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Chapter 2 continued

Chapter 2 continued. 2:1 phyllosilicates. 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”.

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Chapter 2 continued

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  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 = “sameshape”

  3. 2 Tetrahedral sheets + 1 octahedral sheet http://pubpages.unh.edu/~harter/crystal.htm#2:1%20MINERALS

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

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

  8. Muscovite (mica) Pyrophyllite http://www.a-m.de/images/pyrophyllit_01gre.jpg

  9. www.geoclassroom.com/mineralogy/silicatelayer.gif

  10. High-charge 2:1 mineralsMica 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. = 70-120 m2/g (mostly external) • CEC = 10 - 40 cmol/kg • c-spacing = 1.0 nm • Interlayer held tightly together by K+ fit in ditrigonal cavities of tetrahedral sheet

  13. Mica structure http://www.geoclassroom.com/mineralogy/phyllosilicates.html

  14. http://pubs.usgs.gov/of/2001/of01-041/htmldocs/images/illstruc.jpghttp://pubs.usgs.gov/of/2001/of01-041/htmldocs/images/illstruc.jpg

  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

  16. http://pubs.usgs.gov/of/2001/of01-041/htmldocs/images/monstru.jpghttp://pubs.usgs.gov/of/2001/of01-041/htmldocs/images/monstru.jpg

  17. Low-charge smectite properties • Shrink-swell characteristics • Plastic • High S.A. (both external + internal or interlayer area) = 600-800 m2/g • High CEC; 80-150 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. http://webmineral.com/specimens/picshow.php?id=1285

  19. Quartz. This SEM (scanning electron microscope) photograph shows authigenicquartz crystals and smectiteclaycoating another mineral grain.http://www.glossary.oilfield.slb.com/DisplayImage.cfm?ID=163

  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

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