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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.

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Presentation on theme: "Chapter 2 continued 2:1 phyllosilicates. Isomorphous Substitution Substitution, during formation, of one ion for another of similar SIZE (but not necessarily."— 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

7 Muscovite (Tet’l sub’n) K 2 [(Si 6, Al 2 )] IV [Al 4 ] VI O 20 (OH) 4 (6 x Si 4+ ) + (6 x Al 3+ ) = 42+ (20 x O 2- ) +(4 x OH 1- ) = = -2 Net charge = -2 Satisfied by 2 x K 1+ ions that are “fixed” or tightly held in interlayer Pyrophyllite (no sub’n) [Si 8 ] IV [Al 4 ] VI O 20 (OH) 4 (8 x Si 4+ ) + (4 x Al 3+ ) = 44+ (20 x O 2- ) +(4 x OH 1- ) = = 0 Net charge = 0

8 Pyrophyllite Muscovite (mica)

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10 High-charge 2:1 minerals Micax = 2 Dioctahedral: MuscoviteK x [Si,Al] 8 [Al 4 ]O 20 (OH, F) 4 Paragonite Na x [Si,Al] 8 [Al 4 ]O 20 (OH, F) 4 Trioctahedral: Biotite K x [Si,Al] 8 [Mg, Fe, Al] 6 O 20 (OH, F) 4 Phlogopite K x [Si,Al] 8 [Mg 6 ]O 20 (OH, F) 4 LepidoliteK x [Si,Al] 8 [Li,Al] 6 O 20 (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. = m 2 /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 (100) Plane)(001) Plane Mica structure

14 pubs.usgs.gov/of/2001/of01-041/htmldocs/images/illstruc.jpg

15 2:1 minerals with low layer charge (x) Smectitesx = 0.4 – 1.2 Dioctahedral Montmorillonite M x,H 2 O [Si 8 ][Al,Mg] 4 O 20 (OH) 4 Beidellite M x,H 2 O [Si,Al] 8 [Al 4 ]O 20 (OH) 4 Nontronite M x,H 2 O [Si,Al] 8 [Fe +3 ] 4 O 20 (OH) 4 Trioctahedral SaponiteM x,H 2 O [Si,Al] 8 [Mg 6 ]O 20 (OH) 4 HectoriteM x,H 2 O [Si 8 ][Mg,Li] 6 O 20 (OH) 4

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17 Low-charge smectite properties Shrink-swell characteristics Plastic High S.A. (both external + internal or interlayer area) = m 2 /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. authigenicquartzsmectiteclaycoatingmineral

20 Montmorillonite www-esd.lbl.gov/sposito/figure created by Dr. Sung-Ho Park

21 surface cracks in Vertisol (Utah) soils.ag.uidaho.edu/soilorders/i/Vert_07b.jpg

22 soils.ag.uidaho.edu/soilorders/vertisols_07.htm Damage to buildings on Vertisols


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