R.M.Cornell U. Schwertmann The Iron Oxides Structure, Properties, Reactions, Occurrence and Uses Weinheim • New York VCH Basel • Cambridge • Tokyo Contents 1 Introduction to the iron oxides 1 2 Crystal structure 7 2.1 General 7 2.2 Iron oxide structures 7 2.2.1 Close packing of anion layers 10 2.2.2 Linkages of octahedra or tetrahedra 12 2.3 Structures of the individual iron oxides 14 2.3.1 The oxide hydroxides 14 2.3.1.1 Goethite a-FeOOH 14 2.3.1.2 Lepidocrocite y-FeO(OH) 16 2.3.1.3 Akaganeite ß-FeO(OH) and schwertmannite Fe16016(OH)y(S04)z • n H20 18 2.3.1.4 5-FeOOH and 8'-FeOOH (feroxyhyte) 20 2.3.1.5 High pressure FeOOH 21 2.3.1.6 Ferrihydrite 22 2.3.2 The hydroxides 24 2.3.2.1 Bernalite Fe(OH)3 • nH20 24 2.3.2.2 Fe(OH)2 25 2.3.2.3 Green rusts 25 2.3.3 The oxides 26 2.3.3.1 Haematite a-Fe203 26 2.3.3.2 Magnetite Fe304 28 2.3.3.3 Maghemite y-Fe203 30 2.3.3.4 Wüstite Fe^O 31 2.4 The Fe-Ti oxide System 33 3 Cation Substitution 35 3.1 General 35 3.2 Goethite 38 3.2.1 AI Substitution 38 3.2.1.1 Synthetic goethites 38 3.2.1.2 Natural goethites 43 3.2.2 Other substituting cations 43 3.3 Haematite 48 3.4 Other Fe oxides 50 VIII Contents 4 Crystal morphology and size 53 4.1 General 53 4.1.1 Crystal growth 53 4.1.2 Crystal morphology 55 4.1.3 Crystal size 57 4.2 The iron oxides 58 4.2.1 Goethite 59 4.2.1.1 General 59 4.2.1.2 Domainic character 64 4.2.1.3 Twinning 66 4.2.1.4 Effect of additives on goethite morphology 68 4.2.2 Lepidocrocite 70 4.2.3 Akaganeite and schwertmannite 71 4.2.4 Ferrihydrite 73 4.2.5 Haematite 74 4.2.6 Magnetite 82 4.2.7 Maghemite 85 4.2.8 Other iron oxides 85 5 Surface area and porosity 87 5.1 Surface area 87 5.2 Porosity 91 5.3 Surface roughness and fractality 93 5.4 The iron oxides 94 5.4.1 Goethite 95 5.4.2 Lepidocrocite 97 5.4.3 Akaganeite and schwertmannite 97 5.4.4 5-FeOOH and feroxyhyte 98 5.4.5 Ferrihydrite 99 5.4.6 Haematite 101 5.4.7 Magnetite 102 5.4.8 Maghemite 102 6 Electronic, electrical and magnetic properties 103 6.1 Electronic properties 103 6.1.1 Free Fe3+ and Fe2+ ions 103 6.1.2 Bound Fe ions 104 6.1.3 Molecular orbital description of bonding in iron oxides 107 6.2 Electrical properties 108 6.2.1 Semiconductor properties of iron oxides 110 6.3 Magnetic properties 110 6.3.1 Basic definitions 112 6.3.2 Types of magnetism 113 6.3.3 Magnetic behaviour of iron oxides 115 6.3.4 The different iron oxides 118 6.3.4.1 Goethite 118 6.3.4.2 Lepidocrocite 119 Contents IX 6.3.4.3 Akaganeite 119 6.3.4.4 8-FeOOH and feroxyhyte 119 6.3.4.5 High pressure FeOOH 120 6.3.4.6 Ferrihydrite 120 6.3.4.7 Haematite 120 6.3.4.8 Magnetite 123 6.3.4.9 Maghemite 123 6.3.4.10 e-Fe203 124 6.3.4.11 Fe(OH)2 124 6.3.4.12 Wüstite 125 7 Characterization 127 7.1 Introduction 127 7.2 Infrared spectroscopy 128 7.2.1 Goethite 129 7.2.2 Lepidocrocite 132 7.2.3 Ferrihydrite 132 7.2.4 Haematite 133 7.2.5 Other iron oxides 134 7.3 Raman spectroscopy 135 7.4 Ultraviolet-visible spectroscopy 136 7.4.1 UV-Vis spectra 138 7.4.2 Colour and optical properties 141 7.4.2.1 Colour 144 7.4.2.2 Pigment properties 146 7.5 Mössbauer spectroscopy and magnetometry 146 7.5.1 Mössbauer spectra and parameters 146 7.5.1.1 Goethite 148 7.5.1.2 Lepidocrocite 148 7.5.1.3 Ferrihydrite 148 7.5.1.4 Haematite 151 7.5.1.5 Magnetite and maghemite 152 7.5.1.6 Other iron oxides 153 7.5.2 Magnetometry 154 7.5.2.1 General 154 7.5.2.2 Initial magnetic susceptibility 155 7.5.2.3 Magnetic anisotropy, coercivity and Saturation magnetization .... 155 7.5.2.4 Domain type 158 7.5.2.5 Curie temperature analysis 161 7.5.2.6 Applications 161 7.6 Other spectroscopic techniques 162 7.7 Diffraction techniques 165 7.7.1 X-ray diffraction 165 7.7.2 Other diffraction techniques 168 7.8 Microscopy 170 7.9 Thermoanalysis 171 7.9.1 Thermal gravimetric analysis (TGA) 171 X Contents 7.9.2 Differential thermal analysis (DTA) 171 7.10 Dissolution methods 173 8 Thermodynamics of the Fe-02-H20 system 175 8.1 General 175 8.2 Standard free energy of reaction and the equilibrium constant . 176 8.3 Redox reactions 178 8.4 Effect of complexing agents on redox potential 180 8.5 Stability 181 8.5.1 "Bulk" crystals 181 8.5.2 Effect of particle size and AI Substitution 187 9 Solubility 189 9.1 General 189 9.2 The solubility product 189 9.3 The effect of hydrolysis reactions and pH on solubility 190 9.4 Other factors influencing solubility and the solubility product .... 195 9.4.1 Complexation 196 9.4.2 Redox reactions 196 9.4.3 Ionic strength 198 9.4.4 Properties of the solid 199 9.4.4.1 Particle size 199 9.4.4.2 Ageing and isomorphous Substitution 201 9.5 Methods of determining or calculating the solubility products .... 201 9.6 Solubility products 203 10 Surface chemistry and colloidal stability 207 10.1 Surface functional groups 207 10.2 Surface acidity and acidity constants 213 10.3 The electrical double layer 216 10.4 Electrochemical properties 218 10.5 Surface charge and point of zero charge 221 10.6 Stability of colloidal suspensions 226 10.6.1 General 226 10.6.2 Stability of iron oxide suspensions 229 11 Adsorption of ions and molecules 235 11.1 General 235 11.2 Treatment of adsorption data 236 11.2.1 The Langmuir, Freundlich and Temkin isotherm equations 236 11.2.2 Surface complexation modeis 236 11.3 Anion adsorption 240 11.3.1 Modes of coordination 246 11.3.2 Selected inorganic anions 247 11.3.3 Organic anions and other organic Compounds 251 11.4 Cation adsorption 254 11.5 Ternary adsorption 261 Contents XI 11.6 Adsorption of gases 263 11.7 Photochemical reactions 264 12 Dissolution 267 12.1 Introduction 267 12.2 Dissolution reactions and mechanisms 268 12.2.1 Principles 268 12.2.2 Protonation 270 12.2.3 Complexation 272 12.2.4 Reduction 276 12.2.4.1 General 276 12.2.4.2 Examples of reductants 277 12.2.4.3 Photochemical reduction 281 12.2.4.4 Biological and other reduction reactions in natural environments . 285 12.2.5 Comparison of the three different types of dissolution reactions . 286 12.3 Dissolution modeis 287 12.4 Individual iron oxides 291 12.4.1 Goethite 291 12.4.1.1 Unsubstituted goethite 291 12.4.1.2 Substituted goethite 297 12.4.1.3 Natural goethite and haematite 300 12.4.2 Lepidocrocite and akaganeite 301 12.4.3 Ferrihydrite 303 12.4.4 Haematite 303 12.4.5 Magnetite and maghemite 304 12.4.6 Comparison of different oxides 306 13 Formation 313 13.1 General 313 13.2 Formation in Fe111 Systems 314 13.2.1 Hydrolysis reactions 314 13.2.2 Polynuclear species 317 13.2.3 Crystalline products formed in acid media 319 13.2.4 Mechanisms of formation 320 13.2.5 Transformation of ferrihydrite in neutral to alkaline media 323 13.2.6 Factors affecting the goethite-to-haematite ratio 327 13.2.7 Effect of foreign Compounds 329 13.2.7.1 General 329 13.2.7.2 Anions and neutral molecules in acid media 331 13.2.7.3 Anions and neutral molecules in alkaline media 332 13.2.7.4 Cations 335 13.3 Formation in aqueous Fe11 Systems 339 13.3.1 General 339 13.3.2 Effect of pH 340 13.3.3 Effect of oxidation rate 343 13.3.4 Effect of foreign Compounds 344 13.4 Decomposition of Fe complexes 347 XII Contents 14 Transformations 349 14.1 Introduction 349 14.2 Thermal transformations 351 14.2.1 Dehydration and dehydroxylation 351 14.2.1.1 General 351 14.2.1.2 Goethite 352 14.2.1.3 Lepidocrocite 356 14.2.1.4 Akaganeite and schwertmannite 359 14.2.1.5 8-FeOOH and feroxyhyte 360 14.2.1.6 Ferrihydrite 361 14.2.2 Isochemical transformations 363 14.2.2.1 Maghemite to haematite 363 14.3 Via-solution transformations 364 14.3.1 Lepidocrocite to goethite/haematite 364 14.3.2 Akaganeite to goethite/haematite 365 14.3.3 Schwertmannite to goethite 367 14.3.4 Maghemite and goethite to haematite 368 14.4 Oxidative and reductive transformations 368 14.4.1 Oxidation of magnetite to maghemite or haematite 368 14.4.2 Reduction of Fe111 oxides and oxyhydroxides to magnetite 370 14.4.3 Reduction of iron ores to iron 372 14.5 Interaction of iron oxides with other metal oxides and carbonates 373 15 Rocks and ores 375 15.1 Introduction 375 15.2 Magmatic and metamorphic rocks and ores 376 15.3 Sediments and sedimentary rocks 378 15.3.1 Red beds 379 15.3.2 Sedimentary iron ores 381 15.3.3 Other Sediments 385 15.3.4 Ferricretes and bauxites 386 15.4.