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 - 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) = 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)

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