1. Chapter 2 continued
Inorganic soil solids

2. Soil clay minerals Silica Tetrahedrons – one building block of soil minerals
Crystal pictures are from Bob Harter at Univ. of New Hampshire

3. Aluminum Octahedrons – another building block or layer in minerals

4. 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”

5. Layer charge
Results from isomorphic substitution with ions of different charge:
Al+3 for Si+4 in tetrahedra = -1
Mg+2 for Al+3 in octahedra = -1
Fe+2 for Al+3 “ “ = -1
Li+ for Mg+2 or Al+3 “ = -1 or -2
Negative charge must be neutralized by cations adsorbed on the mineral surface or in the interlayer (between the sheets) region

6. Differentiation of Layer Silicates
Number and sequence of tetrahedral and octahedral sheets.
Layer charge per unit cell of structure.
Type of interlayer bonding and neutralizing ions.
Cations in the octahedral sheet
Al+3 = dioctahedral: 2 out of every 3 sites filled (2 x 3+ = 6+)
Mg+2 = trioctahetral: 3 out of every 3 sites filled (3 x 2+ = 6+)
Type of stacking along the c-dimension.

7. 1:1 mineral formed when 1 tetrahedron bonds with 1 octahedron (sharing O’s)

10. 1:1 Layer Silicates Kaolinite [Si4]IV[Al4]VIO10(OH)8
One tetrahetral sheet [Si4]IV and one octahedral sheet [Al4]VI
Dioctahedral (Al in the octahedral sheet)
Layers of 1:1 tet-oct sheets held together by H-bonding
'Weak' individually, but cumulatively strong
No interlayer space

11. 1:1 Layer Silicates, cont’d
Properties:
Non-expansive, “non-sticky, non-plastic”
C-spacing = 0.72 nm
No layer charge (no isomorphous substitution);
low CEC (2-15 cmol/kg)
Small surface area: m2/g (external only; no internal since non-expanding)
~Hexagonal platy structure
Other kaolin polymorph minerals are Dickite and Nacrite (same chemical formula, different stacking arrangement = different shape)

12. - a typical kaolin mineral
- a typical kaolin mineral. Note the hexagonal stack-of-cards shape (and the “book” form)
ceramics.sdsu.edu/micrographs.html

14. 1:1 Layer Silicates, cont’d
Halloysite [Si4]IV[Al4]VIO10(OH)8. 4H2O
Same as kaolinite except for water molecules in the interlayer
Properties:
Slightly expansive
Surface area = ~40 m2/g
C-spacing = 1.0 nm (when hydrated)
Low CEC (10-40 cmol/kg)
Poorly crystallized (precipitated out of soil solution)
Tubular shape
Can adsorb NH4+

15. 2:1 minerals are formed when 2 tetrahedrons bond with 1 octahedron

16. Neutral end-members (no isomorphic substitution no layer charge):
Pyrophyllite
[Si8]IV[Al4]VIO20(OH)4
(8 x 4) + (4 x 3) = 44+
(20 x 2) +(4 x 1) = 44-
Net charge = 0
Dioctahedral (Al+3 in the octahedral sheet)
Talc
[Si8]IV[Mg6]VIO20(OH)4
(8 x 4) + (6 x 2) = 44+
(20 x 2) +(4 x 1) = 44-
Net charge = 0
Trioctahedral (Mg+2 in the octahedral sheet)

17. Pyrophyllite and Talc properties
Non-expansive; “non-sticky, non-plastic”
C-spacing = 0.93 nm
No layer charge (no isomorphous substitution);
low CEC (<10 cmol/kg)
Small surface area: m2/g (external only);
no internal area since non-expanding

18. Serpentine (Mg,Fe)6Si4O10(OH)8 var. chrysotile; fibrous; trioctahedral

19. Talc http://library.thinkquest.org/05aug/00461/images/talc.jpg

21. 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

22. 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)
K+ and NH4+ fixed in interlayers of smectites with tetrahedral substitution

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