NRES 201 Lectures 25 27 Fall 2014 Colloidal Properties NRES 201 Colloidal Properties 1 Colloidal Properties Today s lecture topics What is a colloid Particle size and surface area Importance of the clay fraction Clay minerals Organic matter 2 1 NRES 201 Lectures 25 27 Fall 2014 Colloidal Properties What is a colloid Defined as Any substance finely dispersed in a gaseous liquid or solid medium such that individual particles are not visible in an ordinary microscope do not settle out Examples Fog Smoke Milk Source http photography nationalgeographic com 3 Particle size and surface area Basic principle Subdivision increases surface area Illustration 1 cm 0 5 cm surface area 6 1 cm 1 cm 6 cm2 48 0 5 cm 0 5 cm 12 cm2 4 2 NRES 201 Lectures 25 27 Fall 2014 Colloidal Properties Importance of the clay fraction Main source of soil surface area Size fraction Sand Silt Clay Diameter mm 0 05 2 0 0 002 0 05 0 002 Surface area m2 g 0 1 0 1 1 10 800 Crucial impact on soil properties Source Brady and Weil 2004 5 Types of clay colloids Silicate colloids Crystalline clay minerals Major type in most soils Noncrystalline clay minerals Derived from volcanic ash Important in Andisols Nonsilicate colloids Hydrous oxides of Fe or Al Important in Ultisols and Oxisols Organic matter humus Most important in the A horizon 6 3 NRES 201 Lectures 25 27 Fall 2014 Colloidal Properties Fundamental aspects of clay minerals Layered structure Classified as phyllosilicates Greek phyllon leaf Occur as thin sheets Like pages in a book Source Brady and Weil 2002 7 Two types of building blocks Tetrahedral sheet Si or Al coordinated to 4 O atoms 3 of the O atoms are shared to form a sheet Source Grim 1968 8 4 NRES 201 Lectures 25 27 Fall 2014 Colloidal Properties Two types of building blocks Tetrahedral sheet Si or Al coordinated to 4 O atoms 3 of the O atoms are shared to form a sheet Octahedral sheet Al Mg or Fe coordinated to 6 O atoms or OH groups All 6 are shared to form a sheet Source Grim 1968 layer 1 octahedral sheet 1 or 2 tetrahedral sheets 9 Ionic adsorption Negative charges attract positive ions cations Source Brady and Weil 2010 10 5 NRES 201 Lectures 25 27 Fall 2014 Colloidal Properties pH dependent charge Due to ionization of external OH groups OHSi Si O H OH Al Al H2O Ionization increases with increasing pH Major source of charge for 1 1 minerals 11 Permanent charge Due to isomorphous substitution Of Al3 for Si4 in the tetrahedral sheet Of Mg2 or Fe 3 2 for Al3 in the octahedral sheet Unaffected by pH Major source of charge for 2 1 minerals Source Brady and Weil 2004 12 6 NRES 201 Lectures 25 27 Fall 2014 Colloidal Properties Types of clay minerals Kaolinite A 1 1 layer silicate Unit layer formula Al4Si4O10 OH 8 No isomorphous substitution Unit layer charge 0 Source Grim 1968 13 Types of clay minerals Kaolinite A 1 1 layer silicate Unit layer formula Al4Si4O10 OH 8 No isomorphous substitution Unit layer charge 0 Cation charge 4 Al3 4 Si4 Anion charge 10 O28 OH Net charge 12 16 20 80 14 7 NRES 201 Lectures 25 27 Fall 2014 Colloidal Properties Types of clay minerals Kaolinite A 1 1 layer silicate Unit layer formula Al4Si4O10 OH 8 No isomorphous substitution Unit layer charge 0 Non swelling due to H bonds Source Grim 1968 15 Types of clay minerals Kaolinite A 1 1 layer silicate Unit layer formula Al4Si4O10 OH 8 No isomorphous substitution Unit layer charge 0 Non swelling due to H bonds Rare in Illinois soils But common in flower pots 16 8 NRES 201 Lectures 25 27 Fall 2014 Colloidal Properties Montmorillonite smectite A 2 1 layer silicate Unit layer formula Al2 xMgx Si4O10 OH 2 nH2O Isomorphous substitution of octahedral Mg2 for Al3 Unit layer charge 0 25 0 6 Swelling Common in Illinois soils Source Grim 1968 17 Montmorillonite smectite A 2 1 layer silicate Unit layer formula Al2 xMgx Si4O10 OH 2 nH2O Isomorphous substitution of octahedral Mg2 for Al3 Unit layer charge 0 25 0 6 Swelling No interlayer H bonds with 2 1 minerals Permanent charge is in the octahedral sheet Interlayer cations are not fixed Common in Illinois soils Source Grim 1968 18 9 NRES 201 Lectures 25 27 Fall 2014 Colloidal Properties Vermiculite 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 with montmorillonite Interlayer fixation of K or NH4 Common in Illinois soils 19 Illite A 2 1 layer silicate Unit layer formula K2 x Ca Mg x Si4 yAly Al2O10 OH 2 nH2O Extensive isomorphous substitution of tetrahedral Al3 for Si4 Unit layer charge 1 Low swelling Due to interlayer fixation of K NH4 can also be fixed Common in Illinois soils Source Grim 1968 20 10 NRES 201 Lectures 25 27 Fall 2014 Colloidal Properties Chlorite A 2 1 1 layer silicate Interlayer hydroxyl sheet Unit layer formula AlMg2 OH 6 x Mg3 Si4 xAlx O10 OH 2 Extensive isomorphous substitution Of tetrahedral Al3 for Si4 Of Al3 for Mg2 in the hydroxyl sheet Unit layer charge 1 Nonswelling Common in Illinois soils particularly Alfisols Source Grim 1968 21 Soil clays versus pure clay minerals Soil clays are usually intergrades with intermediate Structure Properties Soil clays are often interstratified With vertical stacking of different minerals Source http pubs usgs gov 22 11 NRES 201 Lectures 25 27 Fall 2014 Colloidal Properties Hydrous oxides Occur as Ubiquitous coatings on clay silt and sand Crystalline and noncrystalline minerals Present in Ultisols and Oxisols But not in Illinois soils Produced by the weathering of clay minerals No isomorphous substitution Charge is entirely pH dependent OHFe Fe OH O H Al Al H2O 23 Genesis of clay minerals Source Brady and Weil 2004 24 12 NRES 201 Lectures 25 27 Fall 2014 Colloidal Properties Clay minerals and soil orders Complicating factors Profile variability in clay mineralogy caused by Different parent materials Different climatic regimes Generalizations Alfisols and Mollisols dominated by Montmorillonite Vermiculite Illite Vertisols dominated by montmorillonite Ultisols and Oxisols dominated by Kaolinite Hydrous oxides 25 Organic matter A complex mixture of organic compounds derived from plants animals and microbes Acidic HA H ACharge is entirely pH dependent Source Brady and Weil 2004 26 13 NRES 201 Lectures 25 27 Fall 2014 Colloidal Properties Colloidal Properties Today s lecture topics What is a colloid