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

http://pubpages.unh.edu/~harter/crystal.htm#2:1%20MINERALS

+ 1 octahedral sheet

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

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

http://www.a-m.de/images/pyrophyllit_01gre.jpg

Pyrophyllite

Muscovite (mica)

www.geoclassroom.com/mineralogy/silicatelayer.gif

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

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

(100) Plane) (001) Plane

http://www.geoclassroom.com/mineralogy/phyllosilicates.html

Mica structure

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

soils.ag.uidaho.edu/soilorders/vertisols_07.htm

Damage to buildings on Vertisols