Chapter 2 continued
Inorganic soil solids
Soil clay minerals
Silica Tetrahedrons – one building block
of soil minerals
Crystal pictures are from Bob Harter at Univ. of New Hampshire
http://pubpages.unh.edu/~harter/crystal.htm#2:1%20MINERALS
Aluminum Octahedrons – another
building block or layer in minerals
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”
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
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.
1:1 mineral formed when
1 tetrahedron bonds with
1 octahedron (sharing O’s)
http://pubs.usgs.gov/of/2001/of01-041/htmldocs/images/kaostru.jpg
http://www.science.uwaterloo.ca/~cchieh/cact/fig/kaolinite.gif
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
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: 10-20 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)
- a typical kaolin mineral. Note the
hexagonal stack-of-cards shape
(and the “book” form)
ceramics.sdsu.edu/micrographs.html
www.georgiaencyclopedia.org
http://soil.gsfc.nasa.gov/forengeo/aukao2.GIF
http://csmres.jmu.edu/geollab/fichter/Minerals/images/kaolinite.JPG
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+
2:1 minerals are formed when 2
tetrahedrons bond with 1
octahedron
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) = 44Net charge = 0
Talc
[Si8]IV[Mg6]VIO20(OH)4
(8 x 4) + (6 x 2) = 44+
(20 x 2) +(4 x 1) = 44Net charge = 0
Dioctahedral (Al+3 in the
octahedral sheet)
Trioctahedral (Mg+2 in the
octahedral sheet)
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: 10-20 m2/g (external
only);
no internal area since non-expanding
Serpentine (Mg,Fe)6Si4O10(OH)8
var. chrysotile; fibrous; trioctahedral
www.galleries.com/minerals/silicate/
http://library.thinkquest.org/05aug/00461/images/talc.jpg
Talc
http://www.ippnet.com/assets/Product~Images/Fiberglass/Talc.jpg
http://pubs.usgs.gov/of/2001/of01-041/htmldocs/images/monstru.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
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)
• K+ and NH4+ fixed in interlayers of smectites
with tetrahedral substitution
Montmorillonite
www-esd.lbl.gov/sposito/ figure created by Dr. Sung-Ho Park