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Electrum Tarnish by Sulfur

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Electrum Tarnish by Sulfur ELECTRUM TARNISH BY SULFUR: A STUDY OF AN IRREVERSIBLE PROCESS. BY DONALD MICHAEL FRANCIS B.Sc. McGill University, 1968 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE In the Department of Geology We accept this thesis as conforming to the required standard: THE UNIVERSITY OF BRITISH COLUMBIA April 1971 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department The University of British Columbia Vancouver 8, Canada Date ABSTRACT The tarnishing of electrum (40 atom % Ag) at 750°C and a sulfur fugacity of -2.8 to -2.9 1og1Q atmospheres is heterogenous and irreversible. This is the case even though these conditions are less than 37°C and .4 log-jQ atmospheres above the point at which tarnish is first observed on the electrum. The tarnish forms initially on the grain boundaries and lattice defects of the electrum. With time, the interiors of the electrum grains become completely masked by tarnish except for narrow untarnished borders adjacent to the grain boundaries. Microprobe analysis indicates that the untarnished borders are depleted in silver and thus cannot tarnish at 750°C. The conclusion is drawn that the tarnish forms as an external layer which destroys the original composition of the adjacent electrum by preferentially removing silver. The silver depletion near the grain boundaries is thought to be caused by rapid diffusion of silver along grain boundaries to the electrum-tarnish interface.. i. CONTENTS I. INTRODUCTION 1 II. THE ELECTRUM-TARNISH METHOD FOR THE MEASUREMENT OF SULFUR FUGACITIES 1 III THE METHOD OF STUDY 5 IV. OBSERVATIONS AND RESULTS 6 A. MICROSCOPIC EXAMINATION OF TARNISHED ELECTRUMS 6 B. ELECTRON MICROPROBE ANALYSIS 16 i. Analysis of Tarnished Regions .. .. 16 ii. Untarnished Borders of Electrum Grains 22 iii. Analysis of Pyrrhotite 22 V. DISCUSSION .... 25 A. THEORETICAL CONSIDERATIONS OF UNTARNISHING 25 B. A MODEL FOR THE TARNISHING OF ELECTRUM .. 27 i. Nucleation of the Sulfide Tarnish .. .. 27 ii. Growth of the Sulfide Tarnish .. 28 C. UNTARNISHED BORDERS .. .. .. .. .. .. .. ..30 D. PSEUDO-GRID NATURE OF THE EARLY TARNISH ON THE INTERIORS OF ELECTRUM GRAINS .. 31 E. EFFECTS OF PRE-TARNISH TREATMENT 32 VI. CONCLUSIONS 33 VII. REFERENCES .. 35 ii • VIII. APPENDIX .. 37 A. RUN DATA ON PYRRHOTITE USED IN THE STUDY 38 B. ELECTRON MICROPROBE ANALYSIS OF PYRRHOTITE 40 C. ELECTRON MICROPROBE ANALYSIS OF TARNISHED ELECTRUM GRAINS 41 / iii. LIST OF ILLUSTRATIONS i. FIGURE 1 Electrum Tarnish Curves 3 ii. PLATE I A tarnished Electrum Grain Boundary on the El ectrum used in Run 17 9 iii PLATE II A Reflected Light Image of the Electrum used in Run 29 11 iv. PLATE III A reflected Light Image of the Electrum used in Run 30 11 v. PLATE IV A reflected Light Image of the Electrum used in Run 31 .. 13 vi. PLATE V A Reflected Light Image of the Electrum in Plate IV at Higher Magnification .. .. ..13 vii. PLATE VI A Tarnished Electrum Grain from the Electrum used in Run 33 .. 15 viii. FIGURE 2 Variation of Ag/Au Across Tarnished Electrum Grain (Run 30) .. .18 ix. FIGURE 3 Variation of Ag/Au Across Tarnished Electrum Grain (Run 31) 19 x. FIGURE 4 Variation of Ag/Au Across Tarnished Electrum Grain (Run 33) 20 xi FIGURE 5 Variation of Ag/Au Across Tarnished Electrum Grain (Run 32) 21 iv. xi. FIGURE 6 Composition of the Untarnished Borders (Run 31) 23 xii. FIGURE 7 Composition of the Untarnished Borders (Run 33) .. .. ..24 xiii. FIGURE 8 The System Gold-Silver-Sulfur 26 ACKNOWLEDGMENTS Thanks must go to Professors H. J. Greenwood, E. P. Meagher and J. A. Gower who have contributed much in the way of advice and criticism to this study. The author is also indebted to J. Harakal and A. Lacis for the technical aid that they willingly rendered. Financial support for this work was supplied in part by two National Research Council Bursaries for the periods 1969-70 and 1970-71. / 1. I. INTRODUCTION Attempts by the author to measure sulfur fugacities over natural pyrrhotite using the Electrum-Tarnish Method (Barton and Toulmin, 1964) failed because none of the achieved tarnishes could be untarnished. Although the Electrum-Tarnish Method has been used successfully by its innovators, there is no report of an investigation of the kinetics and mechanism of the tarnishing process. This study is an attempt to analyse the tarnishing process and to discover why reversibility could not be achieved. II THE ELECTRUM-TARNISH METHOD FOR THE MEASUREMENT OF SULFUR FUGACITIES The basis for the Electrum-Tarnish Method is the univariant relationship between sulfur fugacity and temperature for the assemblage: silver, silver sulfide (Ag2S tarnish) and sulfur vapour. Diluting the silver with gold reduces the activity of the silver. Therefore at any given temperature, a higher sulfur fugacity will be required to tarnish a gold-silver alloy (electrum) than will be required to tarnish pure silver. As a result, any electrum of a specified composition is characterised by a unique log sulfur fugacity versus temperature curve of tarnishing. This relationship is complicated by the fact that a significant amount of gold can enter the tarnish as Au?S. This amount 2. 1 FIGURE I The curves for the electrum indicators were calculated from data published by Barton and Toulmin (1964). The curve for the pyrrhotite buffer used in these experiments was drawn from experimental data. (Appendix: Runs 17, 20, 21 and 24.) 3. ELECTRUM TARNISH NA G .1 L CURVES 0 G S -21 y y\ -35 S y y U y L y y y y F y -41 y y ^ 1 U y R y / y F -6 U G A y C -8 I // / / T ELECTRUM Y -10 INDICATORS . PYRRHOTITE / BUFFER -12 // / / 200 400 600 800 1000 TEMPERATURE FIGURE 1 is fixed by the composition of the electrum involved; with the ratio of silver to gold of the tarnish always being greater than that of the coexisting electrum. Figure I shows the tarnishing curves for a variety of electrum compositions calculated from data given by Barton and Toulmin (1964). A pyrrhotite is stable at a given temperature only at a specified sulfur fugacity. Thus any pyrrhotite of a specified composition is characterised by a unique, univariant temperature versus sulfur fugacity curve. The initial objective of this study was to determine points on the log sulfur fugacity versus temperature curve of a natural pyrrhotite using electrum indicators. For a measurement, pyrrhotite and an electrum of specified composition are heated in an evacuated silica glass tube. A sulfur fugacity arises which is characteristic of the temperature and the composition of the pyrrhotite. At any temperature the slope of the tarnishing curve for the electrum is less (Figure I) than the slope of the log fugacity versus temperature curve for the pyrrhotite. Thus by increasing the temperature of the assemblage a point will be reached at which the respective curves for the pyrrhotite and the electrum intersect. At this temperature, a tarnish should form on the surface of the electrum. A measure of the sulfur fugacity at this temperature can be obtained from the calculated tarnishing curve of the electrum. Performing this operation with a variety of electrum compositions gives a series of points which define the log sulfur fugacity versus temperature curve of the pyrrhotite. 5. For practical and theoretical purposes complete reversibility of the tarnishing process is required. This is necessary not only to pinpoint the temperature at which tarnishing occurs by a series of telescoping temperature brackets, but also to establish that the initial tarnishing could have been a stable reaction. Equilibrium requires that the tarnish have a silver-gold ratio that is stable with the coexisting electrum. In addition, the tarnish layer must be thin enough that continuous equilibration can take place between the electrum, tarnish and sulfur atmosphere. If the ratio of silver to gold of the tarnish becomes greater than the ratio which is specified by the composition of the coexisting electrum for equil• ibrium conditions; then a lower temperature (and thus sulfur fugacity) would be required to untarnish the electrum than was required to tarnish the electrum. This would cause the tarnishing process to be irreversible. In the experiments reported here, no electrum once tarnished could be untarnished. III. THE METHOD OF STUDY A special method of preparing the electrum was used to facilitate later microprobe and reflected light study. Barton and Toulmin's (1964) procedure involved the use of electrum chips simply cut off a stock rod. In the experiments reported here, the electrum chips were flattened to round disks of a thickness slightly less than one millimeter in a cold press. One surface of each disk was polished with tin oxide abrasive. The natural pyrrhotite used in this study was obtained from the 6. Bluebell Mine in southern British Columbia. The °f this pyrrhotite was determined to be 2.0666 ± .0002 A° on a Philips Diffractometer using a Ni filter with Cu Ko<radiation. Three oscillations were made at 1/8 of a degree 29 per minute using potassium bromide as an internal standard. Combining this with data given by Barton and Toulmin (1964) the composition of the pyrrhotite was calculated to be .4736 ± .0002 atom fraction iron.
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