chapter 2 continued
DESCRIPTION
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”. - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 2 continued
2:1 phyllosilicates
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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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2 Tetrahedral sheets
http://pubpages.unh.edu/~harter/crystal.htm#2:1%20MINERALS
+ 1 octahedral sheet
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Form 2:1 minerals
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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
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Two Oreo cookies can be split and reconstructed to
form a 1:1 mineral
Kaolinite
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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 = -2Satisfied 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- = 0Net charge = 0
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http://www.a-m.de/images/pyrophyllit_01gre.jpg
Pyrophyllite
Muscovite (mica)
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www.geoclassroom.com/mineralogy/silicatelayer.gif
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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
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Mica hand samples
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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
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(100) Plane) (001) Plane
http://www.geoclassroom.com/mineralogy/phyllosilicates.html
Mica structure
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http://pubs.usgs.gov/of/2001/of01-041/htmldocs/images/illstruc.jpg
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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
TrioctahedralSaponite Mx,H2O [Si,Al]8[Mg6]O20(OH)4
Hectorite Mx,H2O [Si8][Mg,Li]6O20(OH)4
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http://pubs.usgs.gov/of/2001/of01-041/htmldocs/images/monstru.jpg
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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)
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Authigenic smectite (montmorillonite) overgrown on pore spaces and authigenicly-overgrown quartz grains in a sandstone.
http://webmineral.com/specimens/picshow.php?id=1285
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Quartz. This SEM (scanning electron microscope) photograph shows authigenic quartz crystals and smectite clay coating another mineral grain.http://www.glossary.oilfield.slb.com/DisplayImage.cfm?ID=163
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Montmorillonite
www-esd.lbl.gov/sposito/ figure created by Dr. Sung-Ho Park
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surface cracks in Vertisol (Utah)
soils.ag.uidaho.edu/soilorders/i/Vert_07b.jpg
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soils.ag.uidaho.edu/soilorders/vertisols_07.htm
Damage to buildings on Vertisols