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”

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

Form 2:1 minerals

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

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

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 (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

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

www.geoclassroom.com/mineralogy/silicatelayer.gif

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

Mica hand samples

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

Mica structure (100) Plane) (001) Plane http://www.geoclassroom.com/mineralogy/phyllosilicates.html

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

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