Syntheses and Crystallization of Mineralogically RelevantChalcogenide Glasses

Hui Tao,z Allan Pring,y,z Fang Xia,y,z Joel Brugger,y,z Jing Zhao,y,z Shufen Wang,z and Guorong Chenw,z

zKey Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering,East China University of Science and Technology, Shanghai 200237, China

yDepartment of Mineralogy, South Australian Museum, North Terrace, Adelaide, SA 5000, Australia

zSchool of Earth and Environmental Sciences, University of Adelaide, North Terrace, Adelaide, SA 5005, Australia

Mineralogically relevant chalcogenide glasses with the compo-sition CuxBi5Ag5 (As0.33S0.335Se0.335)90�x (x5 2, 6, and 10, inmol%) that approximate the chemistry of some unusual sulfidemineral assemblages were synthesized. The differential thermalanalyses revealed that the glass transition temperature (Tg), on-set crystallization temperature (Tc), and peak temperature ofcrystallization (Tp) first increase significantly with an increasingCu content from 2 to 6 mol%, and then remain almost constantwhen the Cu content further increases to 10 mol%; the glassthermal stability also increases with an increasing Cu content.Both dry annealing and hydrothermal treatments were carriedout in the temperature range of 2071–3311C for up to 96 h.X-ray diffraction analyses on annealed samples showed thatdry annealing and hydrothermal treatments produced differentphase assemblages. Dry annealing produced crystallized phasesAgBiSe2 (bohdanowiczite), Cu2Se (berzelianite), Ag3AsS3(xanconthite), Cu3AsS4 (luzonite), AgBi3S5, and As4CuS9,while hydrothermal treatments produced phases AgBiSe2(bohdanowiczite), AgBi3S5, and AgAsSe2. This case studypresents a new way for studying the formation and alterationof minerals in nature.

I. Introduction

RECENT years have witnessed a growth in research on theformation and alteration of sulfide, selenide, and telluride

minerals under hydrothermal conditions in nature.1–3 Generally,the formation of minerals is a complex process and the processescan be classified into four broad mechanisms: formation frommelts, from hydrothermal fluids, by the actions of living cells, andby recrystallization in the solid state. The broad grouping of for-mation processes are not always distinct; for example, sulfide min-erals form from hydrothermal fluids and the melts cancrystallize into a mineral assemblage by solid-state diffusion pro-cesses or by dissolution–reprecipitation processes.4,5 Because meltsin nature are formed initially by quenching, a process similar tothe formation of chalcogenide glasses,6–10 it is therefore interestingto investigate the syntheses of some chalcogenide glasses of min-eralogically relevant compositions and their subsequent crystalli-zation. As subsequent crystallization can be practically achievedby both thermal annealing under dry and hydrothermal condi-tions, it will be interesting to see if these comparatively simplethermal-treatment regimes can be used to study the crystallization

of chalcogenide glasses. Such chalcogenide glasses also have greatpotential as homogeneous trace element standards for electronprobe microanalysis and laser ablation inductively coupled plasmamass spectroscopy (LA-ICP-MS).

Arsenic and bismuth are the major constituents of manymineral deposits, particularly those containing sulfides andsulfo-salts.7 In these and other deposits, arsenic commonly ac-companies with Cu, Ag, Au, Zn, Cd, Fe metals, etc. In this pa-per, we prepared Cu–Ag–Bi–As–S–Se glasses and then annealedthem under various conditions, both in dry condition and in thepresence of a hydrothermal fluid, in order to study the forma-tion of crystallized mineral assemblages.11 In this way, we cansee whether we obtain the same assemblage and whether thecompositions of the mineral phases are the same in both cases.

II. Experimental Procedure

The glasses with the composition CuxBi5Ag5 (As0.33S0.335Se0.335)90�x (x5 2, 6, and 10, in mol%, denoted as G1, G2,and G3, respectively) were chosen as starting materials for ther-mal annealing in the present study. Here, the elements Cu, Ag,and Bi were introduced intentionally into the stoichiometricbase glass As0.33S0.335Se0.335

12 so that the glass compositions areclose to some mineralogical assemblages. The glasses weresynthesized from high-purity elements (As, S, and Se, 5N;Ag, Bi, and Cu, 3N) using the conventional melt-quenchingmethod.13 Accurately weighed 5 g batches of each compositionwere sealed into silica ampoules in an oxy-coal gas flame under avacuum of 10�3 Pa and melted in a rocking furnace at 8501C for10 h. After that, the melts were rapidly quenched in cold waterand bulk glass samples were obtained. The glasses were thenannealed at temperatures 201C below the glass transition tem-perature (Tg) for 2 h to remove the inner stress.

The dry condition treatments were carried out by heating20 mg glass samples that were sealed into quartz tubes undervacuum in a muffle furnace at the peak temperature of crystal-lization (Tp) for 5 h. After that, the furnace was turned off andthe samples were allowed to cool to room temperature in thefurnace. For comparison, the hydrothermal annealing was per-formed in 25 mL polytetrafluoroethylene-lined stainless-steelautoclaves containing 40 mg of glass sample and 15 mL ofdeionized water. The reactions were conducted under autoge-nous pressures (o20 bar) and at a temperature about 301Cbelow the onset crystalline temperature (Tc) in order tocompensate for the effects of higher pressure. After annealingfor up to 96 h, the autoclaves were quenched in cold water andopened. Samples were then removed and rinsed (three times withdeionized water and once with acetone) before characterization.

The glass transition temperature (Tg), onset crystallizationtemperature (Tc), and peak temperature of crystallization (Tp) ofthe glasses were determined by differential thermal analysis

Z. Yang—contributing editor

This work was supported by Shanghai Leading Academic Discipline Project, No. B502,and Shanghai Rising-Star Program, No. 09QA1401500.

wAuthor to whom correspondence should be addressed. e-mail: grchen@ecust.edu.cn

Manuscript No. 27275. Received 18 December 2009; approved 4 March 2010.

Journal

J. Am. Ceram. Soc., 93 [9] 2434–2437 (2010)

DOI: 10.1111/j.1551-2916.2010.03772.x

r 2010 The American Ceramic Society

2434

(DTA), which was carried out on a Cary 1 DTA analyzer(Shanghai Tianping factory, Shanghai, China, precision70.21C). X-ray diffraction (XRD) patterns were collected ona Bruker D8 Advance powder X-ray diffractometer (Karlsruhe,Germany) with CuKa radiation. Scanning electron micrographsof the as-prepared glasses were taken on a Philips XL30 fieldemission scanning electron microscope (Eindhoven, the Nether-lands) (FESEM) operated at 15 kV acceleration voltage.Samples with a size of F10mm� 2mm were polished and evap-oratively coated with 15-nm-thick carbon films, and wereexamined under the backscatter electron (BSE) mode.

III. Results and Discussion

The as-synthesized samples are homogeneous glasses. This isproved by BSE micrographs (Fig. 1) and XRD analysis. Low

magnification micrographs (e.g., Fig. 1(a)) show that thesamples are chemically homogeneous but with a few bubbles(the dark spots) distributed across the sample. From high mag-nification images (Fig. 1(b)) and horizontal line scanning(Fig. 1(c)), we further confirm that the samples are chemicallyhomogenous. XRD patterns of the glasses (Figs. 2(a), 3(a), and4(a) present only a very broad diffraction peak, which is a typ-ical characteristic of the amorphous state.

The DTA curves for the chalcogenide glasses are shown inFig. 5. Clearly, all three samples show two exothermal peaks.For the glass with the lowest Cu content (G1), the two exother-mal peaks appear below 230 1C and are separated by only 231C.For glasses with a higher Cu content (G2 and G3), much-enhanced exothermal peaks were observed above 250 1C andtheir separation is about 601C.

Table I shows the characteristic temperatures for the glasses.Tg is determined as the extrapolated onset temperatures of theglass transition, while Tc is the initial temperature of the crys-tallization, and Tp is determined as the temperatures of themaximum crystallization rate. It is clear that all Tg, Tc, and Tp

first increase significantly with an increasing Cu content from2 mol% to 6 mol%, and then remain almost constant when theCu content further increases to 10 mol%. The difference be-tween Tg and Tc, DT, is a common measure of glass thermalstability. The values of DT of the three glasses are all below1001C but increase with an increasing Cu content. This meansthat glasses in the Cu–Ag–Bi–As–S–Se system have a relativelylow thermal stability but the introduction of Cu enhances thethermal stability of the glass samples.

Figure 2 shows the XRD patterns of glass G1 before dryannealing (a), after annealed at 2071C (b), and at 2301C (c) for5 h, and hydrothermally annealing at 1801C for 96 h (d),respectively. Under hydrothermal conditions, 96 h was foundto be the minimum time required to produce a crystalline as-semblage, as samples annealed with a hydrothermal fluid for72 h showed no evidence of crystallization. Dry annealing at alower temperature (2071C) (Fig. 2(b)) crystallizes the AgBiSe2phase (JCPD No. 74-0842) which corresponds to the mineralbohdanowiczite.11 Increasing the temperature of annealing un-der the dry condition to the second crystallization peak (2301C)(Fig. 2(c)) crystallizes Cu2Se (JCPD No. 65-2982) (the mineralberzelianite) and AgBi3S5 phases (JCPD No. 42-0559) (a com-position not known in nature). Annealing under the hydrother-mal condition (Fig. 2(d)), the principal phase found is AgBiSe2(bohdanowiczite), the same as that after dry annealing 2071C(Fig. 2(b)). In other words, the crystallization of AgBiSe2 in theCu–Ag–Bi–As–S–Se glass system under these lower temperature

Fig. 1. Back-scattered electron images of cross section of the sample G1with (a) low and (b) high magnifications. (c) A horizontal line scan of theG1 sample shows its chemical homogeneity.

Fig. 2. X-ray diffraction patterns of the G1 sample before dry anneal-ing (a), dry annealed at 2071C for 5 h (b), dry annealed at 2301C for 5 h(c), and hydrothermal treatment at 1801C for 96 h (d).

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dry annealing and hydrothermal conditions is consistent withAgBiSe2 being the most thermodynamically stable phase in themineralogical systems in nature.

For glass G3, similar heat treatments were undertaken ac-cording to the DTA curve. Annealing at the temperature corre-sponding to the first crystallization peak (2701C) for 5 h(Fig. 3(b)) crystallizes an assemblage consisting of AgBiSe2(JCPD No. 89-3673) (bohdanowiczite), As4CuS9 (JCPD No.47-1131), AgBi3S5 (JCPD No. 50-1645), and Ag3AsS3 (JCPDNo. 86-2354) (xanconthite). By increasing the annealing tem-perature to 3311C (Fig. 3(c)), the same diffraction patterns areobserved but all peaks become more intense, indicating a sig-nificant enhancement of the degree of crystallinity. Comparedwith dry annealing, hydrothermal annealing (Fig. 3(d)) is rela-tively less effective on glass G3 and only AgBi3S5 was found.Apparently, long-duration annealing at a high temperature isrequired under hydrothermal conditions to obtain an assem-blage similar to that by dry annealing.

The same annealing regime was employed on glass G2. Fig-ure 4 presents the XRD patterns of dry-annealed and hydro-thermally annealed samples. AgBi3S5 (JCPD No. 42-0559),

Cu3AsS4 (JCPD No. 10-0450) (luzonite), and AgBiSe2 (JCPDNo. 74-0842) (bohdanowiczite) phases are identified in the dry-annealed samples both at 2621 and 3301C for 5 h (Figs. 4(b) and(c)), and with an increase in the annealing temperature, the in-tensity of the diffraction peaks increases significantly. However,the hydrothermally annealed sample (Fig. 4(d)) shows an as-semblage quite different from those of glasses G1 and G3 in thattwo crystalline phases AgBiSe2 and AgAsSe2 (JCPD No. 34-0702) emerge.

Based on the above results, the effects of Cu content exerts oncrystallization behaviors of Cu–Ag–Bi–As–S–Se glasses can beimplied. With the different Cu content, thermally annealedsamples yield distinctly different crystalline assemblages. Theincreasing Cu content reduces the degree of crystallinity of thesamples when annealed under hydrothermal conditions, asevidenced by the weaker and broader lines in the diffractiontraces. This phenomenon could attribute to the enhancedthermal stability of the glass with increasing Cu content, andthis is also reflected in the increase of the DT values (Table I).

IV. Conclusion

We prepared Cu–Ag–Bi–As–S–Se glasses with compositionssimilar to mineralogical systems. DTA and XRD were used tocharacterize the crystallization behaviors of glass samples beforeand after thermal treatment. The crystallization of glass com-position G1 dry annealed at 2071C for 5 h yielded AgBiSe2 (themineral bohdanowiczite) in a glassy matrix phase. With the in-creasing of the Cu content, phases not found in nature wereformed during dry annealing. Hydrothermal treatments pro-duced similar phases. In particular, glass G1 hydrothermallytreated at 1801C for 96 h also yielded the mineral bohdanowic-zite. The present work presents a new way for studyingchalcogenides formation and alteration in nature. Furtherwork will be followed by systematic comparisons of dry andhydrothermal conditions to investigate the effect of the compo-sition of hydrothermal fluid on crystallization processes.

Fig. 3. X-ray diffraction patterns of the G3 sample before dry anneal-ing (a), dry annealed at 2701C for 5 h (b), dry annealed at 3311C for 5 h(c), and hydrothermal treatment at 1801C for 96 h (d).

Fig. 4. X-ray diffraction patterns of the G2 sample before dry anneal-ing (a), dry annealed at 2621C for 5 h (b), dry annealed at 3301C for 5 h(c), and hydrothermal treatment at 1801C for 96 h (d).

Fig. 5. Differential thermal analysis curves of the glass samples.

Table I. A Summary of Thermal Properties of the GlassSamples CuxBi5Ag5(As0.33S0.335Se0.335)90�x

Sample x Tg (1C) Tc (1C) Tp1 (1C) Tp2 (1C) DT (1C)

G1 2 147 197 207 230 50G2 6 176 245 262 330 69G3 10 164 254 270 331 90

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Acknowledgments

This project has benefited greatly from the collaboration between South Aus-tralian Museum and East China University of Science and Technology.

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