Chapter 2 continued
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Transcript of Chapter 2 continued
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”
2 Tetrahedral sheets
http://pubpages.unh.edu/~harter/crystal.htm#2:1%20MINERALS
+ 1 octahedral sheet
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
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- = -2Net 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
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
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)
Authigenic smectite (montmorillonite) overgrown on pore spaces and authigenicly-overgrown quartz grains in a sandstone.
http://webmineral.com/specimens/picshow.php?id=1285
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
Montmorillonite
www-esd.lbl.gov/sposito/ figure created by Dr. Sung-Ho Park