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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 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 = “sameshape”
2 Tetrahedral sheets + 1 octahedral sheet http://pubpages.unh.edu/~harter/crystal.htm#2:1%20MINERALS
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- = -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
Muscovite Pyrophyllite http://www.a-m.de/images/pyrophyllit_01gre.jpg
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
http://pubs.usgs.gov/of/2001/of01-041/htmldocs/images/monstru.jpghttp://pubs.usgs.gov/of/2001/of01-041/htmldocs/images/monstru.jpg
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 authigenicquartz crystals and smectiteclaycoating 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
surface cracks in Vertisol (Utah) soils.ag.uidaho.edu/soilorders/i/Vert_07b.jpg
Damage to buildings on Vertisols soils.ag.uidaho.edu/soilorders/vertisols_07.htm