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Sulfur–selenium isomorphous substitution and morphotropic transition in the Ag3Au(Se,S)2 series
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Russian Geology and Geophysics 54 (2013) 646–651 www.elsevier.com/locate/rgg
Sulfur–selenium isomorphous substitution and morphotropic transition in the Ag3Au(Se,S)2 series Yu.V. Seryotkin a,b,*, G.A. Pal’yanova a, N.E. Savva c a
V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia b Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia c North-East Interdisciplinary Scientific Research Institute, named after N.A. Shilo, Far Eastern Branch of the Russian Academy of Sciences, ul. Portovaya 16, Magadan, 685000, Russia Received 17 May 2012; accepted 14 August 2012
Abstract Gold–silver sulfoselenides of the series Ag3AuSexS2–x (x = 0.25; 0.5; 0.75; 1; 1.5) were synthesized from melts on heating stoichiometric mixtures of elementary substances in evacuated quartz ampoules. According to X-ray _ single-crystal analysis, compound Ag3Au1Se0.5S1.5 has the structure of gold–silver sulfide Ag3AuS2 (uytenbogaardtite) with space group R3c. The volume of this compound is 1.5% larger than that of the sulfide analog. According to powder X-ray diffraction, compounds Ag3AuSe0.25S1.75 and Ag3AuSe0.75S1.25 also show trigonal symmetry. Compounds Ag3AuSeS and Ag3AuSe1.5S0.5 are structurally similar to the low-temperature modification of gold–silver selenide Ag3AuSe2 (fischesserite) with space group I4132. These data suggest the existence_of two solid solutions: petzite-type cubic Ag3AuSe2–Ag3AuSeS (space group I4132) and trigonal Ag3AuSe0.75S1.25–Ag3AuS2 (space group R3c). It was found that fischesserite from the Rodnikovoe deposit (southern Kamchatka) contains 3.5–4 wt.% S. At the Kupol deposit (Chukchi Peninsula), fischesserite contains up to 2.5 wt.% S and uytenbogaardtite contains up to 5.3 wt.% Se. At the Ol’cha and Svetloe (Okhotskoe) deposits (Magadan Region), uytenbogaardtite contains up to 0.5 and 1.8 wt.% Se, respectively. Literature data on the compositions of silver–gold selenides and sulfides from different deposits were summarized and analyzed. Analysis of available data on the S and Se contents of natural fischesserite and uytenbogaardtite confirms the miscibility gap near composition Ag3AuSeS. © 2013, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: uytenbogaardtite; fischesserite; solid-solution series Ag3Au(Se,S)2; morphotropic transition; structural features
Introduction Sulfur–selenium isomorphism is typical of many metal sulfides and selenides. There is experimental and natural evidence for continuous solid solutions PbS–PbSe (galena– clausthalite), FeS2–FeSe2 (pyrite–ferroselite), HgS–HgSe (metacinnabarite–tiemannite), and others (Earley, 1950; Sindeeva, 1964). The scattered available data on the compositions of uytenbogaardtite Ag3AuS2 and fischesserite Ag3AuSe2 (Botova et al., 1981; Greffié et al., 2002; Savva et al., 2012; Warmada et al., 2003) suggest S–Se isomorphism. Nekrasov et al. (1990) synthesized Ag3AuS2–Ag3AuSe2 compounds of both end and intermediate compositions. According to diffraction data, the phase synthesized during the crystallization of
* Corresponding author. E-mail address: [email protected] (Yu.V. Seryotkin)
Fe-containing sulfide melts at 150 °C (Pal’yanova et al., 2012) was uytenbogaardtite (low-temperature tetragonal modification of Ag3AuS2) (card no. 04-004-6511 (Kabekkodu, 2010)). However, the structure of uytenbogaardtite and the intermediate compositions Ag3AuS2–Ag3AuSe2 was not determined. Liujinyinite, discovered at three deposits in China (Chen et al., 1979), was described as another low-temperature polymorphic modification of uytenbogaardtite (Wei, 1981). According to recent determination, the crystal structure of synthesized sulfide Ag3AuS2 (Seryotkin et al., 2011) differs from that of fischesserite and belongs to a new type. This contradicts the statement about the continuous series of Ag3Au(Se,S)2 solid solutions. This work is aimed at summarizing literature and authors’ data on the compositions of uytenbogaardtite and fischesserite, synthesizing Ag3AuS2–Ag3AuSe2 sulfoselenides, studying S–Se isomorphism, and detecting the structure of the synthesized compounds.
1068-7971/$ - see front matter D 201 3, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.201 + 3.04.014
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Fig. 1. Sulfur and selenium contents (wt.%) of fischesserite and uytenbogaardtite from different deposits (authors’ and literature data). 1, ideal compositions of fischesserite, uytenbogaardtite, and sulfoselenide Ag3AuSeS; deposits: 2, Kupol; 3, Rodnikovoe; 4, Ulakhan; 5, Sergeevskoe; 6, Ozernovskoe; 7, Kubaka; 8, Al’fa; 9, Yakutskoe; 10, Krutoe; 11, Ol’cha; 12, Svetloe; 13, De Lamar (United States); 14, Nazareño (Peru); 15, Broken Hills (Australia); 16, Pongkor; 17, Cirotan (Indonesia).
Synthesis conditions and study methods To obtain Au–Ag sulfoselenides Ag3AuSexS2–x, a stoichiometric mixture of elements was prepared, with a slight excess of the chalcogen (~0.04 wt.% of the total sample mass) (Pal’yanova and Kokh, 2012). In the first run, samples with x = 0.5; 1.0; 1.5 were synthesized, and in the second one, with x = 0.25; 0.75. The mixture was put in a quartz miniampule, which was then placed at the bottom of a large ampoule; after that, a quartz rod was introduced to minimize the evaporation of volatiles and the outer ampoule was evacuated and sealed. The initial substances were Au and Ag (99.99%) as well as S and Se (99.9%); the total mass of the mixture was 500 mg (Mettler Instrument AG CH-8606, Greifensee, Zurich; weighing accuracy ±0.05 mg). The ampoules were heated to 1050 °C for three days at a rate of 0.2–0.5°/min, kept for 12 h, cooled to 500 °C at a rate of 0.2°/min, and annealed for seven days. After the annealing, the furnace was switched off and the ampoules cooled to room temperature for ~7 h. Uytenbogaardtite Ag3AuS2 and fischesserite Ag3AuSe2 were obtained at the same temperature of synthesis, annealing, and quenching (Pal’yanova and Kokh, 2012; Pal’yanova et al., 2011). The solid products of the experiments (a quarter of the samples annealed were mounted in a pellet, and microcrystals were placed on a substrate) and ore samples (polished sections) from the Kupol (Chukchi Peninsula), Rodnikovoe (southern Kamchatka), Ol’cha, and Svetloe (Okhotskoe) deposits (Magadan Region) were studied by optical and electron microscopy. The chemical composition of the phases was determined under a LEO 1430VP SEM with an INCA Energy 350+ EDS (Oxford Instruments Analytical) (accelerating volt-
age, 20 kV; current ~1 nA; live recording time in the analysis mode, 15 s). Standards used: for Au and Ag, a 750‰ Au–Ag alloy; for S, CuFeS2; and for Se, PbSe. In the study of natural samples, mainly large grains (>10 µm in size) were analyzed to avoid background amounts of elements from the surrounding phases. The detection limits of elements were tenths of a percent under these conditions. The analytical accuracy was 1.0–2.5 rel.% (for macrocomponents). The phases synthesized in the first run were studied on an Oxford Diffraction Xcalibur Gemini X-ray single-crystal diffractometer with a CCD detector (MoKα radiation, graphite monochromator, 50 kV, 30 mA). The samples in the second run were studied using a Thermo Scientific ARL X’TRA X-ray powder diffractometer with a Peltier-cooled Si(Li) solid-state detector (CuKα radiation, 40 kV, 40 mA).
Composition of synthesized and natural Au–Ag sulfoselenides The studies of the synthesized phases under optical and scanning electron microscopes showed their homogeneity and coincidence with the given compositions. The only exception is the experiment with the initial composition Ag3AuS1.5Se0.5. The SEM studies of the synthesis products revealed Ag5Au3Se microinclusions (<20 µm). Individual microhemispheres of native Se (~10 µm) and two microcrystals of a fine Au–Ag alloy (950‰) were present on the walls of the ampoule from this experiment. They might be the products of gas phase condensation. Microhemispheres of native Se were also present on the walls in the rest of the experiments.
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Table 1. Comparative data on chalcogenides Ag3AuX2 (X = Te, Se, (Se,S), S) System, space group
Vu.c., Å3
ρ, g/cm3
References
Cubic I4132
1120.0
9.20
(Khamid et al., 1978)
Cubic I4132
989.5
9.11
(Bindi and Cipriani, 2004)
Cubic I4132
977.41
8.90
Our data
–
Cubic I4132
959.54
8.75
Our data
(89.24)
Trigonal*
_ Trigonal R3c
257.2
8.60
Our data
942.74
8.57
Our data
Trigonal*
_ Trigonal R3c
938.2
8.44
Our data
928.80
8.36
(Seryotkin et al., 2011)
9.78
Tetragonal
931.6
8.34
(Barton et al., 1978)
9.81
Tetragonal
919.2
8.45
(Barton et al., 1978)
Compound
a, Å
c, Å (α, º)
Petzite Ag3AuTe2
10.385
Fischesserite Ag3AuSe2
9.965
–
Ag3AuSe1.5S0.5
9.924
–
Ag3AuSeS
9.863
Ag3AuSe0.75S1.25
9.856
Ag3AuSe0.5S1.5
9.806
(89.24)
Ag3AuSe0.25S1.75
9.790
(89.33)
Ag3AuS2
9.758
(89.26)
Uytenbogaardtite Ag3AuS2 9.76 Uytenbogaardtite Ag3AuS2 9.68 Ag3AuS2
9.75
9.85
Ag3AuS2
9.72
Liujinyinite Ag3AuS2
10.01
11.11
Tetragonal
936.4
8.30
(Graf, 1968)
Cubic P4132
918.33
8.46
(Messien et al., 1966)
Tetragonal
1113.2
6.98
(Wei, 1981)
Ag3AuSe1.6S0.4
9.894
–
Cubic
968.5
9.05
(Nekrasov et al.,, 1990)
Ag3AuSeS
9.844
–
Cubic
953.9
8.80
(Nekrasov et al., 1990)
Ag3AuSe0.6S1.4
9.804
–
Cubic
942.3
8.64
(Nekrasov et al., 1990)
Ag3AuSe0.25S1.75
9.763
–
Cubic
930.6
8.51
(Nekrasov et al., 1990)
Ag3AuS2
9.737
–
Cubic
923.2
8.41
(Nekrasov et al., 1990)
Note. The names of natural compounds are in bold. * X-ray powder diffraction.
Our study of the chemical composition of Au–Ag selenides and sulfides from the Kupol deposit (Chukchi Peninsula) revealed up to 1.2 wt.% S in fischesserite and up to 5.6 wt.% Se in uytenbogaardtite (Savva et al., 2012). Higher S contents (3.5–4.0 wt.%) were observed in fischesserite from the Rodnikovoe deposit (southern Kamchatka). Uytenbogaardtite from the Ol’cha and Svetloe (Okhotskoe) deposits contains up to 0.5 and 1.8 wt.% Se, respectively. Literature data were summarized on the compositions of fischesserite and uytenbogaardtite with admixtures of other chalcogenides. The S and Se contents of these minerals at some deposits in Russia and worldwide are shown in Fig. 1. Sulfur microadmixtures (0.02–0.2 wt.%) are present in fischesserite from the De Lamar (United States) (Bindi and Cipriani, 2004) and Ozernovskoe (Kamchatka) (Spiridonov et al., 2009) deposits. Fischesserite with up to 2.5 wt.% S was found at the Sergeevskoe (Botova et al., 1981) and Broken Hills (Australia) (Cocker et al., 2013) deposits. Uytenbogaardtite contains 0.3–1.1 wt.% Se at the following deposits: Cirotan, Pongkor (Indonesia) (Marcoux et al., 1993; Warmada et al., 2003), Nazareño (Peru) (Greffié et al., 2002), Al’fa (Nekrasov, 1997), Krutoe (Savva et al., 2010), and Ulakhan (Russia) (Savva and Pal’yanova, 2007). Up to 5.3 wt.% Se was observed in uytenbogaardtite from the Kubaka (Savva, 1995), Yakutskoe (Samusikov et al., 2002), and Broken Hills (Cocker et al., 2013) deposits. A miscibility gap is present near Ag3AuSeS (Fig. 1) despite the data scatter in the S and Se contents of fischesserite and uytenbogaardtite, which might be due to the micron size of the grains, inhomogeneous structure, and increased porosity of these minerals.
Results of X-ray single-crystal analysis and powder X-ray diffractometry The synthesized samples Ag3AuSe0.5S1.5, Ag3AuSeS, and Ag3AuSe1.5S0.5 were subjected to X-ray single-crystal analysis. The resulting data on the structure of synthesized sulfoselenides are shown in Table 1. Samples Ag3AuSe1.5S0.5 and Ag3AuSeS are isostructural with fischesserite (Bindi and Cipriani, 2004). Sample
Fig. 2. Variation in the unit-cell parameters of the synthesized samples of sulfoselenides Ag3AuSe2–Ag3AuS2. Circles show the values of the a-parameter for a cubic (pseudocubic) unit cell, and squares, those of the α-angle. Dashed line shows the hypothetical area of the morphotropic transition.
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Fig. 3. Powder XRD patterns (CuKα radiation) of uytenbogaardtite no. 3 (Barton et al., 1978) and no. 4 (Greffié et al., 2002), as well as its synthetic analog no. 2 (Graf, 1968), compared with the calculated XRD patterns of the trigonal phase Ag3AuS2 (no. 1) (Seryotkin et al., 2011) and the petzite-type cubic phase Ag3AuSeS (no. 5). Asterisks on pattern no. 4 show metal peaks.
_ Ag3AuSe0.5S1.5 has a trigonal structure (space group R3c) and is similar to the end-member Ag3AuS2, studied in (Seryotkin et al., 2011). In sample Ag3AuSeS, a small amount of the trigonal modification Ag3AuX2 coexists with the main (cubic) phase. The regular intergrowth of these phases is determined by the common topology of the anion lattice in both structural types (body-centered pseudocubic with cube edge variations between 4.04 and 4.27 Å). Comparison between the experimental and calculated values of the peak intensity yields a trigonal-phase content of 1–2%. The powder X-ray diffraction (XRD) patterns of the samples from the second run with Se/S < 1 correspond to the trigonal modification. The parameters of the unit cell, determined in the GSAS software (Larson and Von Dreele, 2000), are shown in Table 1. Thus, the cubic-phase boundary is near the composition Ag3AuSeS. At Se:S = 0.6 :1, only the trigonal phase exists, and its boundary is within the interval 0.6 :1 < Se:S < 1:1. Parameter a and the unit-cell volume in our samples decrease linearly with the portion of Se and increasing S portion in sulfoselenides (Fig. 2). The morphotropic transition is manifested in a jumpy decrease in the α-angle from 90° to 89.2°.
Discussion Compounds of the series Ag3AuX2 (X = S,Se,Te) have similar structures. Their anionic subsystem is a somewhat distorted body-centered cubic packing. The difference between fischesserite and Ag3AuS2 structures consists in the distribution of Ag+ and Au+ cations, while the anionic subsystems are topologically identical; this is the cause of their different symmetries (Seryotkin et al., 2011). The preserved anionic subsystem determines the linear dependences of the unit-cell volume throughout the composition interval. The morphotropic transition is due to the dimensional compression of the anion matrix and, correspondingly, shortening of Ag–Ag distances in edge-sharing tetrahedra to critical values of ≤3.0 Å (Bakakin and Seryotkin, 2012). Petzite and fischesserite structures are well-studied (Bindi and Cipriani, 2004; Khamid et al., 1978), whereas the data on uytenbogaardtite structure are scarce and ambiguous. They amount to the results of the powder X-ray diffraction, based on which a tetragonal unit cell is suggested for uytenbogaardtite (Barton et al., 1978). Synthetic samples of composition Ag3AuS2 were described both as tetragonal (Graf, 1968) and as cubic (Folmer et al., 1976; Llabres and Messien, 1968; Messien et al., 1966; Nekrasov et al., 1990; Smit et al.,
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Fig. 4. Powder XRD patterns (CuKα radiation) of synthetic samples Ag3AuS2 (no. 1), Ag3AuSe0.6S1.4 (no. 2), and Ag3AuSe2 (no. 3) (Nekrasov et al., 1990) compared with the calculated fischesserite XRD pattern (no. 4) (Bindi and Cipriani, 2004).
1970). The structural characteristics of Ag3AuX2 chalcogenides (X = Te, Se, (Se,S), S) are shown in Table 1. The only exception is liujinyinite (Chen et al., 1979) and its synthetic analog, described as another tetragonal polymorphic modification of uytenbogaardtite (Wei, 1981). Evidently, it has an abnormally low density with the given characteristics. The results of its study should be redetermined. The other data are close, and the differences are related to the unit-cell metrics choice. Powder XRD patterns of uytenbogaardtite and its synthetic analogs are compared with model diffraction patterns for the structure of the trigonal phase Ag3AuS2 (Seryotkin et al., 2011) and cubic phase Ag3AuSeS (Fig. 3). For the cubic phase, there are clear differences between the experimental and calculated data. On the other hand, close similarity to the trigonal-phase XRD pattern is observed in both the diffraction angles and intensities of the peaks. It is very likely that the selection of the tetragonal system for synthetic Ag3AuS2 by R.B. Graf (1968) and then for uytenbogaardtite by M.D. Barton (1980) was wrong but understandable. Their powder XRD data are described in a trigonal (rhombohedral) unit cell with a ≈ 9.76 Å, α ≈ 89.3°. Thus, uytenbogaardtite structure is identical to that of synthetic Ag3AuS2 (Seryotkin et al., 2011). The situation with the sulfoselenide samples synthesized by Nekrasov et al. (1990) is different. Their powder XRD patterns (Fig. 4) are much closer to the calculated diffraction pattern of fischesserite. The differences consist in the shift of the
diffraction peaks, owing to a regular change in the unit-cell metrics with composition, and in somewhat different intensity ratios for individual peaks. Note that an attempt to correlate the diffraction angles with the cubic parameters of the unit cell for the data from (Nekrasov et al., 1990) was unsuccessful. This could be attributed to the deviation from the cubic metrics of the unit cell. However, since the inconsistency extends to the synthetic analog of fischesserite, which must be cubic, the cause might be dramatic determination error. Nevertheless, these data indicate the existence of a continuous solid solution, which is isostructural with fischesserite, within the entire interval of compositions, from Se to S. Such dramatic differences might be due to the synthesis conditions: temperature, heating and annealing duration, and quenching regime. Most likely, it was the last factor that affected the results. For example, Nekrasov et al. (1990) cooled the synthesized samples in flowing cold water after their annealing at 300 °C. In our case, the furnace was switched off and the samples cooled to room temperature for 7 h. The slow cooling, probably, causes cation redistribution with the formation of the equilibrium trigonal phase. On the other hand, quenching preserves a metastable state, with a different distribution of cations over the sites in a cubic or pseudocubic structure. This hypothesis seems particularly convincing if there is a phase transition to Ag3Au(Se,S)2 at 160–270 °C depending on composition (Smit et al., 1970).
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Conclusions The study of the series Ag3AuSe2–Ag3AuS2 shows the existence of two solid solutions Ag3AuSe2–Ag3AuSeS with a petzite-type cubic structure (space group I4132) and Ag3AuSe0.75S_1.25–Ag3AuS2 with a trigonal structure of another type (R3c). Analysis of available data on the S and Se contents of natural fischesserite and uytenbogaardtite confirms the miscibility gap near composition Ag3AuSeS. We thank K.A. Kokh (Sobolev Institute of Geology and Mineralogy) for performing high-temperature experiments, N.S. Karmanov (Sobolev Institute of Geology and Mineralogy) for electron microprobe determination of the composition of minerals and synthetic phases, and V.M. Okrugin (Institute of Volcanology and Seismology) for providing Au–Ag ore samples from the Rodnikovoe deposit. The study was supported by the Russian Foundation for Basic Research (grant no. 11-05-00504a).
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Editorial responsibility: N.V. Sobolev
Russian Geology and Geophysics 54 (2013) 646–651 www.elsevier.com/locate/rgg
Sulfur–selenium isomorphous substitution and morphotropic transition in the Ag3Au(Se,S)2 series Yu.V. Seryotkin a,b,*, G.A. Pal’yanova a, N.E. Savva c a
V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia b Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia c North-East Interdisciplinary Scientific Research Institute, named after N.A. Shilo, Far Eastern Branch of the Russian Academy of Sciences, ul. Portovaya 16, Magadan, 685000, Russia Received 17 May 2012; accepted 14 August 2012
Abstract Gold–silver sulfoselenides of the series Ag3AuSexS2–x (x = 0.25; 0.5; 0.75; 1; 1.5) were synthesized from melts on heating stoichiometric mixtures of elementary substances in evacuated quartz ampoules. According to X-ray _ single-crystal analysis, compound Ag3Au1Se0.5S1.5 has the structure of gold–silver sulfide Ag3AuS2 (uytenbogaardtite) with space group R3c. The volume of this compound is 1.5% larger than that of the sulfide analog. According to powder X-ray diffraction, compounds Ag3AuSe0.25S1.75 and Ag3AuSe0.75S1.25 also show trigonal symmetry. Compounds Ag3AuSeS and Ag3AuSe1.5S0.5 are structurally similar to the low-temperature modification of gold–silver selenide Ag3AuSe2 (fischesserite) with space group I4132. These data suggest the existence_of two solid solutions: petzite-type cubic Ag3AuSe2–Ag3AuSeS (space group I4132) and trigonal Ag3AuSe0.75S1.25–Ag3AuS2 (space group R3c). It was found that fischesserite from the Rodnikovoe deposit (southern Kamchatka) contains 3.5–4 wt.% S. At the Kupol deposit (Chukchi Peninsula), fischesserite contains up to 2.5 wt.% S and uytenbogaardtite contains up to 5.3 wt.% Se. At the Ol’cha and Svetloe (Okhotskoe) deposits (Magadan Region), uytenbogaardtite contains up to 0.5 and 1.8 wt.% Se, respectively. Literature data on the compositions of silver–gold selenides and sulfides from different deposits were summarized and analyzed. Analysis of available data on the S and Se contents of natural fischesserite and uytenbogaardtite confirms the miscibility gap near composition Ag3AuSeS. © 2013, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: uytenbogaardtite; fischesserite; solid-solution series Ag3Au(Se,S)2; morphotropic transition; structural features
Introduction Sulfur–selenium isomorphism is typical of many metal sulfides and selenides. There is experimental and natural evidence for continuous solid solutions PbS–PbSe (galena– clausthalite), FeS2–FeSe2 (pyrite–ferroselite), HgS–HgSe (metacinnabarite–tiemannite), and others (Earley, 1950; Sindeeva, 1964). The scattered available data on the compositions of uytenbogaardtite Ag3AuS2 and fischesserite Ag3AuSe2 (Botova et al., 1981; Greffié et al., 2002; Savva et al., 2012; Warmada et al., 2003) suggest S–Se isomorphism. Nekrasov et al. (1990) synthesized Ag3AuS2–Ag3AuSe2 compounds of both end and intermediate compositions. According to diffraction data, the phase synthesized during the crystallization of
* Corresponding author. E-mail address: [email protected] (Yu.V. Seryotkin)
Fe-containing sulfide melts at 150 °C (Pal’yanova et al., 2012) was uytenbogaardtite (low-temperature tetragonal modification of Ag3AuS2) (card no. 04-004-6511 (Kabekkodu, 2010)). However, the structure of uytenbogaardtite and the intermediate compositions Ag3AuS2–Ag3AuSe2 was not determined. Liujinyinite, discovered at three deposits in China (Chen et al., 1979), was described as another low-temperature polymorphic modification of uytenbogaardtite (Wei, 1981). According to recent determination, the crystal structure of synthesized sulfide Ag3AuS2 (Seryotkin et al., 2011) differs from that of fischesserite and belongs to a new type. This contradicts the statement about the continuous series of Ag3Au(Se,S)2 solid solutions. This work is aimed at summarizing literature and authors’ data on the compositions of uytenbogaardtite and fischesserite, synthesizing Ag3AuS2–Ag3AuSe2 sulfoselenides, studying S–Se isomorphism, and detecting the structure of the synthesized compounds.
1068-7971/$ - see front matter D 201 3, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.201 + 3.04.014
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Fig. 1. Sulfur and selenium contents (wt.%) of fischesserite and uytenbogaardtite from different deposits (authors’ and literature data). 1, ideal compositions of fischesserite, uytenbogaardtite, and sulfoselenide Ag3AuSeS; deposits: 2, Kupol; 3, Rodnikovoe; 4, Ulakhan; 5, Sergeevskoe; 6, Ozernovskoe; 7, Kubaka; 8, Al’fa; 9, Yakutskoe; 10, Krutoe; 11, Ol’cha; 12, Svetloe; 13, De Lamar (United States); 14, Nazareño (Peru); 15, Broken Hills (Australia); 16, Pongkor; 17, Cirotan (Indonesia).
Synthesis conditions and study methods To obtain Au–Ag sulfoselenides Ag3AuSexS2–x, a stoichiometric mixture of elements was prepared, with a slight excess of the chalcogen (~0.04 wt.% of the total sample mass) (Pal’yanova and Kokh, 2012). In the first run, samples with x = 0.5; 1.0; 1.5 were synthesized, and in the second one, with x = 0.25; 0.75. The mixture was put in a quartz miniampule, which was then placed at the bottom of a large ampoule; after that, a quartz rod was introduced to minimize the evaporation of volatiles and the outer ampoule was evacuated and sealed. The initial substances were Au and Ag (99.99%) as well as S and Se (99.9%); the total mass of the mixture was 500 mg (Mettler Instrument AG CH-8606, Greifensee, Zurich; weighing accuracy ±0.05 mg). The ampoules were heated to 1050 °C for three days at a rate of 0.2–0.5°/min, kept for 12 h, cooled to 500 °C at a rate of 0.2°/min, and annealed for seven days. After the annealing, the furnace was switched off and the ampoules cooled to room temperature for ~7 h. Uytenbogaardtite Ag3AuS2 and fischesserite Ag3AuSe2 were obtained at the same temperature of synthesis, annealing, and quenching (Pal’yanova and Kokh, 2012; Pal’yanova et al., 2011). The solid products of the experiments (a quarter of the samples annealed were mounted in a pellet, and microcrystals were placed on a substrate) and ore samples (polished sections) from the Kupol (Chukchi Peninsula), Rodnikovoe (southern Kamchatka), Ol’cha, and Svetloe (Okhotskoe) deposits (Magadan Region) were studied by optical and electron microscopy. The chemical composition of the phases was determined under a LEO 1430VP SEM with an INCA Energy 350+ EDS (Oxford Instruments Analytical) (accelerating volt-
age, 20 kV; current ~1 nA; live recording time in the analysis mode, 15 s). Standards used: for Au and Ag, a 750‰ Au–Ag alloy; for S, CuFeS2; and for Se, PbSe. In the study of natural samples, mainly large grains (>10 µm in size) were analyzed to avoid background amounts of elements from the surrounding phases. The detection limits of elements were tenths of a percent under these conditions. The analytical accuracy was 1.0–2.5 rel.% (for macrocomponents). The phases synthesized in the first run were studied on an Oxford Diffraction Xcalibur Gemini X-ray single-crystal diffractometer with a CCD detector (MoKα radiation, graphite monochromator, 50 kV, 30 mA). The samples in the second run were studied using a Thermo Scientific ARL X’TRA X-ray powder diffractometer with a Peltier-cooled Si(Li) solid-state detector (CuKα radiation, 40 kV, 40 mA).
Composition of synthesized and natural Au–Ag sulfoselenides The studies of the synthesized phases under optical and scanning electron microscopes showed their homogeneity and coincidence with the given compositions. The only exception is the experiment with the initial composition Ag3AuS1.5Se0.5. The SEM studies of the synthesis products revealed Ag5Au3Se microinclusions (<20 µm). Individual microhemispheres of native Se (~10 µm) and two microcrystals of a fine Au–Ag alloy (950‰) were present on the walls of the ampoule from this experiment. They might be the products of gas phase condensation. Microhemispheres of native Se were also present on the walls in the rest of the experiments.
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Table 1. Comparative data on chalcogenides Ag3AuX2 (X = Te, Se, (Se,S), S) System, space group
Vu.c., Å3
ρ, g/cm3
References
Cubic I4132
1120.0
9.20
(Khamid et al., 1978)
Cubic I4132
989.5
9.11
(Bindi and Cipriani, 2004)
Cubic I4132
977.41
8.90
Our data
–
Cubic I4132
959.54
8.75
Our data
(89.24)
Trigonal*
_ Trigonal R3c
257.2
8.60
Our data
942.74
8.57
Our data
Trigonal*
_ Trigonal R3c
938.2
8.44
Our data
928.80
8.36
(Seryotkin et al., 2011)
9.78
Tetragonal
931.6
8.34
(Barton et al., 1978)
9.81
Tetragonal
919.2
8.45
(Barton et al., 1978)
Compound
a, Å
c, Å (α, º)
Petzite Ag3AuTe2
10.385
Fischesserite Ag3AuSe2
9.965
–
Ag3AuSe1.5S0.5
9.924
–
Ag3AuSeS
9.863
Ag3AuSe0.75S1.25
9.856
Ag3AuSe0.5S1.5
9.806
(89.24)
Ag3AuSe0.25S1.75
9.790
(89.33)
Ag3AuS2
9.758
(89.26)
Uytenbogaardtite Ag3AuS2 9.76 Uytenbogaardtite Ag3AuS2 9.68 Ag3AuS2
9.75
9.85
Ag3AuS2
9.72
Liujinyinite Ag3AuS2
10.01
11.11
Tetragonal
936.4
8.30
(Graf, 1968)
Cubic P4132
918.33
8.46
(Messien et al., 1966)
Tetragonal
1113.2
6.98
(Wei, 1981)
Ag3AuSe1.6S0.4
9.894
–
Cubic
968.5
9.05
(Nekrasov et al.,, 1990)
Ag3AuSeS
9.844
–
Cubic
953.9
8.80
(Nekrasov et al., 1990)
Ag3AuSe0.6S1.4
9.804
–
Cubic
942.3
8.64
(Nekrasov et al., 1990)
Ag3AuSe0.25S1.75
9.763
–
Cubic
930.6
8.51
(Nekrasov et al., 1990)
Ag3AuS2
9.737
–
Cubic
923.2
8.41
(Nekrasov et al., 1990)
Note. The names of natural compounds are in bold. * X-ray powder diffraction.
Our study of the chemical composition of Au–Ag selenides and sulfides from the Kupol deposit (Chukchi Peninsula) revealed up to 1.2 wt.% S in fischesserite and up to 5.6 wt.% Se in uytenbogaardtite (Savva et al., 2012). Higher S contents (3.5–4.0 wt.%) were observed in fischesserite from the Rodnikovoe deposit (southern Kamchatka). Uytenbogaardtite from the Ol’cha and Svetloe (Okhotskoe) deposits contains up to 0.5 and 1.8 wt.% Se, respectively. Literature data were summarized on the compositions of fischesserite and uytenbogaardtite with admixtures of other chalcogenides. The S and Se contents of these minerals at some deposits in Russia and worldwide are shown in Fig. 1. Sulfur microadmixtures (0.02–0.2 wt.%) are present in fischesserite from the De Lamar (United States) (Bindi and Cipriani, 2004) and Ozernovskoe (Kamchatka) (Spiridonov et al., 2009) deposits. Fischesserite with up to 2.5 wt.% S was found at the Sergeevskoe (Botova et al., 1981) and Broken Hills (Australia) (Cocker et al., 2013) deposits. Uytenbogaardtite contains 0.3–1.1 wt.% Se at the following deposits: Cirotan, Pongkor (Indonesia) (Marcoux et al., 1993; Warmada et al., 2003), Nazareño (Peru) (Greffié et al., 2002), Al’fa (Nekrasov, 1997), Krutoe (Savva et al., 2010), and Ulakhan (Russia) (Savva and Pal’yanova, 2007). Up to 5.3 wt.% Se was observed in uytenbogaardtite from the Kubaka (Savva, 1995), Yakutskoe (Samusikov et al., 2002), and Broken Hills (Cocker et al., 2013) deposits. A miscibility gap is present near Ag3AuSeS (Fig. 1) despite the data scatter in the S and Se contents of fischesserite and uytenbogaardtite, which might be due to the micron size of the grains, inhomogeneous structure, and increased porosity of these minerals.
Results of X-ray single-crystal analysis and powder X-ray diffractometry The synthesized samples Ag3AuSe0.5S1.5, Ag3AuSeS, and Ag3AuSe1.5S0.5 were subjected to X-ray single-crystal analysis. The resulting data on the structure of synthesized sulfoselenides are shown in Table 1. Samples Ag3AuSe1.5S0.5 and Ag3AuSeS are isostructural with fischesserite (Bindi and Cipriani, 2004). Sample
Fig. 2. Variation in the unit-cell parameters of the synthesized samples of sulfoselenides Ag3AuSe2–Ag3AuS2. Circles show the values of the a-parameter for a cubic (pseudocubic) unit cell, and squares, those of the α-angle. Dashed line shows the hypothetical area of the morphotropic transition.
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Fig. 3. Powder XRD patterns (CuKα radiation) of uytenbogaardtite no. 3 (Barton et al., 1978) and no. 4 (Greffié et al., 2002), as well as its synthetic analog no. 2 (Graf, 1968), compared with the calculated XRD patterns of the trigonal phase Ag3AuS2 (no. 1) (Seryotkin et al., 2011) and the petzite-type cubic phase Ag3AuSeS (no. 5). Asterisks on pattern no. 4 show metal peaks.
_ Ag3AuSe0.5S1.5 has a trigonal structure (space group R3c) and is similar to the end-member Ag3AuS2, studied in (Seryotkin et al., 2011). In sample Ag3AuSeS, a small amount of the trigonal modification Ag3AuX2 coexists with the main (cubic) phase. The regular intergrowth of these phases is determined by the common topology of the anion lattice in both structural types (body-centered pseudocubic with cube edge variations between 4.04 and 4.27 Å). Comparison between the experimental and calculated values of the peak intensity yields a trigonal-phase content of 1–2%. The powder X-ray diffraction (XRD) patterns of the samples from the second run with Se/S < 1 correspond to the trigonal modification. The parameters of the unit cell, determined in the GSAS software (Larson and Von Dreele, 2000), are shown in Table 1. Thus, the cubic-phase boundary is near the composition Ag3AuSeS. At Se:S = 0.6 :1, only the trigonal phase exists, and its boundary is within the interval 0.6 :1 < Se:S < 1:1. Parameter a and the unit-cell volume in our samples decrease linearly with the portion of Se and increasing S portion in sulfoselenides (Fig. 2). The morphotropic transition is manifested in a jumpy decrease in the α-angle from 90° to 89.2°.
Discussion Compounds of the series Ag3AuX2 (X = S,Se,Te) have similar structures. Their anionic subsystem is a somewhat distorted body-centered cubic packing. The difference between fischesserite and Ag3AuS2 structures consists in the distribution of Ag+ and Au+ cations, while the anionic subsystems are topologically identical; this is the cause of their different symmetries (Seryotkin et al., 2011). The preserved anionic subsystem determines the linear dependences of the unit-cell volume throughout the composition interval. The morphotropic transition is due to the dimensional compression of the anion matrix and, correspondingly, shortening of Ag–Ag distances in edge-sharing tetrahedra to critical values of ≤3.0 Å (Bakakin and Seryotkin, 2012). Petzite and fischesserite structures are well-studied (Bindi and Cipriani, 2004; Khamid et al., 1978), whereas the data on uytenbogaardtite structure are scarce and ambiguous. They amount to the results of the powder X-ray diffraction, based on which a tetragonal unit cell is suggested for uytenbogaardtite (Barton et al., 1978). Synthetic samples of composition Ag3AuS2 were described both as tetragonal (Graf, 1968) and as cubic (Folmer et al., 1976; Llabres and Messien, 1968; Messien et al., 1966; Nekrasov et al., 1990; Smit et al.,
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Fig. 4. Powder XRD patterns (CuKα radiation) of synthetic samples Ag3AuS2 (no. 1), Ag3AuSe0.6S1.4 (no. 2), and Ag3AuSe2 (no. 3) (Nekrasov et al., 1990) compared with the calculated fischesserite XRD pattern (no. 4) (Bindi and Cipriani, 2004).
1970). The structural characteristics of Ag3AuX2 chalcogenides (X = Te, Se, (Se,S), S) are shown in Table 1. The only exception is liujinyinite (Chen et al., 1979) and its synthetic analog, described as another tetragonal polymorphic modification of uytenbogaardtite (Wei, 1981). Evidently, it has an abnormally low density with the given characteristics. The results of its study should be redetermined. The other data are close, and the differences are related to the unit-cell metrics choice. Powder XRD patterns of uytenbogaardtite and its synthetic analogs are compared with model diffraction patterns for the structure of the trigonal phase Ag3AuS2 (Seryotkin et al., 2011) and cubic phase Ag3AuSeS (Fig. 3). For the cubic phase, there are clear differences between the experimental and calculated data. On the other hand, close similarity to the trigonal-phase XRD pattern is observed in both the diffraction angles and intensities of the peaks. It is very likely that the selection of the tetragonal system for synthetic Ag3AuS2 by R.B. Graf (1968) and then for uytenbogaardtite by M.D. Barton (1980) was wrong but understandable. Their powder XRD data are described in a trigonal (rhombohedral) unit cell with a ≈ 9.76 Å, α ≈ 89.3°. Thus, uytenbogaardtite structure is identical to that of synthetic Ag3AuS2 (Seryotkin et al., 2011). The situation with the sulfoselenide samples synthesized by Nekrasov et al. (1990) is different. Their powder XRD patterns (Fig. 4) are much closer to the calculated diffraction pattern of fischesserite. The differences consist in the shift of the
diffraction peaks, owing to a regular change in the unit-cell metrics with composition, and in somewhat different intensity ratios for individual peaks. Note that an attempt to correlate the diffraction angles with the cubic parameters of the unit cell for the data from (Nekrasov et al., 1990) was unsuccessful. This could be attributed to the deviation from the cubic metrics of the unit cell. However, since the inconsistency extends to the synthetic analog of fischesserite, which must be cubic, the cause might be dramatic determination error. Nevertheless, these data indicate the existence of a continuous solid solution, which is isostructural with fischesserite, within the entire interval of compositions, from Se to S. Such dramatic differences might be due to the synthesis conditions: temperature, heating and annealing duration, and quenching regime. Most likely, it was the last factor that affected the results. For example, Nekrasov et al. (1990) cooled the synthesized samples in flowing cold water after their annealing at 300 °C. In our case, the furnace was switched off and the samples cooled to room temperature for 7 h. The slow cooling, probably, causes cation redistribution with the formation of the equilibrium trigonal phase. On the other hand, quenching preserves a metastable state, with a different distribution of cations over the sites in a cubic or pseudocubic structure. This hypothesis seems particularly convincing if there is a phase transition to Ag3Au(Se,S)2 at 160–270 °C depending on composition (Smit et al., 1970).
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Conclusions The study of the series Ag3AuSe2–Ag3AuS2 shows the existence of two solid solutions Ag3AuSe2–Ag3AuSeS with a petzite-type cubic structure (space group I4132) and Ag3AuSe0.75S_1.25–Ag3AuS2 with a trigonal structure of another type (R3c). Analysis of available data on the S and Se contents of natural fischesserite and uytenbogaardtite confirms the miscibility gap near composition Ag3AuSeS. We thank K.A. Kokh (Sobolev Institute of Geology and Mineralogy) for performing high-temperature experiments, N.S. Karmanov (Sobolev Institute of Geology and Mineralogy) for electron microprobe determination of the composition of minerals and synthetic phases, and V.M. Okrugin (Institute of Volcanology and Seismology) for providing Au–Ag ore samples from the Rodnikovoe deposit. The study was supported by the Russian Foundation for Basic Research (grant no. 11-05-00504a).
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Editorial responsibility: N.V. Sobolev