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Chapter 2 continued 2:1 phyllosilicates.

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Presentation on theme: "Chapter 2 continued 2:1 phyllosilicates."— Presentation transcript:

1 Chapter 2 continued 2:1 phyllosilicates

2 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”

3 2 Tetrahedral sheets + 1 octahedral sheet

4 Form 2:1 minerals

5 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

6 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

7 Pyrophyllite (no sub’n) [Si8]IV[Al4]VIO20(OH)4
(8 x Si4+) + (4 x Al3+) = 44+ (20 x O2-) +(4 x OH1-) = 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- = Net charge = -2 Satisfied by 2 x K1+ ions that are “fixed” or tightly held in interlayer

8 Muscovite (mica) Pyrophyllite


10 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

11 Mica hand samples

12 High-charge 2:1 mineral properties
Source of K+ in soils as they weather Non-expansive, non-sticky, non-plastic S.A. = m2/g (mostly external) CEC = cmol/kg c-spacing = 1.0 nm Interlayer held tightly together by K+ fit in ditrigonal cavities of tetrahedral sheet

13 Mica structure (100) Plane) (001) Plane


15 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


17 Low-charge smectite properties
Shrink-swell characteristics Plastic High S.A. (both external + internal or interlayer area) = m2/g High CEC; 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)

18 Authigenic smectite (montmorillonite) overgrown on pore spaces and authigenicly-overgrown quartz grains in a sandstone.

19                                                        Quartz. This SEM (scanning electron microscope) photograph shows authigenic quartz crystals and smectite clay coating another mineral grain.

20 Montmorillonite
figure created by Dr. Sung-Ho Park

21 surface cracks in Vertisol (Utah)

22 Damage to buildings on Vertisols

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