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

strongNo interlayer space

1:1 Layer Silicates, cont’dProperties:• 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

interlayerProperties:

Slightly expansiveSurface area = ~40 m2/gC-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

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)

Neutral end-members (no isomorphic substitution no layer charge):

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/

Talc

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

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