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Building Soil Minerals. EXPECTED ION CORRDINATION . BOND VALENCE . BOND VALENCE . Clay Minerals. BUILDING BLOCKS. Si 6 O 18. Tetrahedral Sheet. Side View (100) plane. Top View (001) plane. Structural Views: Anion Sheets. Creating Octahedral Sheets. - Filling of Anion Sheets.
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Tetrahedral Sheet Side View (100) plane Top View (001) plane
Creating Octahedral Sheets - Filling of Anion Sheets Dioctahedral (trivalent ions) and Trioctahedral (divalent ions)
(di)OCTAHEDRAL SITE FILLING Anion Sheet with Trivalent (Al3+) Cations
(di)OCTAHEDRAL SITE FILLING Dioctahedral Sheet Without Basal Hydroxyls
Octahedral Sheets Trioctahedral (divalent cation) View down [001] axis x y Dioctahedral (trivalent cation)
Charge Development • Isomorphic substitution • Terminal (unsatisfied) bonds
Charge Development • Isomorphic Substitution Tetrahedral Sites: Al3+ for Si4+g (-) charge Octahedral Sites: Mg2++ for Al3+g (-) charge Al3+ for Mg2+g (+) charge
Charge Development • Terminal (unsatisfied) bonds surface surface - bond valence considerations
Rock Ideal Weathering Series Entisol Inceptisol Mollisol Vertisol (clay mineralogy) Alfisol Ultisol Oxisol
Rock Entisol Inceptisol Mollisol Alfisol Ultisol Oxisol
Phyllosilicates:Clay Minerals Entisol Inceptisol Mollisol Alfisol Ultisol - Phyllosilicates dominate the clay size particles of most soils
Oct. Octahedral Minerals Brucite, Mg(OH)2 Gibbsite, Al(OH)3
1:1 Phyllosilicates Sheets } tet 1:1 layer oct H-bonds H-bonds Kaolinite and Serpentine
Kaolinite: Al2Si2O5(OH)4 - dioctahedral, 1:1 mineral
Sharing of Apical Oxygens in Tetrahdral Sheet with Hydroxyls of Two Octahedral Sheets Talc (2:1 trioctahedral mineral) tet oct tet
tet tet tet tet oct oct Micas Phlogopite: KMg3(AlSi3O10)(OH)2 Biotite: KFe3(AlSi3O10)(OH)2 Muscovite: KAl2(AlSi3O10)(OH)2 K+ K+ K+ Unit layer of charge
Ca2+ H2O K+ H2O H2O tet tet tet tet oct oct Expandable 2:1 Layer Phyllosilicates Illite (hydrous mica) Vermiculite Smectite Illite: KAl1.3Fe0.4Mg0.2Si3.4Al0.6O10(OH)2
Ca2+ H2O K+ H2O H2O tet tet tet tet oct oct Expandable 2:1 Layer Phyllosilicates Vermiculite dioctahedral:Nax(Al,Fe)2(Si4-xAlx)O10(OH)2nH2O trioctahedral: Nax(Mg,Fe)3(Si4-xAlx)O10(OH)2nH2O
Expandable 2:1 Layer Phyllosilicates Smectite Dioctahedral Forms: MontmorilloniteNax(Al 2-xMgx)Si4O10(OH)2 Beidellite:Nax(Al 2)(AlxSi4-x)O10(OH)2 Trioctahedral Forms: Saponite: Nax-y(Mg3-yAly)3(Si4-xAlx)O10(OH)2 Hectorite:Nax(Mg3-xLix)3Si4O10(OH)2
Interlayer Expansion Two Dominant Factors: 1. Structural Charge 2. Interlayer Ion
2:1:1 Layer Phyllosilicates [Mg2Al1(OH )6]Mg3(Si3Al)O10(OH)2
tet tet tet tet oct (b) oct oct 2:1:1 Layer Phyllosilicates Hydroxy Interlayer Vermiculite (HIV) Hydroxy Interlayer Smectite (HIS) - pedogenic chlorites versus true chlorites [Mg2Al1(OH )6]Mg3(Si3Al)O10(OH)2
Accessory Minerals Iron Oxides Aluminum Oxides Manganese Oxides
Accessory Minerals Andisol Hydrous Silicates and Aluminosilicates - volcanic ash
100 nm Accessory Minerals Hydrous Silicates and Aluminosilicates
Identifying Soil Minerals X-ray Diffraction
Useful (Common) Methods/Approaches Optical Microscopy Infrared Spectroscopy Thermal Analysis X-ray Diffraction (XRD) Physical Properties
X-ray Diffraction Attributes Limitations
X-ray Diffraction: Foundation Consider wave properties
X-ray Diffraction: Foundation Constructive versus Destructive Interference
X-ray Diffraction: Foundation Bragg’s Law: n l = 2d sin q • = wavelength (0.5 to 2.5 Å) d q
Octahedral Layer Only: Brucite, Mg(OH)2 Oct. z y 1:1 Mineral: Serpentine, Mg3Si2O5(OH)4 Tet. Oct. 2:1 Mineral: Talc, Mg3Si4O10(OH)2 Tet. Oct. Layer Build-up: Trioctahedral Phyllosilicate Minerals Sheets Tet.
Mineral Mg2+ K+ Glycerol 550 C Kaolinite 7 7 7 - Mica 10 10 10 10 Vermiculite 14 10 14 10 Smectite 14-16 12-14 18 10 HIV 14 14 14 10-11 HIS 14-16 14 18 10-11 XRD: Chemical Treatment d-spacing, Angstroms
Oct. Octahedral Minerals Brucite, Mg(OH)2 Gibbsite, Al(OH)3
Serpentine: Mg3Si2O5(OH)4 - trioctahedral Kaolinite: Al2Si2O5(OH)4 - dioctahedral
tet tet tet tet oct oct Micas Phlogopite: KMg3(AlSi3O10)(OH)2 Biotite: KFe3(AlSi3O10)(OH)2 Muscovite: KAl2(AlSi3O10)(OH)2 K+ K+ K+