continued
2:1 phyllosilicates

2. Isomorphous Substitution
Substitution, during formation, of one ion for another of similar SIZE (but not necessarily the same charge) in an ionic solid without changing the structure (shape, morphology) of the crystal.
Isomorphic = “same shape”

3. 2 Tetrahedral sheets + 1 octahedral sheet

4. Form 2:1 minerals

5. Oreo cookies serve as models for layer minerals (phyllosilicates)
One Oreo can be a mica or smectite-type mineral
First proposed by Jerry Irvin at UCR

6. Two Oreo cookies can be split and reconstructed to form a 1:1 mineral
Kaolinite
Two Oreo cookies can be split and reconstructed to form a 1:1 mineral

7. Pyrophyllite (no sub’n) [Si8]IV[Al4]VIO20(OH)4
(8 x Si4+) + (4 x Al3+) = 44+
(20 x O2-) +(4 x OH1-) = 44-
= 0
Net charge = 0
Muscovite (Tet’l sub’n)
K2[(Si6, Al2)]IV[Al4]VIO20(OH)4
(6 x Si4+) + (6 x Al3+) = 42+
(20 x O2-) +(4 x OH1-) = 44-
= Net charge = -2
Satisfied by 2 x K1+ ions that are “fixed” or tightly held in interlayer

8. Muscovite (mica) Pyrophyllite

10. High-charge 2:1 minerals Mica x = 2
Dioctahedral:
Muscovite Kx[Si,Al]8 [Al4]O20(OH, F)4
Paragonite Nax[Si,Al]8 [Al4]O20(OH, F)4
Trioctahedral:
Biotite Kx[Si,Al]8 [Mg, Fe, Al]6O20(OH, F)4
Phlogopite Kx[Si,Al]8 [Mg6]O20(OH, F)4
Lepidolite Kx[Si,Al]8 [Li,Al]6O20(OH, F)4

11. Mica hand samples

12. High-charge 2:1 mineral properties
Source of K+ in soils as they weather
Non-expansive, non-sticky, non-plastic
S.A. = m2/g (mostly external)
CEC = cmol/kg
c-spacing = 1.0 nm
Interlayer held tightly together by K+ fit in ditrigonal cavities of tetrahedral sheet

13. Mica structure
(100) Plane)
(001) Plane

15. 2:1 minerals with low layer charge (x)
Smectites x = 0.4 – 1.2
Dioctahedral
Montmorillonite Mx,H2O [Si8][Al,Mg]4O20(OH)4
Beidellite Mx,H2O [Si,Al]8[Al4]O20(OH)4
Nontronite Mx,H2O [Si,Al]8[Fe+3]4O20(OH)4
Trioctahedral
Saponite Mx,H2O [Si,Al]8[Mg6]O20(OH)4
Hectorite Mx,H2O [Si8][Mg,Li]6O20(OH)4

17. Low-charge smectite properties
Shrink-swell characteristics
Plastic
High S.A. (both external + internal or interlayer area) = m2/g
High CEC; cmol/kg
Expansive - c-spacing variable with cation saturation and heat (1.0 –2.0 nm)
Very small particles (fine clay)
Flakey shape (e.g., corn flakes)

18. Authigenic smectite (montmorillonite) overgrown on pore spaces and authigenicly-overgrown quartz grains in a sandstone.

19.                                                        Quartz. This SEM (scanning electron microscope) photograph shows authigenic quartz crystals and smectite clay coating another mineral grain.

20. Montmorillonite www-esd.lbl.gov/sposito/
figure created by Dr. Sung-Ho Park

21. surface cracks in Vertisol (Utah)
soils.ag.uidaho.edu/soilorders/i/Vert_07b.jpg

22. Damage to buildings on Vertisols
soils.ag.uidaho.edu/soilorders/vertisols_07.htm