Fluid-Enhanced Coarsening of Mineral Microstructures in

Article http://pubs.acs.org/journal/aesccq

Fluid-Enhanced Coarsening of Mineral Microstructures in Hydrothermally Synthesized Bornite−Digenite Solid Solution Jing Zhao,†,‡,¶ Joel̈ Brugger,†,§ Benjamin A. Grguric,†,⊥ Yung Ngothai,‡ and Allan Pring*,†,¶ †

Department of Mineralogy, South Australian Museum, North Terrace, Adelaide, South Australia 5000, Australia School of Chemical Engineering, University of Adelaide, Adelaide, South Australia, 5005, Australia § School of Earth, Atmosphere, and the Environment, Monash University, Clayton, Victoria 3800, Australia ⊥ Centre for Exploration Targeting, University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, Australia ¶ School of Chemical and Physical Sciences, Flinders University, Bedford Park, South Australia 5042, Australia ‡

ABSTRACT: Symplectic microstructures are abundant in copper−iron−sulfide minerals and are conventionally considered to form by solid-state diffusion processes. Here we experimentally demonstrate that coarsening of exsolution lamellae occurs ∼1000 times faster in the presence of a fluid compared to the equivalent dry system. Bornite−digenite solid solutions (Cu5FeS4−Cu8.52Fe0.11S4.88) were synthesized hydrothermally via the replacement of chalcopyrite, and we compared the microtextures in the product subjected to different cooling histories: (i) dry annealing after synthesis; (ii) cooling to an annealing temperature immediately following hydrothermal synthesis; and (iii) annealing in a hydrothermal fluid following quenching to room temperature and then reheating. We interpret the rapid coarsening of the exsolution lamellae in the presence of a fluid phase to result from recrystallization associated with healing of the open porous microstructure in the parent phase. The porosity is a consequence of the synthesis of the parent bornite−digenite solid solutions via interface coupled dissolution reprecipitation. The texture coarsening is accompanied by the destruction of the transient open porous microstructure via coalescence of the pores and their migration to lamellae and grain boundaries. As a result, the final microstructure and the kinetics of textural coarsening depend upon the crystallization and cooling history of the parent mineral. Such fluid-driven textural evolution may be a major mode of reaction in ore systems, and is likely to affect oxide and silicate systems alike in the presence of aqueous fluids. KEYWORDS: symplectite, exsolution, microstructure, copper sulfide, hydrothermal

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INTRODUCTION There is scarcely an ore deposit anywhere on Earth that has not been formed directly from hydrothermal fluids flowing through the crust, or modified to varying degrees by such fluids.1,2 Much of our knowledge about phase relations for sulfide mineral systems in ore deposits, however, comes from phase diagrams that were almost exclusively derived from experiments undertaken under dry, rather than hydrothermal conditions.3,4 Recently Zhao et al.5 showed that the compositions of bornite (Cu5FeS4)−digenite (Cu8.52Fe0.11S4.88) solid solutions (bdss) formed under hydrothermal conditions via mineral replacement reactions depended principally on the temperature of the reaction (between 240 and 320 °C) but not on the bulk composition of the fluid. Compositions in the range of Bn60Dg40 to Bn100 were formed by the replacement of chalcopyrite in the presence of Cu(I)- and S(-II)-rich hydrothermal fluids. The composition of the solid solution becomes richer in Cu with decreasing temperatures. The formation of bdss from chalcopyrite proceeds via an interface coupled dissolution reprecipitation (ICDR) reaction and displays a characteristic sharp reaction front between the parent chalcopyrite and the product phase, in this case bdss.6−9 For an ICDR reaction to proceed to completion, the product © XXXX American Chemical Society

phase needs to contain pores and/or cracks that make it permeable to the hydrothermal fluid, connecting the reaction front to the bulk solution. This reaction-generated permeability enables mass transfer to and from the reaction front.9−11 The successful hydrothermal synthesis of bdss made it practical to experimentally study the effect of fluid on the phase separation of bdss, and in particular, to explore if and how the presence of a fluid influences the coarsening kinetics of the exsolution textures as reported by Grguric and Putnis.12,13 Exsolution, the separation of a solid solution into two components, is perhaps the most common phase separation phenomenon during the re-equilibration of minerals in both nature and the laboratory. As discussed by Edwards,14 exsolution produces “intergrowths (that) consist of minute inclusions of one mineral, generally, in the crystallographic directions of the host mineral (solvent) so that the orientation of the intergrowth varies with the orientation of the host crystals. The compositions and crystal structures of the two Received: Revised: Accepted: Published: A

April 3, 2017 June 12, 2017 August 7, 2017 August 7, 2017 DOI: 10.1021/acsearthspacechem.7b00034 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

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ACS Earth and Space Chemistry

weave textures (Figure 1c).12,13,19 The degree of structural similarity between the two phases is very high, and despite the changes in stoichiometry from Cu 5 FeS 4 (bornite) to Cu8.52Fe0.11S4.88 (digenite) the dimensions of the cubic subcell change by less than 1% across the series.3 Bornite and digenite share the antifluorite structure type. The Cu atoms occupy tetrahedral interstices in the face-centered cubic sulfur matrix, and in this configuration the Cu ions are relatively mobile, even down to room temperature; as a consequence, a number of differently ordered superstructures exist.3 Grguric and Putnis12 found that many intermediate compositions of bdss were unquenchable. All experiments on the exsolution of Cu−Fe sulfide minerals reported to date have been performed under dry conditions. In this study, we aim to explore the roles of fluid and cooling history on the phase separation of bdss synthesized hydrothermally via the replacement of chalcopyrite. Another goal is to assess the role of fluid in the coarsening of the phase separation microtextures. The resulting textures are compared with those observed in experiments under dry conditions12,13 and in natural samples.17

minerals involved must be such that solid solution between them is feasible at high temperatures and there must be a general absence of textures indicating replacement relationships between the two.” Hence, exsolution is defined both in terms of process (decomposition of a homogeneous solid solution) and a resulting texture. The bdss decomposition studied here follows the seven criteria for exsolution given by Schwartz.15,16 Note that we use the term exsolution in a textural sense rather than as implying a solid-state mechanism. The phase decomposition of bdss has been observed in natural Cu−Fe sulfides textures (Figure 1a)17,18 and studied experimentally in detail by Grguric and Putnis (Figure 1b)12,13 under dry conditions. For bdss, the solid solution is completed above 265 °C in the absence of a fluid, but below this temperature, bornite-rich compositions decompose via exsolution into bornite and digenite-like phases giving basket-

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EXPERIMENTAL METHODS Synthesis and Characterization of Starting bdss. Compositions in the bdss were synthesized using chalcopyrite as the parent mineral under hydrothermal conditions following the procedure of Zhao et al.5 For the synthesis, a 1 M borate buffer solution (pH25°c ∼ 10) was prepared with 0.532 M H3BO4 and 0.480 M NaOH. 1 M NaCl was added to the buffer solution to prevent disproportionation of Cu+ to Cu0 and Cu2+ in the solution and to mimic a low salinity hydrothermal fluid.20 The solutions were prepared, stored, loaded, and sealed into the autoclaves in an argon-filled anoxic glovebox. In each run, 10 mg (55 μmoles) of pure chalcopyrite crystal fragments (SA Museum sample G22623) (125−150 μm in size), 23.5 mg (237 μmoles) CuCl(s), and 189 mg (2516 μmoles) thioacetamide were carefully weighed and placed in an 8 mL titanium autoclave together with 5 mL of buffer solution. The sealed cells were placed in electric muffle furnaces during synthesis and for subsequent annealing. The temperatures of the furnaces were controlled within ±2 °C. Following the experiments, the samples were separated from solution, washed three times with acetone, dried at room temperature, and characterized with the analytical methods described below. Three different bdss compositions (Bn60Dg40, Bn80Dg20, and Bn87Dg13) were obtained by varying the initial reaction temperatures and durations. The reaction conditions and the composition of the products are summarized in Table 1. The compositions of the synthetic bdss were determined by Electron Probe Micro-Analysis (EMPA) using a Cameca SX-51 instrument (20 kV, 20 nA) at Adelaide Microscopy, University of Adelaide. Rietveld quantitative phase analysis (QPA)21 of powder X-ray diffraction (XRD) data was used for phase quantification using the program Topas.22 Data were obtained using a Huber Guinier Image Plate G670 with CoKα1 radiation (λ = 1.78892 Å). A Pseudo-Voigt function and a sixth order Chebychev polynomial were used to model the peak shapes and the background, respectively. Zero shifts were taken from refinements of the powder diffraction pattern for the parent chalcopyrite material. The diffraction pattern parameters refined also include the scale factor (S) and cell parameters of product phases. Crystal structural data of the minerals for QPA were taken from the ICSD database: chalcopyrite #94554;

Figure 1. (A) Cu-Rich scales in the Reykjanes Geothermal System, Iceland (modified from Hardardottir et al.17), showing exsolution lamellae of bornite (dark blue to violet) in a digenite matrix (light blue). (B) Optical photomicrograph of the dry synthesis Bn90Dg10 sample annealed at 180 °C under dry condition for 30 weeks, showing the lamellar texture associated with annealing within the low bornite (red-brown phase) + 1a-ss (bluish-white phase), modified after Grguric et al.13 (C) Phase diagram of the bornite−digenite system (modified after Grguric et al.13). “1a-ss” is the solid solution phase with 1a cubic unit cell (high digenite). “Int. bn” indicates the intermediate bornite. “Low bn” indicates the low bornite and “Low dg” means low digenite. The three crosses (pink) refer to the compositions of three bornite−digenite solid solutions (bdss) synthesized hydrothermally by replacement of chalcopyrite at different temperatures. Three synthetic bdss are all in the zone of 1a-ss, and the bulk compositions do not change significantly during hydrothermal annealing (Method 2). B

DOI: 10.1021/acsearthspacechem.7b00034 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

Article

The ratio between bornite and digenite was calculated from Electron Probe Micro-Analysis (EMPA) data, measured with a Cameca SX-51 instrument (20 kV, 20 nA) at Adelaide Microscopy, University of Adelaide. bThe compositions of the synthetic bdss were determined by EMPA. cThe values were achieved by Rietveld quantitative phase analysis (QPA) of powder X-ray diffraction data21 using the program of Topas.22 Crystal structural data of intermediated bornite (#24174) was taken from the ICSD database and used for the data analysis. dThe weight percentage of bdss in the synthesized product was calculated by using Topas.

bornite #24174 (intermediated bornite); digenite #42709 (simple subcell structure). The extent of exsolution or unmixing, expressed as wt % bornite and digenite, is provided together with the unit cell data in Table 2. The terms bornite and digenite refer to compositionally Fe-rich and Cu-rich phases, respectively, rather than to the end-member compositions of these minerals. It is also important to note that the compositions of the Fe-rich and Cu-rich phases may vary depending on the annealing temperature, although this cannot be quantified from the unit cell parameters. Characterization of the morphological and textural features of samples was undertaken using a QUANTA 450 High resolution field emission gun environmental scanning electron microscopy (SEM) coupled with a TEAM EDS silicon drift energy dispersive spectrometer at Adelaide Microscopy, University of Adelaide. Images of the lamellae were taken in backscattered electron mode (BSE); the practical resolution was ∼50−100 nm given the excitation volume of the electron beam. Attempts to obtain transmission electron microscopy (TEM) and scanning TEM (STEM) images were not successful as the Cu-rich digenite component renders the samples unsuitable to high resolutions TEM (HRTEM) imaging due to their decomposition or recrystallization during ion thinning. Since only very limited amounts of sample were produced (∼10 mg for each run) and the fluid content in the samples was below 1 wt %, direct measurement of the fluid content in the sample was not possible by thermogravimetric or CHN (carbon−hydrogen−nitrogen) analysis. Instead, the fluid content within the synthetic bdss was estimated by calculating the total volume of the pores within the samples. ImageJ software was used to measure the surface area of the pores visible on the SEM image of freshly broken grain surfaces. The results showed that the average pore density was around 2% by surface area based on the measurements of 5 grains. Assuming that these pores were spheres and filled with the hydrothermal fluid, we calculated a maximum content of 3000 ppm water trapped in the synthetic grains after quenching to room temperature. Annealing Experiments and the Exsolution of Bornite−Digenite Solid Solution (bdss). The annealing of bdss was undertaken using one of three methods (Figure 2). Method 1: Annealing under Dry Conditions. This method was devised as a control to explore the effect of fluid on the kinetics of lamellae coarsening. Following hydrothermal synthesis, bdss samples were quenched to room temperature, washed, dried at room temperature, and then sealed in silica tubes under vacuum, so only the fluid trapped in fluid inclusions in the bdss grains remained (Figure 2). The samples were then annealed at 150 °C and the silica tubes were then quenched to room temperature in cold water (runs D1 to D4; Table 2). Method 2: Direct Annealing. Here the samples were annealed in the mother solution without quenching after the initial synthesis period (Figure 2). The autoclaves, after the synthesis period, were rapidly transferred to a preheated oven at the annealing temperature. In these experiments the autoclaves rapidly re-equilibrated to the annealing temperatures (over approximately 10 min) (runs A1−A4, B1−B4, and C1−C11; experimental conditions are listed in Table 2). After the annealing period, the autoclaves were quenched in a large volume of cold water to room temperature.

a

1349.0(2) 1327.2(1) 1320.02(6) 11.0493(5) 10.9895(4) 10.9697(2) 41 80 86 24.30(23.66−24.78) 24.90(24.63−25.14) 25.40(25.13−25.69) 4.79(4.53−5.11) 8.04(7.86−8.19) 8.19(7.87−8.47) Cu5.89Fe0.45S4.00 Cu5.43Fe0.74S4.00 Cu5.28Fe0.74S4.00 240 280 300 A B C

9 2 2

Bn60Dg40 Bn80Dg20 Bn87Dg13

70.91(70.11−70.56) 67.06(66.66−67.23) 66.41(65.83−66.79)

a (Å) wt %d S Fe Cu formulab bdssa time (day) T/°C composition

synthesis conditions

Table 1. Compositions of Three bdss Compositions

weight percentages of elements in products (wt %) [mean (range)]b

bdssc

Cell volume (Å3)


C


D

2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 1 1 1 1 3 3 3 3

A1 A2 A3 A4 B1 B2 B3 B4 C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 D1 D2 D3 D4 E1 E2 E3 E4

125 150 170 190 125 150 170 190 125 150 150 150 150 150 150 170 190 220 250 150 150 150 150 150 150 150 150

T (°C) 1 1 1 1 1 1 1 1 1 0.04 0.08 1 3 5 10 1 1 1 1 1 15 30 45 1 3 5 15

time (day) 37 33 30 31 68 36 60 59 74 62 56 47 67 63 65 64 71 58 69 70 72 58 68 71 64 62 53

wt % 10.9562(7) 10.9582(7) 10.999(2) 10.998(4) 10.9522(3) 10.9545(4) 10.9550(3) 10.9559(6) 10.9518(4) 10.9519(2) 10.9520(5) 10.9529(5) 10.9514(2) 10.9538(2) 10.9550(2) 10.9573(2) 10.9562(2) 10.9641(6) 10.9669(3) 10.9535(3) 10.9524(5) 10.9508(5) 10.9491(5) 10.9569(3) 10.9563(3) 10.9569(5) 10.9557(3)

a (Å)

borniteb 63 67 70 69 32 64 40 41 26 38 44 53 33 37 35 36 29 42 31 30 28 42 32 29 36 38 31

wt % 5.5362(2) 5.5344(2) 5.5257(2) 5.5280(3) 5.5398(4) 5.5343(1) 5.5305(4) 5.506(3) 5.535(2) 5.5545(3) 5.536(1) 5.5353(4) 5.5281(3) 5.539(2) 5.5391(3) 5.5360(4) 5.5380(5) 5.493(2) 5.5283(3) 5.5775(2) 5.5497(2) 5.507(3) 5.536(2) 5.5537(4) 5.5492(3) 5.486(1) 5.5436(3)

a (Å)

digeniteb

7.94(4.40−10.24) 4.32(1.59−7.58) 6.00(3.00−9.15) 10.91(7.10−12.14) 7.58(3.31−9.51) 2.36(1.45−3.43) 10.67(6.85−16.08) 9.73(2.62−18.04) 11.02(5.89−14.23) 11.36(6.98−19.58) 7.85(4.36−17.02) 8.69(2.68−15.01) 5.30(3.56−7.88) 1.00(0.40−1.40)

0.90(0.48−1.31) 0.82(0.52−1.26) 0.87(0.59−1.36) 1.03(0.49−1.56) 1.16(0.68−1.70) 0.50(0.26−0.87) 1.31(0.88−2.04) 1.31(0.89−2.67) 1.65(1.19−2.22) 1.46(0.96−2.34) 1.39(0.94−2.14) 1.12(0.75−2.19) 0.67(0.39−0.90) 0.30(0.20−0.40)

length (μm) 1.86(0.87−3.03) 2.82(1.46−8.97)

0.48(0.26−0.58) 0.79(0.50−1.04)

width (μm)

size of bn lamellaec

2.62(1.41−4.48) 3.40(1.16−6.65) 4.01(1.89−7.78) 3.59(2.26−4.74) 5.73(5.43−8.01) 4.12(1.43−6.13)

0.24(0.16−0.35) 0.15(0.10−0.23) 0.23(0.16−0.35) 0.15(0.10−0.19)

length (μm)

0.16(0.09−0.25) 0.18(0.11−0.41)

width (μm)

size of dg lamellaec

Experiments of sample numbers A1−A4 were conducted using Bn60Dg40 under hydrothermal conditions (Method 2); experiments of sample numbers B1−B4 were conducted using Bn80Dg20 under hydrothermal conditions (Method 2); experiments of sample numbers C1−C11 were conducted using Bn87Dg13 under hydrothermal conditions (Method 2). D1−D4 experiments were conducted using Bn87Dg13 under dry conditions (Method 2), and experiments E1−E4 were a Bn87Dg13 sample quenched to room temperature after synthesis procedure and annealed in the mother solution at 150 °C for 1−15 days (Method 2). bBornite and digenite refer to compositionally Fe-rich and Cu-rich phases, respectively, rather than to the end-member compositions of these minerals. The compositions of Bn and Dg can be different in different runs, which could be related to the annealing temperatures. All the values listed in these six columns were calculated using the program of Topas. The crystal structural data of intermediated bornite (#24174) and digenite (#42709) was taken from the ICSD database and used for the data analysis. cAll the values listed in these four columns were estimated using Nikon NIS-Elements software using the SEM images, as shown in Figure 7a. The mean value is based on at least 30 measurements in each sample. The standard deviation was not estimated.

a

method

run no.a

annealing conditions

Table 2. Reaction Conditions of All Runs, XRD Quantitative Analysis Results, and The Size of Lamellae

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generated by the traditional dry synthesis and annealing experiments of Grguric et al.13 The exsolution of bdss was observed in this set of experiments, with textures very similar to those of Grguric et al.13 The textures in Method 1 experiments were compacted check patterns (Figure 4). For example, after 1

Figure 4. SEM images showing the exsolution textures within Bn87Dg13 annealed using Method 1 for 1 day (A, Run D1) and 15 days (B, Run D2). Digenite lamellae are illustrated in light gray and the bornite domains are in dark gray.

Figure 2. Summary of the three different annealing methods.

Method 3: Annealing Following Quenching and Reheating in the Hydrothermal Fluid. Here the bdss was quenched in the autoclave to room temperature following the completion of synthesis at 300 °C and then, after 1 h at room temperature, the unopened autoclaves were transferred to a preheated oven and annealed at 150 °C for up to 15 days, with the bdss remaining within the “mother” solution for the entire duration of the experiment (runs E1 to E4; Figure 2; Table 2). This quench and reheat method was designed to explore how quickly the permeability in the bdss decayed upon cooling.

day of annealing at 150 °C, the lamellae were ∼0.16 μm (range 0.09−0.25 μm) wide (Figure 4a; the size of the lamellae were measured using the Nikon NIS-Elements software). If the dry annealing period was increased to 15 days, the width of the digenite lamellae increased slightly to 0.18 μm (range 0.11− 0.41 μm). Several broader lamellae of digenite (around 1 μm in width) were found developing at the reaction interface between the unreacted chalcopyrite and the bdss grain (Figure 4b). After 45 days of annealing at 150 °C, 4.0 μm long (range 1.89−7.78 μm; Table 1) bornite lamellae were formed. SEM images of samples annealed under hydrothermal conditions (Method 1) showed that bdss exsolved and coarsened rapidly into bornite and digenite lamellae at annealing temperatures between 125 and 220 °C. At 150 °C (Figure 5), the exsolution of bdss was readily observable after only 1 h of annealing, and the lamellae of bornite (dark gray) had average widths of ∼0.5 μm (range 0.26−0.87 μm) and average lengths of 2.4 μm (range 1.45−3.43 μm) (Figure 5a). After 2 h of annealing, the average width of the bornite lamellae increased to 1.3 μm (range 0.88−2.04 μm), and the length to 10.7 μm (range 6.85−16.08 μm) (Figure 5b), such that bornite could now be considered the dominant phase and therefore the host, rather than the lamellae. One might expect that “exsolution” coarsening would further increase with annealing time, but no significant coarsening of the texture was observed beyond 2 h (Figure 5b,c; Table 2). This implies that coarsening was effectively over after the first 2 h, afterward the microstructural evolution of the sample appeared to involve the coarsening of the digenite (light gray) domains along the boundaries of the bornite (Figure 5b,c), but this may be an effect of the orientation of the grain. Textural coarsening was essentially completed after 1 day (Figure 5c), and no significant difference in microstructure was observed upon further annealing (up to 10 days). The final resulting microstructure was a typical basket-weave texture consisting of an intergrowth of bornite and digenite lamellae (Figure 5c), showing a neat basket-weave pattern. The bornite lamellae (dark gray) occurred in two orientations at ∼90° to each other, surrounded by digenite domains (light gray) around 1 μm in width. The appearance of the texture depends on the orientation of the grains; bdss being cubic, will have three equivalent directions, but the three sets of lamellae will only be seen in grain sectioned close to being perpendicular to [111]. Under high resolution SEM, the bornite phase was homogeneous while the

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RESULTS The XRD results for hydrothermally synthesized bdss quenched from 300 °C (Figure 3) indicate it is a mixture of nanoscale

Figure 3. X-ray patterns of hydrothermally synthesized bdss of the three compositions listed in Table 1. The main peak of bdss shifts to the right as the percentage of bornite increases within the bdss. The short lines at the bottom of the pattern indicate the crystal structural data of intermediated bornite (#24174, blue line) and digenite (#42709, red line) from the ICSD database.

bornite and digenite domains. The reflections for bdss in the Xray diffraction patterns were broad. They displayed full widths at half-maximum height (FWHM) values almost double those of the parent chalcopyrite, and were not resolved into separate reflections for bornite and digenite. These results are consistent with nucleation and nanoscale phase separation being very rapid during quenching. Grguric and Putnis12,13 similarly observed that most bdss compositions synthesized under dry conditions were unquenchable. The results of the “Annealing under dry conditions” (Method 1) runs can be compared directly with those E


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Figure 5. SEM images showing the exsolution pattern of Bn87Dg13, (A) annealed at 150 °C for 1 h (Run C2), (B) 2 h (Run C3), and (C) 1 day (Run C4) using annealing Method 2. Bornite lamellae are shown in dark gray while the digenite domains are illustrated in light gray.

digenite phase showed a mixture of nanocrystals and fine pores (Figure 6). The porosity along the bornite lamellae boundaries was clearly visible (Figure 6c). This porosity can only be observed on freshly broken surfaces because the polishing process fills the smaller pores on the polished sections. The annealing temperature has a strong effect on the size of the lamellae (Figure 7). XRD results confirmed that all samples separated into bornite and digenite after 24 h of annealing Bn87Dg13 at 125 to 250 °C (Table 2). However, the textures were coarsest for temperatures between 125 and 190 °C (Figure 7), presenting typical basket-weave patterns. Samples annealed at 220 °C showed only very fine lamellae, and no obvious exsolution textures were obtained at temperatures higher than this. Experiments conducted using the three bdss compositions by Method 2 showed similar kinetic and textural evolution (runs A1−A4, B1−B4, C1, C4, and C8−C9). With prolonged annealing when coarsening appears to cease (after 2 h), one would expect compositional readjustment. However, there was no evidence for this; the lamellae were too small for accurate elemental analysis by EPMA and there was no significant change in the cell dimensions with annealing time (Table 2). In this respect the cell dimensions of our hydrothermally annealed samples (Table 2) are fully consistent with those reported by Grguric and Putnis.12 Note, however, that cell parameters are relatively insensitive to compositional change. Quantitative phase analysis confirmed that the proportions of bornite and digenite did not change with annealing time, within the analytical uncertainty on the order of ±10 wt %.

Figure 6. Backscattered electron image of the surface of one cracked sample (Run C4). The surface was prepared by directly breaking the grain with a needle, allowing the structure of the sample to be preserved without any damage caused by polishing. (A) The bornite lamellae are indicated by Bn, and the arrows show the high porosity in the digenite domains around bornite lamellae; the nonflat surface of the cracked sample, illustrating (B) the nanopores within the digenite domains and (C) the porosity along the bornite lamellae boundaries.

Samples annealed under Method 3 (“Annealing following quenching and reheating in the hydrothermal fluid”) showed exsolution textures at the center of the samples that were very similar to the patterns obtained in the dry annealing runs (Method 1), with the digenite lamellae (light gray) being fine F


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permeability to solution due to the reaction-generated porosity.8,10 This permeability was required to enable mass transport via the fluid to the reaction front from the bulk solution, without which the reaction would rapidly stall (e.g., Qian et al.23). The reaction-generated porosity created a large internal surface area. As the system entered the two-phase region, the surface energy term associated with the high porosity in the system became energetically significant; as a result, the bdss began to recrystallize, reducing surface area in the process. This recrystallization resulted in the coarsening of the bornite and digenite lamellae such that the porosity was now concentrated at the grain boundary between the lamellae and the host phase (Figure 6). A similar coalescence of fine porosity was observed by Putnis et al.24 in the replacement of KBr by K(Cl,Br) by an ICDR reaction. Once the composition of the solid equilibrated with that of the fluid, the textures of the K(Cl,Br) crystals further evolved via coarsening of pores and loss of permeability, starting at the outer surface of the crystal. After 12 days of re-equilibration, the outer rim of the K(Cl,Br) crystal became clear, with larger trapped fluid inclusions replacing the initial finely connected porosity. When samples were quenched to room temperature and then reheated to the annealing temperature in the hydrothermal fluid (Method 3), the resulting exsolution textures were very similar to those obtained via dry annealing (Method 1 and Grguric et al.13) except around the margins of the grains actually in contact with the fluid. This points to a coarsening process being driven by the presence of fluid and permeability inside the grains. The results of the Method 3 annealing runs indicated how rapidly the permeability decayed during quenching, and how sensitive the observed coarsening rates were to the cooling history of the grains. Figure 7 shows the coarseness of the exsolution textures for a series of Bn87Dg13 samples annealed under Method 2 for 24 h. It is clear from these images that the coarseness of the lamellae is dependent on the annealing temperatures, and decreases rapidly at 190 °C and above. This suggests that the separation of “Intermediate bornite” expected for bn-rich compositions above 190 °C (Figure 1c) is slow, and at these temperatures a single bdss phase remains and exsolution does not occur. The very fine exsolution textures we see in samples annealed at 220 °C with Method 2 are most likely formed during the process of quenching to room temperature. Altogether, these observations indicate that it is the fluid within the permeability microstructure of bdss formed via ICDR5,24,25 that plays the essential driving role in the coarsening process. Milke et al.26 determined that very small amounts of fluid (tens of ppm) in a solid silicate system are highly effective in catalyzing mineral reactions, via a process that they named “enhanced grain boundary diffusion”. In their case there was no free water phase, all H existing as OH in the silicates. However, in our experiments the differences between the three different methods strongly suggest that a free fluid is present in pores, and drives the coarsening and recrystallization processes. The quenching at the end of the hydrothermal bdss synthesis procedure in Methods 1 and 3 caused a very rapid breakdown in the porosity, and a sealing of the mineral system.24 This happened without a recrystallization-driven coarsening of the lamellae, but the trapped solution nevertheless coalesced into fluid inclusions at the grain boundaries, which is consistent with the observation of Zhao et al.5 for the same system. For Methods 1 and 3, the annealing of permeability microstructures commenced from the external

Figure 7. Backscattered electron SEM images showing the variation of exsolution coarseness with temperature. The exsolution patterns are for composition Bn87Dg13 annealed at (A) 125 °C (Run C1), (B) 150 °C (Run C4), (C) 170 °C (Run C8), (D) 190 °C (Run C9), (E) 220 °C (Run C10), and (F) 250 °C (Run C11) using annealing Method 2 for 1 day. Bornite lamellae are shown in dark gray while the digenite domains are illustrated in light gray. The lines and text boxes are the example of the measure of the length and width of the lamellae using Nikon images analysis software NIS-Elements.

and sharp (≤0.35 μm in width) (Figure 8; Table 2). However, the exsolution textures around the surface of the grains were similar to the coarse lamellae obtained in the hydrothermal runs (Method 2).

Figure 8. SEM images showing the exsolution pattern of Bn87Dg13 annealed using Method 3 for 15 days (run E4), with very fine lamellae of bornite (dark gray) and digenite (light gray) around the center of the sample (A), and the coarsening of lamellae around the surface of the sample (B).

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DISCUSSION The coarsening of the lamellae and the evolution of their microstructure were very different for the dry system (Method 1) compared to Method 2 used in this study. Method 1 produced 4.0 μm long bornite lamellae (range 1.89−7.78 μm; Table 1) over a 45-day (1080 h) annealing period. This is consistent with the study of Grguric et al.,13 who produced 10 μm long bornite lamellae via annealing under dry conditions for 30 weeks (5040 h) of bdss obtained by dry synthesis (Figure 1b).13 A similar coarseness was obtained in just 2 h in our hydrothermal experiments under Method 2 for a bdss of similar composition (Figure 5b). This implies that the coarsening of the lamellae under hydrothermal conditions using Method 2 is some 1000 times faster than via solid-state diffusion under dry conditions. We propose that the rapid coarsening process of hydrothermally synthesized bdss was driven by the presence of a fluid in the reaction-generated porosity in the bdss. Following the lowering of the temperature from the synthesis temperature (300 °C) to the annealing temperature, the bdss became metastable and nuclei of bornite and digenite formed. At annealing temperatures of 190 °C and below, all bdss compositions were now in the two-phase region of the phase diagram (Figure 1c). Since our bdss was synthesized via an ICDR reaction under hydrothermal conditions,5 it had high G


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In summary, in bdss, very fine-grained, but essentially complete phase separation occurs during quenching, as revealed by the powder diffraction data. The highly connected porosity associated with the formation of bdss under hydrothermal conditions via an ICDR mechanism facilitates rapid coarsening of the bornite and digenite lamellae by a recrystallization process during annealing.

surface of the grain; within the grains, some coarsening of the microtexture was observed locally around fluid inclusions, that is, as a result of the solution locked within the fluid inclusions including those at the reaction front with the unreacted chalcopyrite. When hydrothermally synthesized samples were cleaned and dried at room temperature, only the solution at the surface was removed. Within the samples annealed by Method 1 in evacuated silica tubes, the remaining fluid inclusions could have catalyzed the coarsening of the digenite lamellae within the metastable bdss. It is the same process as the coarsening of digenite lamellae in Method 2, but the reaction in Method 1 was relatively slowed down due to the limited volume of fluid in the fluid inclusions associated with the bdss-digenite interface. Sample annealing remobilized fluid at the reaction front, promoting textural coarsening. Hence the degree of coarsening decreased from the bdss-chalcopyrite reaction front to the grain surface (Figure 4b). Unfortunately, it was very difficult to directly measure the amount of fluid inclusions within the sample, because the water contents were low, around 3000 ppm according to the pore density estimated from SEM images of Bn87Dg13, and the fact that the pores were mostly sub-micron in size. The exsolution textures in the center of the samples annealed via Method 3 were similar to those obtained in Method 2. However, once the material quenched was reheated in the mother solution (Method 3), the outer layer of the grains would not be in chemical equilibrium with the solution, and the bulk solution would drive the coarsening of the lamellae from the grain surface inward toward the center, resulting in coarser microstructures at the grain surface, as observed in Figure 8. Since the permeability within bdss broke down during the quenching process, and the formation of new porosity would be slower at the annealing temperature, the coarsening rate was expected to be significantly lower than in the case of Method 2, as was indeed observed. The role of water in coarsening textures in alkali feldspars was established first in the 1970s.9,27−29 For example, Walker et al.28 described the transformation of a braided cryptoperthite (a fine-scale exsolution texture) to a coarser patch perthite in terms of a fluid-driven recrystallization that involved the dissolution of the cryptoperthite and the precipitation of the patch perthite, with little or no bulk compositional or volume change. There are two important differences between the coarsening of cryptoperthite and the very rapid coarsening of bdss described here: (i) in the coarsening of cryptoperthite to form a patch perthite there is a distinct reaction front that separates the parent cryptoperthite and the product patch perthite; (ii) the patch perthite has a distinctly microporous texture contrary to bdss, reflecting the fact that the porosity that drives the coarsening reaction in bdss is transient and essentially unquenchable even on laboratory time scales. These features are consistent with an ICDR reaction breaking down the cryptoperthite to a coarser mixture of albite and K-feldspar. The lack of a reaction front for the decomposition of bdss indicated that multiple nucleation sites were available for phase separation via the recrystallization reaction, and is consistent with the coarsening not being a classical ICDR process. The evidence of microporosity in the “exsolution” texture from the phase separation of bdss, as seen in Figure 6, and the sluggishness of the reaction using Method 3 are explained by the rapid coalescing of the porosity in the system into fluid inclusions during quenching.

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IMPLICATIONS An important feature revealed by our work is that in systems where ions are relatively mobile (such as many Cu−Fe sulfides), the nano- to microporosity and associated fluids that drive textural evolution are very challenging to study, since they are unquenchable. This prevents the direct observation of the mechanism driving textural evolution (e.g., ICDR associated with nanoscale fluid inclusions versus waterenhanced grains boundary diffusion). This illustrates the importance of experiments linking reaction kinetics and microstructural observations to decipher the nature of these reactions and identify the controls on the resulting mineral compositions and textures. This work reinforces the need for the interpretation of mineralogical textures in terms of geological processes to be based on sound and realistic experimental data, because in many cases reaction mechanisms are not preserved in the rock record. For example, the conclusions of the recent study of bornite microtextures by Ciobanu et al.18 were based on the dry phase diagram, ignoring the major effects of fluids and annealing history in controlling reaction pathways and textural evolution in these minerals. We also illustrate how the final microstructure and the kinetics of textural coarsening depend upon the crystallization history of the parent mineral−in this case the nanoporosity associated with the formation of bdss via ICDR reactions controls the subsequent evolution of the microtexture during cooling. Liu et al.30 provide another recent example in which dynamic recrystallization driven by the internal energy stored in the mineral (in their case most likely lattice strain due to the incorporation of As(III) in the apatite structure, as well as nanoscale chemical inhomogeneities, in addition to porosity) rapidly obliterated the original mineral composition and texture. On a fundamental level, our experiments provide a new example of the interaction among solid-state and fluid-driven reactions.31 After the different phases had nucleated, the rate of coarsening was strongly controlled by the presence of fluid. This suggests that the coarsening of exsolution-like textures does not necessarily arise via solid-state diffusion processes, but can be fluid-driven. The textures are similar in both cases, but the kinetics of coarsening differs by orders of magnitude, and the final microstructure and the kinetics of textural coarsening depend upon the crystallization and cooling history of the parent mineral. Given that hydrothermal alteration of orebodies is very widespread, it seems likely that many textures, such as symplectites, are the result of hydrothermal recrystallization involving dissolution−reprecipitation and/or fluid-enhanced grain boundary diffusion reactions associated with healing of porosity rather than solid-state processes sensu-stricto. It also seems likely to us that many inclusions embedded within sulfide mineral grains may also form by the same type of recrystallization processes, where porosity and chemical impurities coalesce into fluid inclusions and mineral inclusions. H


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(6) Tenailleau, C.; Pring, A.; Etschmann, B.; Brugger, J.; Studer, A. Transformation of pentlandite to violarite under mild hydrothermal conditions. Am. Mineral. 2006, 91, 706−709. (7) Xia, F.; Brugger, J.; Chen, G.; Ngothai, Y.; O’Neill, B.; Putnis, A.; Pring, A. Mechanism and kinetics of pseudomorphic mineral replacement reactions: A case study of the replacement of pentlandite by violarite. Geochim. Cosmochim. Acta 2009, 73, 1945−1969. (8) Putnis, A. Mineral replacement reactions. Rev. Mineral. Geochem. 2009, 70, 87−124. (9) Putnis, A. Mineral replacement reactions: from macroscopic observations to microscopic mechanisms. Mineral. Mag. 2002, 66, 689−708. (10) Putnis, A. Transient Porosity resulting from Fluid-Mineral Interaction and its consequences. Rev. Mineral. Geochem. 2015, 80, 1− 23. (11) Altree-Williams, A. L.; Pring, A.; Ngothai, Y.; Brugger, J. Textural and compositional complexities resulting from coupled dissolution-reprecipitation reactions in geomaterials. Earth-Sci. Rev. 2015, 150, 628−651. (12) Grguric, B. A.; Putnis, A. Rapid exsolution behaviour in the bornite−digenite series, and implications for natural ore assemblages. Mineral. Mag. 1999, 63, 1−14. (13) Grguric, B. A.; Harrison, R. J.; Putnis, A. A revised phase diagram for the bornite−digenite join from in situ neutron diffraction and DSC experiments. Mineral. Mag. 2000, 64, 213−231. (14) Edwards, A. B. Textures of the Ore Minerals and Their Significance; AusIMM: Melbourne, 1947. (15) Schwartz, G. M. Progress in the study of exsolution in ore minerals. Econ. Geol. Bull. Soc. Econ. Geol. 1942, 37, 345. (16) Schwartz, G. M. Textures due to unmixing of solid solutions. Econ. Geol. Bull. Soc. Econ. Geol. 1931, 26, 739−761. (17) Hardardóttir, V.; Hannington, M.; Hedenquist, J.; Kjarsgaard, I.; Hoal, K. Cu-Rich Scales in the Reykjanes Geothermal System, Iceland. Econ. Geol. Bull. Soc. Econ. Geol. 2010, 105, 1143−1155. (18) Ciobanu, C. L.; Cook, N. J.; Ehrig, K. Ore minerals down to the nanoscale: Cu-(Fe)-sulphides from the iron oxide copper gold deposit at Olympic Dam, South Australia. Ore Geol. Rev. 2017, 81, 1238−1235. (19) Brett, R. Experimental data from the system Cu-Fe-S and their bearing on exsolution textures in ores. Econ. Geol. Bull. Soc. Econ. Geol. 1964, 59, 1241−1269. (20) Brugger, J.; Etschmann, B.; Liu, W.; Testemale, D.; Hazemann, J. L.; Emerich, H.; van Beek, W.; Proux, O. An XAS study of the structure and thermodynamics of Cu(I) chloride complexes in brines up to high temperature (400 ◦C, 600 bar). Geochim. Cosmochim. Acta 2007, 71, 4920−4941. (21) Rietveld, H. M. A profile refinement method for nuclear and magnetic structures. J. Appl. Crystallogr. 1969, 2, 65−71. (22) TOPAS V4.2: General profile and structure analysis software for powder diffraction data; Bruker AXS GmbH: Karlsruhe, Germany, 2009. (23) Qian, G.; Brugger, J.; Skinner, W. M.; Chen, G.; Pring, A. An experimental study of the mechanism of the replacement of magnetite by pyrite up to 300 °C. Geochim. Cosmochim. Acta 2010, 74, 5610− 5630. (24) Putnis, C. V.; Tsukamoto, K.; Nishmura, Y. Direct observations of pseudomorphism: compositional and textural evolution at a fluid solid interface. Am. Mineral. 2005, 90, 1909−1912. (25) Xia, F.; Zhao, J.; Etschmann, B. E.; Brugger, J.; Garvey, C. J.; Rehm, C.; Lemmel, H.; Ilavsky, J.; Han, Y. S.; Pring, A. Characterization of porosity in sulfide ore minerals: A USANS/SANS study. Am. Mineral. 2014, 99, 2398−2404. (26) Milke, R.; Neusser, G.; Kolzer, K.; Wunder, B. Very little water is necessary to make a dry solid silicate system wet. Geology 2013, 41, 247−250. (27) Parsons, I. Feldspar and fluids in cooling plutons. Mineral. Mag. 1978, 42, 1−17. (28) Walker, F. D. L.; Lee, M. R.; Parsons, I. Micropores and micropermeable texture in alkali feldspars: geochemical and geophysical implications. Mineral. Mag. 1995, 59, 505−534.

For example, Figure 1a shows graphic intergrowths of digenite and bornite formed as scaling in the Reykjanes Geothermal System, Iceland.17 Indeed, many of the commonly occurring sulfide intergrowths in Cu deposits (e.g., Montana; Olympic Dam, Australia18) formed at temperatures above the miscibility gap of the mineral assemblage from hydrothermal fluids with complex composition and flow history; hence the original mineral assemblage must have exsolved into its endmembers upon cooling/late fluid flow. Note that protracted and/or multiple fluid flow events are characteristic of many large ore deposits (e.g., gold mineralization from Warrawoona Syncline, Pilbara, Western Australia,32 and the Olympic Dam Cu−Au−U−Ag deposit, South Australia33). Since the kinetic barriers for the nucleation of exsolved phases are lowered in the presence of fluids, resulting in high rates of cation transport at geologically low temperatures, the exsolution and coarsening process of bdss could be very rapid. Fluids could also play a significant role in the formation of exsolution textures among silicate minerals, for example in dehydration reactions during prograde metamorphism. It is widely recognized that myrmekites are of metasomatic origin,34 and it seems likely that myrmekitization may be driven by a process similar to the one discussed here. In the case of myrmekitization, there is a change in composition between parent and product. This is contrary to the coarsening of exsolution of bdss studied here, which is largely an isochemical process. In both systems, however, the presence of fluids increases element mobility and controls reaction pathways and kinetics.35

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AUTHOR INFORMATION

Corresponding Author

*E-mail: allan.pring@flinders.edu.au. ORCID

Jing Zhao: 0000-0002-5268-3646 Joel̈ Brugger: 0000-0003-1510-5764 Funding

Australian Research Council (Grants DP1095069 and DP140102765). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Aoife McFadden and Benjamin Wade from Adelaide Microscopy Center for their assistance in using the FESEM and electron microprobe, and Dr. Vigdis Hardardottir of the Iceland GeoSurvey for Figure 1a. We thank Associate Editor Chakraborty and the three anonymous reviewers for their constructive comments on the manuscript. This work has been made possible by the financial support of the Australian Research Council (Grants DP1095069 and DP140102765).

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REFERENCES

(1) Barnes, H. L. Geochemistry of Hydrothermal Ore Deposits, 3rd ed.; John Wiley & Sons: USA, 1997. (2) Robb, L. Introduction to Ore Forming Processes; Blackwell Publishing: Malden USA, 2005; pp 373. (3) Vaughan, D. J.; Craig, J. R. Mineral chemistry of Metal Sulfides; Cambridge University Press: U.K., 1978. (4) Vaughan, D. J. Sulfide Mineralogy and Geochemistry: Introduction and Overview. Rev. Mineral. Geochem. 2006, 61, 1−14. (5) Zhao, J.; Brugger, J.; Ngothai, Y.; Pring, A. The replacement of chalcopyrite by bornite under hydrothermal conditions. Am. Mineral. 2014, 99, 2389−2397. I


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