M121 GEOLOGICAL and MICROBIOLOGICAL FORMATION
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m121 GEOLOGICAL AND MICROBIOLOGICAL FORMATION OF IRON SULPHIDES A Thesis submitted for the degree of Doctor of Philosophy of the University of London by DAVID TERENCE RICKARD Imperial College, London. May, 1968. 2. ABSTRACT An experimental investigation into the chemistry of the iron sulphides in terms of the chemistry of iron and sulphur in aqueous solution demonstrates that the six mineral iron sulphides, mackinawite, emythite, griegite, pyrite, marcasite and pyilhotite, form through a series of distinct reacticns in which factors such as pH, Eh and oxygen activity only play indirect roles. A study of the chemical kinetics of the sulphidation of goethite, which is shwan to be the major source of reactive iron in the natural aqueous environment, demonstrates that the sulphidation mechanism includes the formation of a transitory intaniledlary, ferric sulphide, which causes a structural discontinuity between the reaction product and reactant. Ferric sulphide is shown for the first time tc be a discrete chemical ccnpouna. The production of hydrogen sulphide through the activities,of the sulfeate-reducing bacteria is the main source of this compc-Ind in ancient and modern natural aqueous environments. However although the production of iron sulphides through the reaction of goethite with this biogenic hyarogen sulphide is essential to the biochemistry and geochemistry of the hydrosphere, - n experimental investigation demonstrates that there microorganisms have no effect on the nature cf the iron sulphide produced- 3. The low temperature experimental results are valid for any environment where liquid water is present and where the non- equilibrium iron sulphides may be formed, and these results maybe used to interpret certain aspects of the natural occurrence of iron sulphides in mineral deposits. Furthermore, this chemical approach to the formation of iron sulphides leads to a new interpretation of the nature of ore-forming solutions and the mechanisms of ore-forming processes. 4. TABLE OF CONTENTS Page INTRODUCTION 7 LIST OF SYMBOLS 10 ACKNO ms JiimiTTS 12 PART ONE CHEMISTRY 14 CHAPTER 1 THE CHEMISTRY OF IRON IN AQUEOUS SOLUTION 15 A. Soluble iron 15 B. The oxides, hydroxides and oxyhydroxides of iron and their interrelationships 19 C. Carbonates and silicates of iron 29 D. Other iron species and complexing 31 B. Detrital iron-bearing minerals 34 F. Evidence from the natural aqueous environ- ment 34 SUM= 3E CHAPEIR 2 THE CHEMISTRY OF SULPHUR IN AQUEOUS SOLU!PTON 37 A. Theoretical examination of the sulphur- oxygen-water system 37 B. Factors affecting the forms of sulphur in the natural environment 47 SUMMARY 52 CHAPTER 3 T77 CHEMISTRY OF IRON SULPHIDES 54 A. The iron sulphide minerals 55 B. Ferric sulphide: preparation and properties64 5. Page C. Mechanism of the sulphidation of roethite 78 D. Chemistry and conditions of formation of the iron sulphides 90 SUMMARY 117 PART TWO MICROBIOLOGY 120 Malta( 4 'IBE SULPHUR CYCLE IN bhDIMENTS 121 A. The biogeochemical sulphur cycle in sediments 125 B. The aulfUretum 135 C. The distribution of sulfureta in the natural aqueous environment 139 D. The sulphur isotope cycle 142 SUMMARY 146 CHAPTER 5 MICROBIOLOGICAL SULPHATE REDUCTION 150 A. Classification of the dissimilatcry sulphate- reducing bacteria 150 B. The mechanism of the bacterial reduction of sulphate 155 C. The distribution of mterobiological sulphate- reduction in the past 158 SUMMARY 161 CHAPTta 6 THE MICROBIOLOGICAL FORMATION OF IRON alfJPNTDES162 A. Experimental investigation 162 SMEARY 173 PART THREE GEOLOGY CHAPTER 7 THE GEOLOGICAL SIGNIFICANCE OF EXPERIMENTAL RESULTS 175 A. Extent of the validity of the experimentation 175 B. The distribution, chemistry and forms of iron sulphides in modern sediments 187 C. The distribution, chemistry and forms of iron sulphides in ancient rocks 196 SUMMAAY 202 CHAPTA1 8 MECHANISMS INVOLVED IN Mill FORMATION OF IRON SULPHIDE DEPOSITS 204 A. The four elements of the mechanism of the formation of mineral deposits 205 B. A discussion of the hypotheses for the for- mation of iron sulphide deposits 218 SUMMARY 229 CHAPTER 9 ¢ TS OF METAMORPHISM AND DIAGENESIS 231 A. Processes involved in metamorphism and diagenesis 232 B. Diffusion and deformation 235 C. Effects of metamorphism and diagenesis on iron sulphides 237 SUMMARY 244 SUMMARY AND CONCLUSIONS 245 APPENDIX 1. CALCULATED X-RAY DATA FOR "GREEN RUST 2" 257 APPENDIX 2. X-RAY DATA FOR THE UNKNOWN SYNTHETIC SODIUM FERROUS SULPHIDE 260 APPENDIX 3. X-RAY DATA FOR THE SYNTHETIC IRON SULPHIDES, MACK1NAWITE, GRIEGITE, SMYTHITE AND PYRITE 261 REFERENCES 265 7. INTRODUCTION One of the peculiarities of the present day natural aqueous environment is the preponderance of life. These biological systems effect large scale transformation and distribution of a number of elements, especially carbon, nitrogen, oxygen, phosphorus and sulphur. Of all the organisms of the biosphere, microorganisms are geochemically the most important since they effect a number of reactions which are unknown among metazoan life-forms. One particular reaction which is exclusive to microorganisms is V-- reduction of sulphate to sulphide by certain bacteria. This reaction is of extreme importance in the hydrosphere since it not only provides large amounts of a potent reducing agent, hydrogen sulphide, for an environment where such agents are scarce, but also produces the sulphide radicle for the formation of synsedimentary iron sulphides. Thus in any study which is concerned with the formation of iron sulphides, these bacteria must be considered. The results of an investigation into the effects of microorganisms on the formation, distribution and nature of iron sulphides in the natural environment forms Part 2 of this Thesis. Until the last decade only three iron sulphides were known, pyrite, marcasite and pyrrhotite, although others were suspected. Recently, three more h-- been discovered, griegite, smythite and mackinawite, but very little is as yet known about'. their distribution in the natural environment. Therefore at the present time, six compounds 8. consisting exclusively of iron and sulphur are known with a variety of compositions and an even greater variety of structures. Throughout the past century and a half, a great deal of work has been done on the iron-sulphur system. However, the majOr purpose of this work, particularly at low temperatures, has not been to examine the chemistry of the iron sulphides but only to synthesise minerals. There is a distinct difference in these approaches even though, as written, it looks like a subtlety of semantics. In a strictly chemically-motivated approach, the question asked is always why should one sulphide form under any particular set of conditions as against any other. Experience with other chemical systems has shown that the assumption that there must be a reason for this is valid, and that chemically, reactions do not occur randomly. The reason for the formation of any particular sulphide can only be understood in terms of the chemistry of the iron and sulphur it contains. Such variables as pH, Eh, pressure and temperature can only affect the iron sulphide inasmuchas they affect the chemistry of the iron and sulphur radicles. Experimental synthesis in itself has no necessarily direct relation- ship with the formation of these phases in the natural environment. For example, the abundance of the mineral pyrite in the natural environment indicates that it can be formed under a variety of conditions. Therefore the synthesis of pyrite in a particular experiment does not mean that pyrite will be formed in this way in the natural environment. It is necessary to discover those factors that 9. are essential to the formation of the iron sulphide and to distinguish these from other inessential or spurious factors. And it is only possible to do this in terms of the iron and sulphur species that are the essential elements of an iron sulphide. Once this is established it is possible to predict the conditions necessary for the formation of the sulphide in any environment. Such a project has never been attempted before, and therefore, before investigating the effect of microorganisms on iron sulphides, it is necessary to establish the chemistry of iron sulphides. The chemistry and conditions of formation of the six mineral iron sulphides, described in terms of the chemistry of iron and sulphur in aqueous solutions, is the subject of Part 1 of this thesis. Part 2 then considers the effect of microorganisms on the formation, distribution and nature of these iron sulphides, and Part 3 demonstrates how the results of the chemical and microbiological investigation may lead to a completely new interpretation of phenomena related to the geological occurrence and formation of these minerals. 10. LIST OF SYMBOLS angstrom units. ao , bo co unit cell parameters. atm atmospheric pressure. degrees Centigrade. c, aq, g suffixes indicating the crystalline, aqueous or gaseous state of a substance. cm centimetre. 834s the variation in 34S concentration relative to the 34S/32S ratio of the Canon Diablo meteoritic troilite. electron E potential of an electrode or cell, in volts. o E standard potential of an electrode or cell, in volts. Eh potential of an electrode or cell relative to the hydrogen half—cell. F the Faraday. 6fF° difference in free energy of products and reactants at 25°C. aro standard free energy of formation of a substance at 25°C g gram 614 width of half the signal height / 2 HV Vickers indentation microhardness in kg mm . equilibrium constant. kg kilogram. rw equilibrium constant for the dissociatierr of water. kcal. kilocalorie, 11. 1 litre mM millimolar M molar mg. milligram ml millilitre mV millivolt min. minute mmM milliatcromolar n /limber of electrons taking part in a redox reaction. nm nanometer Tu normal pH log of reciprocal of hydrogen ion concentration ppm part per million poi pcund per square inch product of the activities of the products divided by the product of the activities of the reactants.