THE ELECTRICAL PROPERTIES of IRON RICH SILICATES by DUANE
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THE ELECTRICAL PROPERTIES OF IRON RICH SILICATES by DUANE CHRISTIAN UHRI B. S., Michigan State University (1953) M. S., Michigan State University (1954) SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY February, 1961 Signature of Author ..... ............. Department of Geology and Geophysics, January 23, 1961 Certified by . ThesisL Supervisor Accepted by .......... 0..................................... Chairman, Departmental Committee on Gr duate Students ABSTRACT Title: The Electrical Properties of Iron Rich Silicates Author: Duane Christian Uhri Submitted to the Department of Geology and Geophysics on January 23, 1961 in partial fulfillment of the re- quirements for the degree of Doctor of Philosophy at Massachusetts Institute of Technology. The electrical conductivities of a number of minerals (primarily garnets, but also enstatite, hypersthene, horn- blende, olivine, and quartz) are measured in a controlled atmosphere as a function of the temperature. The design of the apparatus is applicable to the measurement of A.C. and D.C. conductivities at elevated temperatures and the atmos- phere is efficiently regulated by a wastite buffer so as to prevent alteration of the iron contents of the specimens. Although the majority of specimens measured exhibit a conductivity peak (as great as 3 orders of magnitude) or a suggestion of a peak in the plots of the logarithm of the conductivity versus the temperature, garnets are the only specimens investigated in detail. Experimentation with garnets indicates that conductivity curves obtained from heating and cooling measurements do not coincide, but a given set of curves may be reproduced exactly providing an- other sample from the same specimen is measured under iden- tical conditions. The presence of an aqueous vapor phase tends to eliminate the electrical conductivity peak and suggests that the water vapor in the laboratory air may be responsible for the absence of the conductivity peak asso- ciated with a sample run in air. Oxidation of the sample appears to increase the overall conductivity. Therefore, a buffered atmosphere is an absolute necessity in measuring rock-forming minerals containing ferrous 'iron. For those garnets run in a dry buffered atmosphere, a possible dependence of the electrical conductivity upon chemical composition is demonstrated. It appears that the lower temperature conductivities depend upon the concentrations of FeO and Fe203 , the highest temperature conductivities upon "MgO" (CaO plus MgO as equibalent MgO), and the intermediate temperature conductivities possibly upon "Mg0" and/or "MgO"/ MnO. Polarization and/or thermo-electric effects become predominant in the higher temperature regions for many of the minerals investigated, especially quartz and enstatite. How- ever, as the iron contents of the minerals increase, these iii effects become suppressed to the point of being unobservable. Corrections are attempted where polarization and thermo- electric effects occur and the accuracies of all the measure- ments are estimated to be within 10% of the actual values. The majority of the curves obtained for log (r vs l/T are quite different from those obtained by other investi- gators and this may be due to the following: 1) a dry silicate run in air produces no conductivity peak; 2) a dry silicate run in a dry buffered atmosphere yields a conductivity peak; and 3) a dry silicate run in an aqueous buffered atmosphere gives no conductivity peak. If the conductivity peaks observed in this investigation occur in nature, they may possess considerable geophysical significance. Until the presence or absence of a conductivity peak is verified for the minerals at depth, a maximum for the iron content in the upper portion of the mantle may not be deter- mined from geomagnetic field observations or magneto- telluric sounding techniques. Finally, on the basis of the magnitudes of the electri- cal parameters and the shapes of the curves for log a- vs 1/T, the following processes probably determine the electrical conductivity in the outer portion of the earth: 1) in the near surface region of the crust - ionic conductivity deter- mined by abundant pore fluids; 2) in the lower portion of the crust - impurity and intrinsic semi-conduction; 3) in the basal portion of the crust or upper mantle - intrinsic semi-conduction and ionic conduction together (peak conduc- tivity region); and 4) in the deeper mantle - intrinsic semi-conduction giving way to ionic conductivity with depth. iv ACKNOWLEDGEMENTS The author wishes to express his gratitude for the interest and counsel of Professor Harry Hughes during this investigation. Professor Hughes suggested this pro- blem which resulted in many enjoyable discussions. The advice of Professor W. H. Dennen and others of the Cabot Spectrographic Laboratory and of Professor W. H. Pinson and Mr. C. Schnetzler of the Geochemical Laboratory is gratefully acknowledged. The author wishes to express his appreciation also to Mrs. Barbara A. Uhri for her assistance in many of the chemical analyses. During his graduate studies at M.I.T., the author was supported by a generous fellowship from the Magnolia Petroleum Company (Socony - Mobil Oil Company) and is indebted to this industrial organization for their sponsorship. Finally, he gratefully acknowledges the efforts of Miss Delores Suter, who has performed all of the secretarial work in this thesis. TABLE OF CONTENTS Page Title Page Abstract ii Acknowledgements iv Table of Contents Table of Tables ix Table of Figures x Chapter I - INTRODUCTION 1 1.1 General 1 1.2 Previous Investigations 3 1.3 Purpose of the Present Investigation 14 Chapter II - REDUCTION - OXIDATION CONSIDERATIONS 16 2.1 General 16 2.2 The Stability of Fayalite Utilizing a 20 Wustite Buffer 2.3 The Stability of Solid Solutions Containing 24 Ferrous Silicates 2.4 Buffer Composition 28 Chapter III - EXPERIMENTAL APPARATUS 29 3.1 General 29 3.2 Apparatus I 29 3.21 General 29 3.22 Furnace Construction 29 3.23 Control Box 34 3.24 Other Components 35 3.3 Development of Apparatus II 35 3.4 Apparatus II 38 TABLE OF CONTENTS (Continued) Page 3.41 General 38 3.42 Furnace Construction 38 3.43 Control Box 40 3.44 Other Components 40 Chapter IV - EXPERIMENTAL PROCEDURE 41 4.1 Sample Preparation 41 4.2 Procedure with Apparatus I 41 4.3 Procedure with Apparatus II 43 Chapter V - CHEMICAL ANALYSES 44 5.1 Spectrographic 44 5.2 Wet Chemical 46 Chapter VI - GARNET SEQUENCE 54 6.1 General 54 6.2 Mineralogy 56 6.3 Experimentation 57 6.31 General 57 6.32 Comparison of Results from Apparatus I 64 and Apparatus II 6.33 Changes in Electrical Conductivity due 71 to the Presence of an Aqueous Vapor Phase 6.34 Efficiency of the Buffer in Preventing 72 Sample Alteration 6.35 Changes in Electrical Conductivity due 73 to Sample Oxidation 6.36 Dependence of the Electrical Conductivity74 upon Chemical Composition vii TABLE OF CONTENTS (Continued) Page Chapter VII - ENSTATITE - HYPERSTHENE SERIES 77 7.1 General 77 7.2 Mineralogy 78 7.3 Experimentation 79 Chapter VIII - HORNBLENDE 83 8.1 General 83 8.2 Mineralogy 83 8.3 Experimentation 84 Chapter IX - OLIVINE SERIES 86 9.1 General 86 9.2 Experimentation 86 Chapter X - Si0 2 92 10.1 General 92 10.2 Experimentation 92 Chapter XI - DISCUSSION 94 11.1 Reproducibility of Measurements 94 11.2 Differences between Heating and Cooling 96 Curves 11.3 Polarization and Thermo-electric Effects 99 11.4 Sources of Error and Accuracy of Results 101 11.5 Comparison with Other Investigations 103 11.6 Geophysical Implications 106 11.61 The Possibility of a Phase Transfor- 106 mation at the Mohorovicic Discontinuity 11.62 The Electrical Conductivity of the 111 Earth's Crust and Upper Mantle viii TABLE OF CONTENTS (Continued) Page 11.63 A Maximum for the Iron Content in 131 the Upper Portion of the Mantle 11.64 Conductivity Mechanisms in the Upper Portion of the Earth Chapter XII - CONCLUSIONS 139 Chapter XIII - FUTURE INVESTIGATIONS 142 APPENDIX A - Plots of log a- vs l/T 144 APPENDIX B - Derivations of the Equations for 189 Temperatures at Depth Biographical Note 191 Bibliography 192 ix TABLE OF TABLES Page I-1 Electrical Conductivity Constants and 5 Excitation Energies 1-2 Directional Dependence of a;' and E' 7 V-1 Chemical Analyses 48 V-2 Results Obtained by Rapid and Conventional 52 Analysis of Two Carefully Studied Rock Samples V-3 Ratios of the Oxides from Rapid Silicate 53 Analyses VI-1 Macroscopic Properties of the Garnet 58 Sequence VI-2 Experimental Conditions Existing During 59 Conductivity Measurements of the Garnet Sequence VI-3 Parameters Extracted from the Plots of Log 65 c- vs. 1/T for the Garnet Sequence VII-i Experimental Conditions Existing During 81 Conductivity Measurements of Enstatite, Hypersthene, Quartz, and Hornblende VII-2 Parameters Extracted from the Plots of 82 log o-vs. 1/T for the Enstatite, Hypersthene, Quartz, and Hornblende Specimens IX-1 Experimental Conditions Existing During 89 Conductivity Measurements of the Olivine Sequence IX-2 Parameters Extracted from the Plots of log 90 a- vs. 1/T for the Olivine Sequence XI-1 Temperature vs Depth for Models A, B, C, 116 and D A-1 Symbols for Figures A-1 through A-42 145 TABLE OF FIGURES Page II-1 The standard free energy of formation of 17 many metal oxides as a function of tempera- ture 11-2 The stability diagram for fayalite at 1 atm 17 pressure 11-3 The temperature-composition diagram for the 21 system Fe - 0 III-1 Experimental apparatus 30 111-2 Control Box 31 111-3 Furnace for Apparatus I 32 111-4 Temperature profile for the furnace of 33 Apparatus I 111-5 Furnace for Apparatus II 39 111-6 Sample holder in the furnace tube of Apparatus 39 II XI-1 Temperature vs.