ABIOTIC CONTROLS ON COPPER ISOTOPE FRACTIONATION DURING THE DISSOLUTION OF COPPER SULFIDE MINERALS

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ABIOTIC CONTROLS ON COPPER ISOTOPE FRACTIONATION DURING THE DISSOLUTION OF COPPER SULFIDE MINERALS

ABSTRACT

Stable isotope measurements have long been used as a geochemical tool in the Earth sciences, and recent advances in analytical techniques have added intermediate mass stable isotopes (e.g. Cu, Zn, Fe, Ni) to this suite of interpretive methods. The Cu isotope system offers particularly high potential to solve geologic problems due to its large natural isotopic variation (~12 ‰).  However, the factors that control the fractionation of Cu isotopes, especially during the dissolution of Cu-sulfide minerals, remain incompletely resolved.

In this dissertation, I explore abiotic controls on Cu isotope fractionation during the dissolution of Cu-sulfide minerals by combining in situ time-resolved X-ray diffraction (TRXRD) coupled with stable isotope analysis.  As a foundational part of this study, I modified a preexisting design for a TR-XRD flow-through cell in order to remove any metal content and to allow for automated sampling of the eluate fluid.  The resulting device (described in Chapter 1) allowed us to correlate Cu isotope fractionation with changes in crystal structure during Cusulfide dissolution.

TR-XRD analyses of oxidative dissolution of chalcocite (Cu2S) and bornite (Cu5FeS4) enabled the development of rate equations that describe these reactions and the identification of the reaction sequences as Cu leached from the solid phase (Chapter 2).  During chalcocite dissolution, XRD analysis revealed mineral transformations involving the following phases: djurleite (Cu1.94S), roxbyite (Cu1.75S), yarrowite (Cu1.13S), and covellite (CuS).  Similarly, the dissolution of bornite by ferric sulfate solutions also produced changes to the mineral structure: a contraction of the bornite unit-cell volume as “non-stoichiometric bornite” formed.  XRD results demonstrated that the structure of non-stoichiometric bornite is similar to mooihoekite (Cu2.25Fe2.25S4). These results clarified the reaction sequences that occur when ferric sulfate solutions dissolve chalcocite and bornite.

By combining time-resolved diffraction data of these Cu sulfide dissolution reactions with real-time sampling and isotopic analysis of the eluates, we were able to discern structural controls on Cu isotope fractionation during dissolution.  As described in Chapter 3, during the initial stages of bornite oxidative dissolution by ferric sulfate (<5 mol% of total Cu leached), dissolved Cu was enriched in isotopically heavy Cu (65Cu) relative to the solid, with an average apparent isotope fractionation (aq-min = δ65Cuaq –  δ65Cumino, where δ65Cuaq  is the isotope composition of the leached Cu and δ65Cumino is the isotope composition of the beginning mineral powder) of  2.20 ± 0.25‰ (Chapter 3).  When >20 mol% Cu was leached from the solid, the difference between the Cu isotope composition of the aqueous and mineral phases approached zero, with aq-mino values ranging from -0.21 ± 0.61‰ to 0.92 ± 0.25‰. We propose that the decrease in the apparent isotope fractionation as the reaction progressed resulted from distillation of isotopically heavy Cu (65Cu) during dissolution or isotope effects associated with the formation of a leached layer on the surfaces of bornite particles.

Similarly, during the initial stages of chalcocite oxidative dissolution (Chapter 4), leached fluids were enriched in heavy Cu (65Cu) with δ65Cu values of ~3‰.  As the dissolution reaction progressed and chalcocite transformed to covellite, the leached Cu became isotopically lighter and δ65Cu values of the leachate decreased to as low as -3.01‰.  Isotope box models are consistent with two isotope effects that influence the degree of fractionation observed during the reaction: one due to oxidation ( ~ 1.003) and another due to changes in bonding during mineral transformations ( ~ 1.001). These results may be useful in interpreting the extent of weathering in Cu ore bodies and the potential for Cu release from acid mine drainage environments.

 

TABLE OF CONTENTS

LIST OF FIGURES………………………………………………………………………………………………. vii

LIST OF TABLES………………………………………………………………………………………………… xii

PREFACE………………………………………………………………………………………………………….. xiii

ACKNOWLEDGEMENTS…………………………………………………………………………………….. xv

 

Chapter 1  A flow-through reaction cell that couples time-resolved X-ray diffraction with

stable isotope analysis…………………………………………………………………………………………1

1.0 Abstract……………………………………………………………………………………………………….1

1.1 Introduction………………………………………………………………………………………………….1

1.2 Experimental Setup……………………………………………………………………………………….3

1.3 Application…………………………………………………………………………………………………..6

1.4 References……………………………………………………………………………………………………8

Chapter 2  Investigating the dissolution of chalcocite and bornite by ferric sulfate using

time-resolved X-ray diffraction…………………………………………………………………………….11

2.0 Abstract……………………………………………………………………………………………………….11

2.1 Introduction………………………………………………………………………………………………….12

2.2 Methods……………………………………………………………………………………………………….14

2.2.1 Material ……………………………………………………………………………………………..14 2.2.2  X-ray diffraction flow-through reaction cells………………………………………….15

2.2.3 Chemical analysis………………………………………………………………………………..16

2.2.4  Crystallographic analysis …………………………………………………………………….16

2.3 Results…………………………………………………………………………………………………………17

2.3.1 Chalcocite leach experiments………………………………………………………………..17

2.3.2 Bornite leach experiments…………………………………………………………………….18

2.4 Discussion……………………………………………………………………………………………………19

2.4.1 Kinetic analysis…………………………………………………………………………………..19

2.4.2 Mineralogical Analysis ………………………………………………………………………..26

2.5  Conclusions…………………………………………………………………………………………………28

2.6  References…………………………………………………………………………………………………..29

Chapter 3  Cu isotope fractionation during bornite dissolution:   An in situ X-ray

diffraction analysis……………………………………………………………………………………………..55

3.0 Abstract……………………………………………………………………………………………………….55

3.1 Introduction………………………………………………………………………………………………….56

3.2 Methods……………………………………………………………………………………………………….59

3.2.1  X-ray diffraction flow-through reaction cells………………………………………….59

3.2.2  Crystallographic analysis …………………………………………………………………….60

3.2.3  Cu isotope analysis……………………………………………………………………………..60

3.3 Results…………………………………………………………………………………………………………62

3.3.1  Chemistry………………………………………………………………………………………….62 3.3.2  Cu Isotope fractionation………………………………………………………………………63

3.3.3 Combined XRD and isotope analysis……………………………………………………..63

3.4 Discussion……………………………………………………………………………………………………64

3.5 Conclusion …………………………………………………………………………………………………..70

3.6 References……………………………………………………………………………………………………71

Chapter 4  Cu isotope fractionation during chalcocite dissolution:  An in situ X-ray

diffraction analysis……………………………………………………………………………………………..83

4.0 Abstract……………………………………………………………………………………………………….83

4.1 Introduction………………………………………………………………………………………………….84

4.2 Methods……………………………………………………………………………………………………….88

4.2.1 Materials…………………………………………………………………………………………….88 4.2.2  X-ray diffraction flow-through reaction cells………………………………………….88 4.2.3  Cu isotope analysis……………………………………………………………………………..89

4.2.4 Scanning Electron Microscopy ……………………………………………………………..91

4.2.5  Density functional theory and Bader calculations……………………………………92

4.3 Results…………………………………………………………………………………………………………93

4.3.1  Dissolution experiments: Cu concentrations…………………………………………..93 4.3.2  Cu isotope results……………………………………………………………………………….93 4.3.3  Time-resolved X-ray diffraction …………………………………………………………..94

4.3.4  SEM………………………………………………………………………………………………….95

4.3.5  DFT/Bader results ………………………………………………………………………………96

4.4 Discussion……………………………………………………………………………………………………96

4.4.1  Apparent fractionation as a function of extent of reaction………………………..96

4.4.2  Mechanisms for isotope fractionation……………………………………………………98

4.5  Model…………………………………………………………………………………………………………101

4.5.1 Box model definitions………………………………………………………………………….102

4.5.2 Box model results………………………………………………………………………………..109

4.5.3 Model interpretation…………………………………………………………………………….111

4.6 Implications………………………………………………………………………………………………….112

4.7 References……………………………………………………………………………………………………112

Appendix A  Butte, MT Cu Isotope Study…………………………………………………………………….142

Appendix B  “Slices of Time: Time Scales of the Environment” Script……………………………147

Chapter 1

 

A flow-through reaction cell that couples time-resolved X-ray diffraction with stable isotope analysis

1.0 Abstract

Here we describe a non-metallic flow-through reaction cell designed for in situ timeresolved X-ray diffraction coupled with stable isotope analysis.  The experimental setup allows us to correlate Cu isotope fractionation with changes in crystal structure during Cu-sulfide dissolution.  This flow-through cell can be applied to many classes of fluid-mineral reactions that involve dissolution or ion exchange.

1.1 Introduction

Environmental sample cells for time-resolved powder X-ray diffraction (TR-XRD) have been designed to study catalysis reactions (Andersen et al., 1998; Chupas et al., 2001), hydrothermal precipitation (Evans et al., 1995; Hummer et al., 2008; Hummer et al., 2009), cation exchange (Lee et al., 1998; Lee et al., 2000; Lee et al., 2001; Celestian et al., 2004; Celestian &

Clearfield, 2007; Lopano et al., 2007; Lopano et al., 2009), and biomineralization (Fischer et al., 2008), among other applications.  These real-time X-ray diffraction techniques allow researchers to quantify kinetic parameters for mineral-fluid and mineral-gas reactions (Andersen et al., 1998; Madsen et al., 2005; Fischer et al., 2008) and to determine reaction mechanisms with high precision at the atomic scale (Evans et al., 1995; Scarlett et al., 2008; Lopano et al., 2009).

Many angle-dispersive TR-XRD investigations have employed capillary-based cell designs innovated by Norby (1996) and Parise et al. (2000).  These cells offer an extremely high degree of versatility.  They serve as autoclaves for closed solid-fluid or solid-gas systems at low and high temperatures, and can accommodate open-system experiments that involve gas or fluid flow-through reactions with powdered solids.  Moreover, a recent design presented in Chupas et al. (2008) permits flow-through solid-fluid or solid-gas experiments at high temperatures.

Although previous researchers have measured reaction products in gas-solid flowthrough reactions (Clausen et al., 1991; Chupas et al., 2001), we are unaware of angle-dispersive TR-XRD flow-through studies that have attempted to correlate real-time measurements of fluid isotope chemistry with changes in the crystal structure of the solid phase. Coupled studies of fluid-mineral behavior are particularly challenging with respect to mineral reactions that produce changes in the stable isotope composition of the ambient fluid, since no in-line methodology currently can extract the isotopic character of the eluate from a flow-through experiment.

Here we report a novel approach that combines in situ, time-resolved synchrotron X-ray diffraction with time-resolved stable isotope analysis of dissolved metals.  In order to make accurate and reproducible isotope measurements, we modified flow-through capillary reaction cells to encompass four key components: 1) a non-metallic sample cell to prevent any contribution of dissolved metals from the cell; 2) a sample cell that can rotate about phi through at least 30° to minimize preferred orientation; 3) an automated fraction collector that can be programmed and operated from outside the protective hutch; and 4) regulated N2 gas pressure to control eluent flow.  Using this sample cell we can directly monitor crystallographic changes in the solid phase and correlate those changes with variations in the isotopic chemistry of the solution.

1.2 Experimental Setup

The flow-through reaction cell includes two commercially available components (union tee and capillary tube) and two machined pieces (stainless steel cap and Teflon® plug) (Fig. 1.1a).  A Chemfluor® mini union tee (Saint-Gobain Inc., part No. 1072002) provides the basis for this inert material sample cell. A glass capillary (ID 0.7 to 1.0 mm; Charles Supper Inc., part # 10QZ), which holds the mineral powder to be interrogated, is affixed to one of the union tee fittings.  A milled cap of 316 stainless steel is attached to the union tee that is located at 180o to the capillary tee.  This stainless steel cap is milled from a single block, but it contains two sections: a post that serves to attach the cell to the goniometer head, and a cup that closes off the union tee.  The post section has a length of 8.15 mm and a diameter of 3.12 mm (Fig. 1.1b).  The cup portion is 5.08 mm long, and it has an outer diameter of 11.11 mm with a bore diameter of 5.41 mm and depth of 3.81 mm.  To accommodate the Teflon® plug, the stainless steel cap has an additional center bore with a diameter of 5.33 mm and a depth of 0.508 mm.  Threads were set using a 1/4″ – 28 tap.  The Teflon® plug, which is required to isolate the stainless steel cap from the reaction fluids, has a diameter of 1.37 mm with a length of 3.94 mm, and its base has a diameter of 5.31 mm and thickness of 0.508 mm (Fig. 1.1a).

A plug of acid-washed glass wool or cotton is placed at the end of the capillary to prevent solid material from passing into solution collectors. The powdered mineral sample is then deposited into the capillary.  Powders that pack densely impede fluid flow, and for some of our experiments we sprinkled the powder onto glass wool, rolled the mixture into a tight wad, and packed the wad into the capillary.

The vertical port on the union tee (marked a, Fig. 1.1a) serves as the fitting for the solution tubing and is not modified. However, the horizontal ports are cut off to be flush with the thread barrel in order to accommodate the capillary fitting and the stainless steel cap.  We note that ferrules are not used for either of these fittings.  A bead of 5-minute epoxy is placed around the exterior surface near the flared opening of the quartz glass capillary, and the capillary is inserted into a Chemfluor® fitting that contains an o-ring.  Once the capillary is epoxied to the fitting, the flared opening of the capillary is crushed and removed from the fitting using tweezers.  An additional o-ring is placed within the threads of the fitting and the fitting is attached to the union tee.  The closed tip at the outlet end of the capillary is cut to allow for fluid flow.  The union tee is attached to the goniometer head using the stainless steel cap with a Teflon plug  (Fig.

1.1b).

The reaction fluid is placed in an HDPE acid-washed bottle that is sealed with a screw cap containing inlet and outlet ports.  Nitrogen gas is fed into the inlet port to provide a backpressure on the reaction fluid, and in response the fluid flows through the outlet port to the union tee via 1.50 mm I.D. HDPE tubing.  A peristaltic pump can be attached to the reaction fluid tubing to regulate the flow rate.  As the fluid reacts with the powdered sample, X-ray diffraction patterns are collected every 2 to 3 minutes using an image plate detector.  We obtained highquality patterns using a MAR345 full-imaging plate detector (beamline X7B, National

Synchrotron Light Source, Brookhaven National Lab) and a MAR165 CCD camera (beamline 13-BM-C, Advanced Photon Source, Argonne National Lab).  Although integration of the diffraction rings helps minimize preferred orientation, our device allows rotation of the capillary about the phi axis by ±30° to diminish this problem further.

The post-reaction eluate drips through the capillary outlet and is collected in HDPE acidwashed 15 ml bottles that are aligned approximately 3 cm apart on a rotating autosampler stage 10 cm directly below the capillary (Fig. 1.1c).  The autosampler stage is a 30-cm diameter stainless steel disk attached to a Newport RTM stage using stepping motor control.  The autosampler is controlled from outside the hutch using EPICS Open Source software.  Sample bottles rotate under the capillary and can be exchanged during an experimental run if flow-rate is high or a large number of fractions is desired.  For our Cu isotope analysis, we required approximately 100 μg of Cu so the volume of solution that we collected depended on the concentration of Cu in the eluate and the flow-rate of the solution.  For a typical experimental run we collected 1 to 2 ml of solution over a five-minute span.  Evaporation of the collected fluid would likely not influence the Cu isotope composition of the fluid, but it could potentially affect the metal concentrations.  This can be prevented by periodically entering the hutch and capping the collection bottles.

In our experiments, we were targeting the fractionation of Cu isotopes during an oxidative transformation of chalcocite (Cu2S) to covellite (CuS). For our isotope analysis, the eluate was acidified with ultrapure HNO3, and the remaining mineral powder in the capillary was digested completely using 1 part HNO3 and 3 parts HCl.  In order to separate Cu from other cations, we employed a wet ion exchange chromatography (IEC) procedure following the protocol adapted from previous work (Marechal & Albarede, 2002; Mathur et al., 2005).  After sample purification we measured 65Cu/63Cu using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS; Thermo Scientific, Neptune) at Washington State University.

A critically important characteristic of our reaction cell is the absence of any metal components that contact the fluid. Prior to the adoption of our cell design, we ran test experiments using more standard cells that employed metallic fittings. The brief exposure of the reaction fluid to the metal fittings resulted in Zn contamination (~ 1 ppm) in the eluate.  Using the non-metallic experimental cell, Zn concentrations in run products dropped below analytical detection limits.  Furthermore, our design differs from other cells (e.g., the SECReTS cell of Parise et al., 2000) by incorporating a shortened exhaust port that eliminates solution mixing and minimizes lag time between diffraction collection and eluate sampling.

1.3 Application

We designed and tested our non-metallic reaction cell to collect reaction eluate for Cu isotope analysis during real-time XRD of Cu-sulfide mineral dissolution.  During oxidative dissolution, chalcocite (Cu2S), a common copper ore mineral, undergoes a series of phase transformations as it forms covellite (CuS) (Whiteside et al., 1986). Mathur et al. (2005) observed that dissolution of chalcocite fractionates stable isotopes of Cu, with the heavier 65Cu preferentially partitioning into the fluid.  Prior studies had demonstrated that bonding environment can strongly influence Cu isotope fractionation (Marechal & Albarede, 2002; Marechal & Sheppard, 2002), and Mathur et al. (2005) proposed that Cu isotope fractionation during the reaction from Cu2S to CuS may be controlled by changes in the Cu sulfide structure.

We developed our flow-through cell to correlate changes in mineral structure with the degree of Cu isotope fractionation during Cu-sulfide dissolution.  Our experiments have allowed us to monitor changes in crystal structure and mineral phase abundance in real time, and we quantitatively relate crystallographic parameters to the changes in the Cu isotope values of the eluate.  In Figure 1.2, time-resolved X-ray diffraction patterns are stacked to reveal the oxidation of chalcocite (Cu2S) to covellite (CuS).  The emergence and disappearance of diffraction peaks between the first and final patterns indicate the formation of transient Cu sulfide phases during this reaction.  To the left of the stacked diffraction patterns, we have plotted the variation in the δ65Cu values of the eluate.  As is standard practice in isotope chemistry, the δ value is a function that expresses the 65Cu/63Cu ratio normalized to a standard, and the normalized ratio is expressed in units of per mil (‰).  In this experiment, the δ65Cu of the starting mineral powder equaled 0 ± 0.14‰. Therefore, δ65Cu values of the eluate are positive when 65Cu fractionates preferentially into the solution and negative when 65Cu is preferentially retained in the solid.

Because our experimental protocol allowed us to correlate the variation in isotopic fractionation with the transformation of phases, the two graphs share the same time axis.  Thus, we observe that aqueous Cu is initially enriched in isotopically heavy Cu relative to the sample and becomes depleted in heavy Cu over time. Coupling the isotope analysis with TR-XRD reveals that the degree of fractionation was heavily dependent on the Cu sulfide phase in coexistence with the solution.  Specifically, fractionation is large and positive when the solid phases are characterized by Cu:S ratios close to 2:1, but as the stoichiometry approaches a 1:1 ratio, the fractionation becomes negative, indicating the preferential release of relatively light 63Cu into solution.

We note that this sample cell can be applied beyond isotopic fractionation during mineral reactions to any structural transformation that involves the release of ions into solution.  Additional applications of this cell thus would include mineral dissolution or ion exchange reactions, and these applications uniquely allow the researcher to couple structural analysis with fluid chemistry. This synergy is ideally suited to studies that attempt to bridge reaction kinetics with crystallographic mechanisms of reaction.

 

1.4 References

Andersen, E. K., Andersen, I. G. K., Norby, P. & Hanson, J. C. (1998). Journal of Solid State

Chemistry 141, 235-240.

Celestian, A. J. & Clearfield, A. (2007). Journal of Materials Chemistry 17, 4839-4842. Celestian, A. J., Parise, J. B., Goodell, C., Tripathi, A. & Hanson, J. (2004). Chemistry of

Materials 16, 2244-2254.

Chupas, P. J., Chapman, K. W., Kurtz, C., Hanson, J. C., Lee, P. L. & Grey, C. P. (2008). J. Appl.

Cryst 41, 822-824.

Chupas, P. J., Ciraolo, M. F., Hanson, J. C. & Grey, C. P. (2001). Journal of the American            Chemical Society 123, 1694-1702.

Clausen, B. S., Steffensen, G., Fabius, B., Villadsen, J., Feidenhansl, R. & Topsoe, H. (1991).

Journal of Catalysis 132, 524-535.

Evans, J. S. O., Francis, R. J., Ohare, D., Price, S. J., Clark, S. M., Flaherty, J., Gordon, J., Nield,              A. & Tang, C. C. (1995). Review of Scientific Instruments 66, 2442-2445.

Fischer, T. B., Heaney, P. J., Jang, J. H., Ross, D. E., Brantley, S. L., Post, J. E. & Tien, M.

(2008). American Mineralogist 93, 1929-1932.

Hummer, D. R., Heaney, P. J. & Post, J. E. (2008). Powder Diffraction 23, 267-267. Hummer, D. R., Kubicki, J. D., Kent, P. R. C., Post, J. E. & Heaney, P. J. (2009). Journal of        Physical Chemistry C 113, 4240-4245.

Lee, Y., Reisner, B. A., Hanson, J. C., Jones, G. A., Parise, J. B., Corbin, D. R., Toby, B. H.,   Freitag, A. & Larese, J. Z. (2001). Journal of Physical Chemistry B 105, 7188-7199.

Lee, Y. J., Carr, S. W. & Parise, J. B. (1998). Chemistry of Materials 10, 2561-2570. Lee, Y. J., Kim, S. J., Schoonen, M. A. A. & Parise, J. B. (2000). Chemistry of Materials 12,

1597-1603.

Lopano, C. L., Heaney, P. J. & Post, J. E. (2009). American Mineralogist 94, 816-826. Lopano, C. L., Heaney, P. J., Post, J. E., Hanson, J. & Komarneni, S. (2007). American           Mineralogist 92, 380-387.

Madsen, I. C., Scarlett, N. V. Y. & Whittington, B. I. (2005). J. Appl. Cryst 38, 927-933. Marechal, C. & Albarede, F. (2002). Geochimica Et Cosmochimica Acta 66, 1499-1509. Marechal, C. N. & Sheppard, S. M. F. (2002). Geochimica Et Cosmochimica Acta 66, A484-

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Mathur, R., Ruiz, J., Titley, S., Liermann, L., Buss, H. & Brantley, S. (2005). Geochimica et

Cosmochimica Acta 69, 5233-5246.

Norby, P. (1996). Materials Science Forum, Vol. 228, European Powder Diffraction: Epdic Iv,     Pts 1 and 2, edited by R. J. Cernik, R. Delhez & E. J. Mittemeijer, pp. 147-152.

Parise, J. B., Cahill, C. L. & Lee, Y. J. (2000). Canadian Mineralogist 38, 777-800.

Scarlett, N. V. Y., Madsen, I. C. & Whittington, B. I. (2008). J. Appl. Cryst 41, 572-583. Whiteside, L. S., Goble, R. J. & Abbott, D. (1986). Canadian Mineralogist 24, 247-258.

 

ABIOTIC CONTROLS ON COPPER ISOTOPE FRACTIONATION DURING THE DISSOLUTION OF COPPER SULFIDE MINERALS

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