NRES 201 Lectures 25-27 (Fall 2014):Colloidal Properties1NRES 201Colloidal Properties12Today’s lecture topics What is a colloid? Particle size and surface area Importance of the clay fraction Clay minerals Organic matterColloidal PropertiesNRES 201 Lectures 25-27 (Fall 2014):Colloidal Properties2What is a colloid?Defined as Any substance finely dispersed in a gaseous,liquid, or solid medium, such that individualparticles-are not visible inan ordinarymicroscope-do not settle outExamples Fog Smoke Milk3Source: http://photography.nationalgeographic.comParticle size and surface areaBasic principle Subdivision increases surface areaIllustration1 cm6  1 cm  1 cm = 6 cm20.5 cm48  0.5 cm  0.5 cm = 12 cm2surface area4NRES 201 Lectures 25-27 (Fall 2014):Colloidal Properties3Importance of the clay fractionMain source of soil surface areaSize Diameter Surface areafraction (mm) (m2/g)_______ ________ ___________Sand 0.05-2.0 < 0.1Silt 0.002-0.05 0.1-1Clay < 0.002 10-800Crucial impact on soil propertiesSource: Brady and Weil (2004)5Types of clay colloidsSilicate colloids Crystalline clay minerals-Major type in most soils Noncrystalline clay minerals-Derived from volcanic ash-Important in AndisolsNonsilicate colloids Hydrous oxides of Fe or Al-Important in Ultisols and Oxisols Organic matter (humus)-Most important in the A horizon6NRES 201 Lectures 25-27 (Fall 2014):Colloidal Properties4Fundamental aspects of clay mineralsLayered structure Classified as phyllosilicates-Greek phyllon = leaf Occur as thin sheets-Like pages in a bookSource: Brady and Weil (2002)7Two types of building blocks Tetrahedral sheet-Si or Al coordinatedto 4 O atoms-3 of the O atoms areshared to form a sheetSource: Grim (1968)8NRES 201 Lectures 25-27 (Fall 2014):Colloidal Properties5Two types of building blocks Tetrahedral sheet-Si or Al coordinatedto 4 O atoms-3 of the O atoms areshared to form a sheet Octahedral sheet-Al, Mg, or Fecoordinated to 6 Oatoms or OH groups-All 6 are shared toform a sheetlayer = 1 octahedral sheet + 1 or 2 tetrahedral sheetsSource: Grim (1968)9Ionic adsorption Negative charges attract positive ions (cations)Source: Brady and Weil (2010)10NRES 201 Lectures 25-27 (Fall 2014):Colloidal Properties6 pH-dependent charge-Due to ionization of external OH groups-Ionization increases with increasing pH-Major source of charge for 1:1 mineralsSiAlOHSiAlO-+ H+OH-H2O11 Permanent charge- Due to isomorphous substitution•Of Al3+for Si4+in the tetrahedral sheet•Of Mg2+ or Fe(3+2+)for Al3+in the octahedralsheet-Unaffected by pH-Major source ofcharge for 2:1mineralsSource: Brady and Weil (2004)12NRES 201 Lectures 25-27 (Fall 2014):Colloidal Properties7Kaolinite A 1:1 layer silicate Unit-layer formula:Al4Si4O10(OH)8 No isomorphoussubstitution  Unit-layer charge = 0Types of clay mineralsSource: Grim (1968)13Kaolinite A 1:1 layer silicate Unit-layer formula:Al4Si4O10(OH)8 No isomorphoussubstitution  Unit-layer charge = 0-Cation charge:4  Al3+= 12+4  Si4+= 16+-Anion charge:10  O2-= 20-8  OH-= 8-____-Net charge: 0Types of clay minerals14NRES 201 Lectures 25-27 (Fall 2014):Colloidal Properties8Kaolinite A 1:1 layer silicate Unit-layer formula:Al4Si4O10(OH)8 No isomorphoussubstitution  Unit-layer charge = 0 Non-swelling(due to H bonds)Types of clay mineralsSource: Grim (1968)15Kaolinite A 1:1 layer silicate Unit-layer formula:Al4Si4O10(OH)8 No isomorphoussubstitution  Unit-layer charge = 0 Non-swelling(due to H bonds) Rare in Illinois soils But common in flower potsTypes of clay minerals16NRES 201 Lectures 25-27 (Fall 2014):Colloidal Properties9Montmorillonite (smectite) A 2:1 layer silicate Unit-layer formula:(Al2-xMgx)Si4O10(OH)2•nH2O Isomorphous substitutionof octahedral Mg2+for Al3+ Unit-layer charge = 0.25-0.6 Swelling Common in Illinois soils Source: Grim (1968)17Montmorillonite (smectite) A 2:1 layer silicate Unit-layer formula:(Al2-xMgx)Si4O10(OH)2•nH2O Isomorphous substitutionof octahedral Mg2+for Al3+ Unit-layer charge = 0.25-0.6 Swelling-No interlayer H bondswith 2:1 minerals-Permanent charge isin the octahedral sheet-Interlayer cations are notfixed Common in Illinois soils Source: Grim (1968)18NRES 201 Lectures 25-27 (Fall 2014):Colloidal Properties10Vermiculite A 2:1 layer silicate Unit-layer formula:Mgn(H2O)6(Si4-xAlx)(Al2-yFey)O10(OH)2•nH2O Isomorphous substitution-Of tetrahedral Al3+for Si4+-Of octahedral Fe3+2+for Al3+ Unit-layer charge = 0.6-0.9 Limited swelling-Permanent charge is largely tetrahedral-Interlayer cations held more tightly than withmontmorillonite Interlayer fixation of K+or NH4+ Common in Illinois soils 19Illite A 2:1 layer silicate Unit-layer formula:[K2-x(Ca,Mg)x](Si4-yAly)Al2O10(OH)2•nH2O Extensive isomorphoussubstitution of tetrahedralAl3+for Si4+ Unit-layer charge = 1 Low swelling-Due to interlayer fixationof K+-NH4+can also be fixed Common in Illinois soils Source: Grim (1968)20NRES 201 Lectures 25-27 (Fall 2014):Colloidal Properties11Chlorite A 2:1:1 layer silicate Interlayer hydroxyl sheet  Unit-layer formula:[AlMg2(OH)6]x[Mg3(Si4-xAlx)O10(OH)2] Extensive isomorphoussubstitution-Of tetrahedral Al3+for Si4+-Of Al3+for Mg2+in thehydroxyl sheet Unit-layer charge = 1 Nonswelling Common in Illinois soils(particularly Alfisols) Source: Grim (1968)21 Soil clays are usuallyintergrades withintermediate-Structure-Properties Soil clays are ofteninterstratified-With vertical stackingof different minerals Soil clays versus pure clay minerals22Source: http://pubs.usgs.govNRES 201 Lectures 25-27 (Fall 2014):Colloidal Properties12 Occur as-Ubiquitous coatings on clay, silt, and sand-Crystalline and noncrystalline minerals•Present in Ultisols and Oxisols•But notin Illinois soils Produced by the weathering of clay minerals No isomorphous substitution  Charge is entirely pH-dependentHydrous oxidesFeAl-OHFeAl-O-+ H+OH-H2O23Genesis of clay mineralsSource: Brady and Weil (2004)24NRES 201 Lectures 25-27 (Fall 2014):Colloidal Properties13Complicating factors Profile variability in clay mineralogy caused by-Different parent materials-Different climatic regimesGeneralizations Alfisols and Mollisols dominated by-Montmorillonite-Vermiculite-Illite Vertisols dominated by montmorillonite Ultisols and