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Vibrational spectra of M3IMIIIS3 type synthetic minerals (MI = Tl or Ag and MIII = As or Sb)
Vibrational Spectroscopy 35 (2004) 59–65
Vibrational spectra of M3IMIIIS3 type synthetic minerals (MI ¼ Tl or Ag and MIII ¼ As or Sb) Petre Makreskia,*, Gligor Jovanovskia, Biljana Minceva-Sukarovaa, Bojan Soptrajanova, Alex Greenb, Bernward Engelenc, Ivan Grzeticd a
Institute of Chemistry, Faculty of Science, ‘‘Sv. Kiril i Metodij’’ University, Arhimedova 5, P.O. Box 162, Skopje MK-1001, Republic of Macedonia b Department of Chemistry, Imperial College of Science, Technology and Medicine, London SW7 2AY, UK c Anorganische Chemie, Universitat Siegen, Siegen 57068, Germany d Faculty of Mining and Geology, University of Belgrade, Djusina 7, Belgrade 11000, Serbia and Montenegro Received 1 October 2003; received in revised form 13 November 2003; accepted 13 November 2003 Available online 15 January 2004
Abstract The vibrational (Raman and far infrared) spectra of four M3IMIIIS3 type synthetic minerals (proustite, Ag3AsS3; ellisite, Tl3AsS3; pyrargyrite, Ag3SbS3; stibioellisite, Tl3SbS3) in the region from 600 to 100 cm1 (Raman) and 600 to 20 cm1 (far infrared) were investigated. Their infrared and Raman spectra were compared with corresponding vibrational spectra of natural orpiment, As2S3, and stibnite, Sb2S3. In general, rather expressed similarity between the Raman spectra of the M3IAsS3 type synthetic minerals, and particularly between the M3ISbS3 type synthetic minerals (MI ¼ Ag or Tl) was observed. This is, most probably, due to the presence of the MIIIS3 pyramids (MIII ¼ As or Sb) as main structural units in all minerals. The observed similarity between the Raman spectra of the M3IAsS3 type minerals and the corresponding spectrum of orpiment, As2S3 (also built up of MIIIS3 pyramids), justifies the treatment of those pyramids as a main vibrational units. The agreement between the Raman spectra of M3ISbS3 type minerals, on the one hand, and the Raman spectrum of stibnite, Sb2S3, on the other hand, is even much more pronounced. It was found, however, that the bands in the infrared spectra in the studied M3IMIIIS3 type minerals, besides the sensitivity to the MIII atom (As or Sb), are significantly influenced by the nature of the MI atom (Ag or Tl). This is most probably related to the different extent of the covalent character of the Ag–S bonds compared to the Tl–S bonds. # 2003 Published by Elsevier B.V. Keywords: Raman spectra; Infrared spectra; Synthetic minerals; Ellisite; Stibioellisite; Stibnite; Proustite; Pyrargyrite; Orpiment
1. Introduction Continuing the vibrational study of the minerals originating from the thallium well-known Allchar locality [1–6], we investigated the Raman and infrared spectra of two synthetic thallium analogues (ellisite, Tl3AsS3; stibioellisite, Tl3SbS3). Their appearance as natural mineral species at Allchar locality is undutiful, but very rare [7]. The vibrational study of the analogues pair of silver synthetic minerals (pyrargyrite, Ag3AsS3; proustite, Ag3SbS3), also present in the Allchar locality, has been parallely carried out in order to bring more relevant band assignation and spectralstructure correlation between Tl3MIIIS3 and Ag3MIIIS3 (MIII ¼ As or Sb) mineral types. For the ease of the interpretation, * Corresponding author. Tel.: þ389-2-3117-055; fax: þ389-2-3226-865. E-mail address: [email protected] (P. Makreski).
0924-2031/$ – see front matter # 2003 Published by Elsevier B.V. doi:10.1016/j.vibspec.2003.11.007
the Raman and infrared spectra [6] of the corresponding natural and simple sulfides, orpiment (As2S3) and stibnite (Sb2S3) which, similarly to the structures of M3IAsS3 [8,9] and M3ISbS3 type minerals [9,10] are built up of MIIIS3 pyramids (MIII ¼ As or Sb) [11,12], are involved. The infrared spectra of M3IMIIIS3 type synthetic minerals (MI ¼ Tl or Ag; MIII ¼ As or Sb), as well as the band assignment in the 500–180 cm1 region have been reported in our previous work [2]. In the present study the assignment of the lower frequency bands in the corresponding FT far-IR spectra of the same polycrystalline samples, was extended down to 20 cm1. For this assignment, the published reflectivity and infrared spectra of proustite, pyrargyrite and ellisite [13–15] were used. The Raman spectra of proustite [16,17] and pyrargyrite have also been published [17–19]. On the other hand, the Raman spectra of ellisite and stibioellisite, to the best of our knowledge, have not been previously studied.
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2. Experimental The synthetic minerals were prepared by the solid-state reactions in sealed quartz tubes [20,21]. The far infrared spectra of the studied samples were recorded on a Bruker 113v FTIR interferometer using mulls in Nujol between polyethylene plates. Jobin Yvon LabRam Infinity spectrometer with 532 nm laser line of a Nd-YAG frequency-double laser was employed for recording the presented Raman spectra. The measurements were carried out at a room temperature and spectral data were analysed with the GRAMS/32 software package. 3. Crystallographic data The most important structural parameters of the synthetic minerals of the type M3IMIIIS3 (MI ¼ Tl or Ag; MIII ¼ As or Sb) have been presented in our previous work [2]. However, to support the assignment of the bands in the Raman and far infrared spectra of these minerals, the structural differences and/or similarities between the Tl and Ag, on the one hand, as well as As and Sb containing minerals, on the other hand, are emphasised and discussed below: (a) The thallium containing minerals are isomorphous and crystallize in the rhombohedral R3m space group. The same crystal system, but different space group (R3c) is structural characteristic for the other isomorphous pair involving the silver atoms (Table 1). (b) In general, the structures of Tl and Ag containing minerals are complex, consisting of two types of MIII–S bonds and variety of MI–S bonds and S–MI–S angles as well as S–MIII–S angles. (Note: only bond distances are shown on Table 1).
(c) For each mineral, the main structural building element are trigonal MIIIS3 pyramids, consisted of one set of three identical shorter, and the other set of three identical longer M III–S (As–S and Sb–S) bond distances [8–10]. Consequently, the shorter As–S and Sb–S distances form the intra-layers which are mutually interconnected by the longer contacts of As S type (377.5 and 383.1 in ellisite and proustite, respectively) and Sb S type (360.0 and 375.9 pm in stibioellisite and pyrargyrite, respectively) (Table 1). The shorter As–S distances in arsenic containing synthetic minerals are very similar to each other (222.5 and 225.4 pm in ellisite and proustite, respectively). Similarly, the shorter Sb–S distances in antimony containing analogues stibioellisite and pyrargyrite are also very close to each other (243.1 and 246.3 pm, respectively) (see Table 1). All As–S and Sb–S distances in the structures of As2S3 [11] and Sb2S3 [12], respectively, are non-equivalent (Table 1). (d) Six sulfur atoms surround each MI atom in ellisite (Tl3AsS3) and pyrargyrite (Ag3SbS3), while in stibioellisite (Tl3SbS3) and proustite (Ag3AsS3), the MI atoms are surrounded with five sulfur atoms. Thus, there are three shorter and three longer MI–S bonds in ellisite and pyrargyrite and three shorter and two longer MI–S in stibioellisite and proustite. Contrary to the two sets of two identical Tl–S distances in ellisite and stibioellisite, there are not identical Ag–S bonds in both silver containing minerals, proustite and pyrargyrite. It is to be noted that the coordination of the MI atoms is different—Tl atoms are surrounded by three S atoms in a trigonal-pyramidal arrangement, contrary to the essentially digonal coordination of Ag atoms by S atoms.
Table 1 Some more important structural parameters in the studied As and Sb containing mineralsa Space group As minerals Orpiment As2S3
P21/n (C52h )
Proustite Ag3AsS3
R3c (C63v )
Ellisite Tl3AsS3
R3m (C53v )
Sb minerals Stibnite Sb2S3
r(MI–S) (pm) MI ¼ Tl or Ag
(1) (1) (1) (2) (2)
244.3, 288.6, 342.1 304.9, 344.3,
R3c (C63v )
Stibioellisite Tl3SbS3
R3m (C53v )
(1) (1) (1) (2) (1)
243.2, 288.4, 354.8, 303.0, 308.4,
Structural type
(6) 224–231 (1) 244.8, (1) 318.4,
(3) 225.4, (3) 383.1
Layer-like structure; As–S–As–S– chains of AsS3 pyramids Three-dimensional network of regular AsS3 pyramids with essentially two-coordinated Ag atoms
(1) 309.8, (1) 389.5
(3) 222.5, (3) 377.5
Three-dimensional network of regular AsS3 pyramids with Tl-polyhedra through As–S–Tl bridges
(8) (6) (3) (3)
Crumpled sheet-type struc-ture; infinite (Sb4S6)n chains of SbS3 and squared pyramids Three-dimensional network of regular SbS3 pyramids with essentially two-coordinated Ag atoms
Pnma (D16 2h )
Pyrargyrite Ag3SbS3
r(MIII–S) (pm) MIII ¼ As or Sb
(1) 245.0, (1) 314.6, (1) 366.8 (2) 346.1
245–285, 311–364 246.3, 375.9
(3) 243.1, (3) 360.0
Three-dimensional network of regular SbS3 pyramids with Tl-polyhedra through Sb–S–Tl bridges
a The values given in Table 1 are newly calculated on the basis of the published unit cell parameters and atomic coordinates for sulfosalts given in refs. [8–10].
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In accordance with the mode predictions made by factor group analysis [6A1 ðIRÞ þ 7A2 þ 13EðIRÞ] [22], six modes of A1 symmetry and 13 modes of E symmetry for E||c and for E?c polarisation, respectively, are observed in the infrared reflectivity spectra of proustite [13]. Due to the common structural properties, the same types and number of spectral modes are expected in the infrared reflectivity spectra of ellisite, pyrargyrite and stibioellisite. It is well known that, for this type of minerals with complex structure, the selection of a unique polyatomic vibrational group to whose the observed infrared bands of
the studied polycrystalline sample could be attributed is not a straightforward task. In spite of that, in our previous work [2], the interpretation of the infrared spectra of these synthetic minerals (in the region from 500 to 180 cm1) was based on the existence of the MIIIS3 pyramids as main vibrational units. In general, one would expect that the common crystal structures of these minerals (Table 1) could be associated with the similarity in their vibrational (infrared and Raman) spectra. This is indeed the case, both with the far infrared (Fig. 1) and also with the Raman spectra (Fig. 2). As it has already been proposed [2], the vibrational bands in the higher frequency region (above 280 cm1), are mainly due to the vibrations involving the identical MIII–S bonds, whereas in the low-frequency range (bellow 130 cm1) the vibrations including MI atom could be expected. It has been confirmed by the detailed analysis of the far infrared spectra
Fig. 1. The FT far IR spectra of the studied arsenic and antimony containing minerals.
Fig. 2. The FT Raman spectra of the studied arsenic and antimony containing minerals.
4. Results and discussion 4.1. Interpretation of the FT IR spectra of Ag3AsS3, Tl3AsS3, Ag3SbS3 and Tl3SbS3
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Table 2 The frequencies (in cm1) and the assignment of the bands in the FT IR spectra of the studied synthetic minerals Assignation
Proustite, Ag3AsS3
n(MIII–S3) n(MIII–S3) d(MIII–S3) n(MI–S) n(MI–S) n(MI–S) d(S–MI–S) d(S–MI–S)
362 335 264 134 118 103 90 62
d(S–MI–S)
sha vs s vw vw w sh vw
34 m 24 w
Ellisite, Tl3AsS3
Pyrargyrite, Stibioellisite, Ag3SbS3 Tl3SbS3
353 325 203 109
315 sh 304 vs 248 s
sh vs vs w
78 s 52 sh
100 vw 90 vw 59 vw
324 295 183 114 94 70 47
sh vs vs w w m sh
34 m 26 sh
a vs: very strong; s: strong; m: medium; w: weak; vw: very weak; sh: shoulder.
shown in Fig. 1. Namely, one weak (expressed as a shoulder) and one strong band are registered in the infrared spectra of both As containing minerals proustite and ellisite in the region above 280 cm1. The corresponding bands at somewhat lower frequency (about 30 cm1) have been also observed in the spectra of the Sb containing minerals pyrargyrite and stibioellisite (Fig. 1 and Table 2). The lower frequency of the bands in the spectra of the Sb containing minerals could be taken as an additional evidence that they are due to the MIII–S3 stretching modes. On the other hand, the frequency closeness between the n(As–S3) modes and also between the n(Sb–S3) modes in the infrared spectra of the threethioarsenates(III) and threethioantimonates(III), respectively, agrees well with the presence of very similar As–S distances in the structure of the former pair as well as between the Sb–S distances in the latter pair of the synthetic minerals (Table 2). It should be mentioned, however, that all n(MIII–S3) modes in the spectra of the Ag containing synthetic minerals are observed at higher frequency compared to the corresponding n(MIII–S3) modes in the spectra of the Tl containing minerals. It could be taken as an indication that As–S and Sb–S stretching vibrations are not pure being partly influenced by the contribution of other types of vibrational modes. Instead of two, even seven bands (for orpiment) and five bands (for stibnite) are assigned in the corresponding infrared spectra in the above discussed region of the stretching MIII–S modes [6] (See Fig. 1). It is due, however, to the presence of six and eight different As–S (224–231 pm) and Sb–S (245–285 pm) distances in the structures of orpiment [11] and stibnite [12], respectively, compared to the structurally equivalent As–S as well as Sb–S distances in the structures of As containing proustite (Ag3AsS3) [9] and ellisite (Tl3AsS3) [8] and Sb containing pyrargyrite (Ag3SbS3) [9] and stibioellisite (Tl3SbS3) [10] (Table 1). Additional six longer Sb–S distances (311–364 pm) are registered in the structures of stibnite [12].
As seen from Fig. 1 and Table 2, going towards the lower frequencies, the strong band at 264 and 248 cm1 in the Ag containing minerals proustite (Ag3AsS3) and pyrargyrite (Ag3SbS3) as well as a very strong band at 203 and 183 cm1 in the Tl containing minerals ellisite (Tl3AsS3) and stibioellisite (Tl3SbS3) appears. As it was previously mentioned [2], the frequency of these bands depends on the nature of the MI atom, being higher (for about 60 cm1) in the infrared spectra of the Ag compounds compared to their Tl analogues. On the other hand, the substitution of Sb by As atom in the silver pair of minerals as well as in the thallium pair of minerals causes blue shifting for about 20 cm1 (Table 2). The above mentioned band frequency dependence on the MIII atom (As and Sb) as well as on the MI atom (Ag and Tl) should be taken as a strong evidence that in addition to the d(S–MIII–S) normal modes, the n(MI–S) coordinates also participate in the potential energy distribution. Thus, the mentioned bands should be basically prescribed to S–As–S or S–Sb–S bending modes mixed with the considerable contribution of the MI–S stretchings, especially pronounced in the case of the Ag compounds. Namely, the more pronounced Ag–S stretching character is reasonable if the stronger Ag–S bonds (compared to the corresponding Tl– S bonds) are assumed (in an approximation of MI–S diatomic oscillators). This assumption is confirmed by following arguments: (i) the smaller difference in electronegativity between Ag and S atoms (compared to Tl and S atoms) implicates higher covalent character of Ag–S bonds; (ii) the shortest observed Ag–S distances (244.3 and 243.2 pm for Ag3AsS3 and Ag3SbS3, respectively) are smaller than the sum of the atomic radii (247 pm) and also smaller than the sum of the effective ionic radii (245 pm) [23] of silver and sulfur, establishing the mostly covalent bond character. On the other hand, experimentally determined Tl–S distances (304.9 and 303.0 for Tl3AsS3 and Tl3SbS3, respectively) are significantly longer than the sum of the corresponding atomic (273 pm) radii but smaller than the sum of the effective ionic radii (334 pm) [23] of thallium and sulfur. The minerals containing the same MI atom in the structure exhibit very common spectral features in the infrared region below 130 cm1, relating the origin of these bands to the MI–S vibrations. Thus, the weak band at 114 cm1 and the strong one at 82 cm1 in the IR spectrum of ellisite, Tl3AsS3 attributed to the Tl–S vibrations [15], in our spectrum are found at somewhat lower frequencies, 109 and 78 cm1, respectively. The very weak band with the same origin observed by Povarennykh and Gerasimenko [15] at 166 cm1 has not been registered in our spectrum, whereas the low frequency band appeared as a shoulder at 52 cm1 in our spectrum, has not been mentioned in the previous work [15]. The same set of two bands at 114 and 70 cm1 are observed in the spectrum of stibioellisite, Tl3SbS3, accompanied by an additional weak band at 94 cm1 (Fig. 1 and Table 2). Similarly to the spectrum of ellisite, these three bands are attributed to the n(Tl–S) modes. The corresponding Ag–S stretching vibrations in the spectra of the Ag
P. Makreski et al. / Vibrational Spectroscopy 35 (2004) 59–65
containing synthetic minerals are prescribed to the bands at 134, 118 and 103 cm1 in proustite and at 100 cm1 in pyrargyrite (no separated bands have been observed around 135 and 120 cm1 in the latter spectrum—the broadening of the lower-frequency side of the intense band at 248 cm1 appears instead) (Fig. 1). The bands (shoulders) at 52 and 47 cm1 in the infrared spectra of ellisite and stibioellisite, respectively, are prescribed to the bending S–Tl–S modes. The corresponding S– Ag–S bending vibrations in the spectra of the analogous Ag compounds proustite and pyrargyrite appear at about 40 cm1 higher frequency (90 cm1). The remaining two bands in the spectrum of proustite (62 and 34 cm1) and pyrargyrite (59 and 34 cm1) are probably due to the d(S– Tl–S) modes (see forthcoming discussion concerning the Raman spectra). The bands below 100 cm1 in the spectra of both Ag containing minerals are observed at very close (almost identical) frequencies (Table 2). Taking this fact into consideration, it seems that Ag–S modes are less influenced by the contribution of the MIII–S coordinates. In other words, these bands are probably mainly due to the pure Ag–S vibrations which additionally justifies the above supported presumption about the existence of stronger Ag–S force constant in the structures of Ag containing synthetic minerals compared to the Tl containing analogues. The evident larger number of the bands due to Ag–S stretchings and S–Ag–S bendings (Fig. 1 and Table 2) corresponds to the larger number of non-equivalent Ag–S distances in the Ag compounds compared to the corresponding Tl–S distances in the Tl analogues (Table 1), especially in the case of pyrargyrite vs. stibioellisite. 4.2. Interpretation of the FT Raman spectra of Ag3AsS3, Tl3AsS3, Ag3SbS3 and Tl3SbS3 The Raman spectra of the studied synthetic minerals accompanied with the spectra of the natural minerals orpiment and stibnite are shown in Fig. 2, whereas the observed band intensities and frequencies are given in Table 3. According to the group theory the first-order Raman spectrum of the investigated minerals in C3v phase should consist of 6A1 and 13E modes, being selected by the X(ZZ)Y and X(ZX)Y scattering geometry, respectively [19]. The previously discussed structural similarities/differences in the studied minerals served as a basic starting point for the interpretation of their Raman spectra. Namely, having in mind that the MIII–S3 pyramids are the main vibrational units, both M3IAsS3 compounds (proustite and ellisite—consisted of AsS3 pyramids) exhibit very similar Raman spectra, whereas in the case of M3ISbS3 type minerals (pyrargyrite and stibioellisite), where SbS3 pyramids are present in the structure, the similarity is even more pronounced and the spectra seems to be almost identical (Fig. 2). In order to facilitate the general assignment of the bands attributed to the MIII–S stretching, S–MIII–S bending as well
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Table 3 The frequencies (in cm1) and the assignment of the bands in the FT Raman spectra of the studied synthetic minerals Assignation
Proustite, Ag3AsS3
Ellisite, Tl3AsS3
n(MIII–S3) n(MIII–S3) n(MIII–S3) n(MIII–S3) d(MIII–S3) d(MIII–S3) n(MI–S) n(MI–S) d(S–MI–S)
– 360 334 265 217 179 128 117 103
390 371 304 – 201 178 – 115 –
a
vs s vw vw m vw vw vw
wa vs w w m vw
Pyrargyrite, Ag3SbS3
Stibioellisite, Tl3SbS3
448 370 325 250 – 187 137 116 107
447 369 324 250 – 186 142 116 104
m w w vs s vw w vw
m w vw vs s w w vw
vs: very strong; s: strong; m: medium; w: weak; vw: very weak.
as MI–S stretching modes, the recently obtained results of the Raman study of orpiment and stibnite (As2S3 and Sb2S3) [6], where the AsS3 and SbS3 pyramids are also the main constituents of their structures, were included in the present study (Fig. 2). The results of the Raman spectral study of Ag containing minerals proustite and pyrargyrite published by Ewen et al. [16–19] were also taken into account. According to the literature data for the Raman spectrum of orpiment [6,24–26], two groupings of the bands are proposed and observed in the 600–100 cm1 spectral region: (i) set of the internal-mode bands from 130 to 200 cm1; (ii) set of the internal-mode bands between 300 and 400 cm1. The Raman spectra of the studied As containing minerals proustite and ellisite in the 600–100 cm1 region are closely related to the corresponding spectrum of orpiment. The similarities in the spectra are influenced by the common layer structures among the studied minerals. Namely, the As–S intra-layer distances are significantly shorter compared to the corresponding ones between the layers (225.4 pm versus 383.1 pm in the proustite and 222.5 pm versus 377.5 pm in the ellisite structure). The corresponding shortest intra-layer As–S distance and smallest inter-layer As S contacts in the structure of orpiment are 224 and 356 pm, respectively [11]. Thus, as in orpiment, the layer symmetry is dominant and determines the symmetry and the activity of the normal modes. However, the strongest Raman band in the spectra of proustite (360 cm1) and ellisite (371 cm1) is attributed to the symmetric stretching of the pyramidal As–S3 groups. Its blue frequency shifting for 11 cm1 in the spectrum of ellisite compared to proustite spectrum (Fig. 2 and Table 3) is due to the shortening of the As–S bonds from 225.4 pm in the structure of proustite [9] to 222.5 pm in the structure of ellisite [8] (Table 1). The both remaining highfrequency bands in the spectrum of proustite (334, 265 cm1) and ellisite (390 and 304 cm1) are also due to the stretching As–S3 Raman active modes. The band at 179 cm1 in proustite spectrum and at 178 cm1 in the spectrum of ellisite is probably mainly due to the d(As– S3) modes which could be mixed with the n(MI–S) modes.
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The decreasing of the number of bands due to the n(As–S3) modes in the Raman spectra of proustite (two) and ellisite (three) compared to five observed bands in the corresponding spectrum of orpiment in the 400–300 cm1 spectral region (Fig. 2 and Table 3) is in consistence with the existence of only one type of As–S distances in the structure of former two minerals, compared to the six different As–S distances in the structure of the latter mineral (Table 1). The evident differences between the Raman spectrum of orpiment, on the one hand, and the Raman spectra of proustite and ellisite, on the other hand, in the spectral region below 160 cm1 could be taken as a serious suggestion that the remaining three weak bands in the Raman spectrum of proustite (128, 117 and 103 cm1) as well as the weak one in the spectrum of ellisite (115 cm1) are due to the modes where MI atoms (Ag or Tl) are included. Thus, they are attributed to the MI–S stretching (first two in proustite and the unique one in ellisite) and to the S–MI– S bending vibrations (the lowest frequency one in proustite) (Table 3). Similarly to the Raman spectra of the As containing samples, the strongest bands in the corresponding spectra of stibnite (247 cm1) [6], pyrargyrite (250 cm1) and stibioellisite (250 cm1) are assigned to the stretching Sb–S vibrations (Table 3). The remaining three higher frequency weak or medium bands in the spectra of the pyrargyrite (448, 370 and 325 cm1) and stibioellisite (447, 369 and 324 cm1) also seems not to be affected from the nature of the MI atom, being therefore attributed to the n(MIII–S) modes. Two analogue higher-frequency bands with the similar frequencies (443 and 366 cm1) are also observed in the Raman spectrum of stibnite (Fig. 2) being previously attributed to the n(Sb–S) modes [6]. It confirms the presumption that the SbS3 and squared pyramids (as the main structural constituents, see Table 1) could be considered as a rather isolated vibrators, justifying the assignation of these bands as mainly pure n(Sb–S) vibrations. However, it should be noted that, very often, different polarisation effects as well as the extent of grinding of the studied samples [27] could influence the spectral view and consequently, the comparison of the Raman spectra is not always a straightforward task. It is well illustrated in the case of our Raman spectrum of stibnite [6] which, due to the above mentioned influences, is somewhat different from the reported Raman spectrum of the single crystal of stibnite [28]. Therefore, in this case, it is not recommendable to compare the number of the bands in the Raman spectrum of stibnite with the number of the observed bands in the corresponding spectra of pyrargyrite and stibioellisite, and to relate them with their structural characteristics. The similarity between the Raman spectra of pyrargyrite and stibioellisite also continues in the lower frequency region (200–100 cm1) where additional four bands appear in each spectrum. The highest frequency one (at 187 cm1, in pyrargyrite and at 186 cm1 in stibioellisite) are strong, whereas the remaining three (at 137, 116 and 107 cm1, in
pyrargyrite and at 142, 116 and 104 cm1, in stibioellisite) are weak (Fig. 2 and Table 3). Only two strong bands, however, appear in this region in the Raman spectrum of stibnite at 184 and 140 cm1 (Fig. 2) [6]. The former one (184 cm1) exhibits practically the same frequency as well as intensity with the highest frequency bands in the spectra of pyrargyrite (187 cm1) and stibioellisite (186 cm1). This could be taken as a strong evidence that their origin is still related to the presence of Sb–S3 pyramids in the structure and consequently could be assigned as mainly due to the d(Sb–S3) modes which could be eventually mixed with the n(MI–S) modes. Similar spectral behaviour has been observed in the case of the Raman spectrum of orpiment for the band at 178 cm1 [6] which appears at the same frequency as the corresponding band in the spectrum of proustite (179 cm1) and ellisite (178 cm1) (Fig. 2 and Table 3). Contrary to our assignation, in the phase transition study of single pyrargyrite crystal, based on its X(ZZ)Y and X(ZX)Y Raman polarizability spectra at various temperatures this band was prescribed solely to the Ag–S stretchings [19,29]. The above mentioned differences between the Raman spectrum of stibnite, on the one hand, and the Raman spectra of pyrargyrite and stibioellisite, on the other hand, in the spectral region below 140 cm1 strongly indicate that the remaining three weak bands in the Raman spectrum of pyrargyrite (137, 116 and 107 cm1) as well as in the spectrum of stibioellisite (142, 116 and 104 cm1) are related to the modes where MI atoms (Ag or Tl) are included and therefore are assigned as being due to the MI–S stretching (first two) and S–MI–S bending vibrations (the lowest frequency ones) (Table 3). The bands with the same origin in the Raman spectrum of pyrargyrite are also registered below 100 cm1 [17–19] but the limited performances of our instrument makes their registration impossible. References [1] M. Trajkovska, B. Soptrajanov, G. Jovanovski, T. Stafilov, J. Mol. Struct. 267 (1992) 191. [2] B. Soptrajanov, M. Trajkovska, I. Grzetic, G. Jovanovski, T. Stafilov, N. Jb. Miner. Abh. 166 (1993) 83. [3] M. Trajkovska, B. Soptrajanov, T. Stafilov, G. Jovanovski, Geol. Maced. 7 (1993) 55. [4] B. Soptrajanov, M. Trajkovska, G. Jovanovski, T. Stafilov, N. Jb. Miner. Abh. 167 (1994) 329. [5] B. Soptrajanov, M. Trajkovska, T. Stafilov, G. Jovanovski, I. Grzetic, Spectrosc. Lett. 30 (1997) 79. [6] B. Minceva-Sukarova, G. Jovanovski, P. Makreski, B. Soptrajanov, W. Griffith, R. Willis, I. Grzetic, J. Mol. Struct. 651–653 (2003) 181. [7] B. Boev, V. Bermanec, T. Serafimovski, S. Lepitkova, S. Mikulcic, M. Soufek, G. Jovanovski, T. Stafilov, M. Najdoski, Geol. Maced. 15–16 (2001–2002) 1. [8] M. Gostojic, Z. Kristallorg. 151 (1980) 249. [9] P. Engel, W. Nowacki, N. Jb. Miner. Mh. 1966 (1966) 181. [10] N. Rey, J.C. Jumas, J. Olivier-Fourcade, E. Philippot, Acta Crystallogr. 40C (1984) 1655. [11] D.J.E. Mullen, W. Nowacki, Z. Kristallogr. 136 (1972) 48. [12] P. Bayliss, W. Nowacki, Z. Kristallogr. 135 (1972) 308.
P. Makreski et al. / Vibrational Spectroscopy 35 (2004) 59–65 [13] N.N. Syrbu, V.Z. Cebotari, M. Bogdanash, Spectrochim. Acta 52 (1996) 1193. [14] D.F. Baisa, D.D. Kolendritskii, M.I. Golovei, I.Y. Roman, A.I. Kovalenko, Fiz. Tverd. Tela 30 (1988) 573. [15] A.S. Povarennykh, V.S. Gerasimenko, Miner. Zh. 3 (1981) 16. [16] W. Taylor, P.J.S. Ewen, T. Han, Ferroelectrics 55 (1984) 751. [17] P.J.S. Ewen, W. Taylor, G.L. Paul, J. Phys. C 16 (1983) 6475. [18] P.J.S. Ewen, W. Taylor, Solid State Commun. 45 (1983) 227. [19] P.J.S. Ewen, T. Han, W. Taylor, Solid State Commun. 52 (1984) 1041. [20] G.M. Moh, L.A. Taylor, N. Jb. Miner. Mh. 450 (1971) 1971. [21] I. Grzetic, XII Congress of Geologists of Yugoslavia, vol. II, Mineralogy and Petrology, Ohrid, 1990, p. 38.
[22] [23] [24] [25] [26] [27]
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L.A. Rebane, K.E. Haller, Fiz. Tverd. Tela 24 (1982) 2351. R.D. Shannon, Acta Crystallogr. A32 (1976) 752. R. Zallen, M.L. Slade, A.T. Ward, Phys. Rev. B 3 (1971) 4257. R. Zallen, M.L. Slade, Phys. Rev. B 9 (1974) 1627. M.L. Slade, R. Zallen, Solid State Commun. 30 (1979) 357. G. Jovanovski, B. Minceva-Sukarova, P. Makreski, B. Soptrajanov, W.P. Griffith, R.L. Willis, in: J. Mink, G. Jalsovszky, G. Keresztury (Eds.), Proceedings of the XVIII International Conference on Raman Spectroscopy, Budapest, Hungary, Wiley, Chichester, 2002, p. 931. [28] T.P. Mernagh, A.G. Trudu, Chem. Geol. 103 (1993) 113. [29] H.H. Byer, L.C. Bobb, I. Lefkowitz, B.S. Deaver, Ferroelectrics 5 (1973) 207.
Vibrational spectra of M3IMIIIS3 type synthetic minerals (MI ¼ Tl or Ag and MIII ¼ As or Sb) Petre Makreskia,*, Gligor Jovanovskia, Biljana Minceva-Sukarovaa, Bojan Soptrajanova, Alex Greenb, Bernward Engelenc, Ivan Grzeticd a
Institute of Chemistry, Faculty of Science, ‘‘Sv. Kiril i Metodij’’ University, Arhimedova 5, P.O. Box 162, Skopje MK-1001, Republic of Macedonia b Department of Chemistry, Imperial College of Science, Technology and Medicine, London SW7 2AY, UK c Anorganische Chemie, Universitat Siegen, Siegen 57068, Germany d Faculty of Mining and Geology, University of Belgrade, Djusina 7, Belgrade 11000, Serbia and Montenegro Received 1 October 2003; received in revised form 13 November 2003; accepted 13 November 2003 Available online 15 January 2004
Abstract The vibrational (Raman and far infrared) spectra of four M3IMIIIS3 type synthetic minerals (proustite, Ag3AsS3; ellisite, Tl3AsS3; pyrargyrite, Ag3SbS3; stibioellisite, Tl3SbS3) in the region from 600 to 100 cm1 (Raman) and 600 to 20 cm1 (far infrared) were investigated. Their infrared and Raman spectra were compared with corresponding vibrational spectra of natural orpiment, As2S3, and stibnite, Sb2S3. In general, rather expressed similarity between the Raman spectra of the M3IAsS3 type synthetic minerals, and particularly between the M3ISbS3 type synthetic minerals (MI ¼ Ag or Tl) was observed. This is, most probably, due to the presence of the MIIIS3 pyramids (MIII ¼ As or Sb) as main structural units in all minerals. The observed similarity between the Raman spectra of the M3IAsS3 type minerals and the corresponding spectrum of orpiment, As2S3 (also built up of MIIIS3 pyramids), justifies the treatment of those pyramids as a main vibrational units. The agreement between the Raman spectra of M3ISbS3 type minerals, on the one hand, and the Raman spectrum of stibnite, Sb2S3, on the other hand, is even much more pronounced. It was found, however, that the bands in the infrared spectra in the studied M3IMIIIS3 type minerals, besides the sensitivity to the MIII atom (As or Sb), are significantly influenced by the nature of the MI atom (Ag or Tl). This is most probably related to the different extent of the covalent character of the Ag–S bonds compared to the Tl–S bonds. # 2003 Published by Elsevier B.V. Keywords: Raman spectra; Infrared spectra; Synthetic minerals; Ellisite; Stibioellisite; Stibnite; Proustite; Pyrargyrite; Orpiment
1. Introduction Continuing the vibrational study of the minerals originating from the thallium well-known Allchar locality [1–6], we investigated the Raman and infrared spectra of two synthetic thallium analogues (ellisite, Tl3AsS3; stibioellisite, Tl3SbS3). Their appearance as natural mineral species at Allchar locality is undutiful, but very rare [7]. The vibrational study of the analogues pair of silver synthetic minerals (pyrargyrite, Ag3AsS3; proustite, Ag3SbS3), also present in the Allchar locality, has been parallely carried out in order to bring more relevant band assignation and spectralstructure correlation between Tl3MIIIS3 and Ag3MIIIS3 (MIII ¼ As or Sb) mineral types. For the ease of the interpretation, * Corresponding author. Tel.: þ389-2-3117-055; fax: þ389-2-3226-865. E-mail address: [email protected] (P. Makreski).
0924-2031/$ – see front matter # 2003 Published by Elsevier B.V. doi:10.1016/j.vibspec.2003.11.007
the Raman and infrared spectra [6] of the corresponding natural and simple sulfides, orpiment (As2S3) and stibnite (Sb2S3) which, similarly to the structures of M3IAsS3 [8,9] and M3ISbS3 type minerals [9,10] are built up of MIIIS3 pyramids (MIII ¼ As or Sb) [11,12], are involved. The infrared spectra of M3IMIIIS3 type synthetic minerals (MI ¼ Tl or Ag; MIII ¼ As or Sb), as well as the band assignment in the 500–180 cm1 region have been reported in our previous work [2]. In the present study the assignment of the lower frequency bands in the corresponding FT far-IR spectra of the same polycrystalline samples, was extended down to 20 cm1. For this assignment, the published reflectivity and infrared spectra of proustite, pyrargyrite and ellisite [13–15] were used. The Raman spectra of proustite [16,17] and pyrargyrite have also been published [17–19]. On the other hand, the Raman spectra of ellisite and stibioellisite, to the best of our knowledge, have not been previously studied.
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2. Experimental The synthetic minerals were prepared by the solid-state reactions in sealed quartz tubes [20,21]. The far infrared spectra of the studied samples were recorded on a Bruker 113v FTIR interferometer using mulls in Nujol between polyethylene plates. Jobin Yvon LabRam Infinity spectrometer with 532 nm laser line of a Nd-YAG frequency-double laser was employed for recording the presented Raman spectra. The measurements were carried out at a room temperature and spectral data were analysed with the GRAMS/32 software package. 3. Crystallographic data The most important structural parameters of the synthetic minerals of the type M3IMIIIS3 (MI ¼ Tl or Ag; MIII ¼ As or Sb) have been presented in our previous work [2]. However, to support the assignment of the bands in the Raman and far infrared spectra of these minerals, the structural differences and/or similarities between the Tl and Ag, on the one hand, as well as As and Sb containing minerals, on the other hand, are emphasised and discussed below: (a) The thallium containing minerals are isomorphous and crystallize in the rhombohedral R3m space group. The same crystal system, but different space group (R3c) is structural characteristic for the other isomorphous pair involving the silver atoms (Table 1). (b) In general, the structures of Tl and Ag containing minerals are complex, consisting of two types of MIII–S bonds and variety of MI–S bonds and S–MI–S angles as well as S–MIII–S angles. (Note: only bond distances are shown on Table 1).
(c) For each mineral, the main structural building element are trigonal MIIIS3 pyramids, consisted of one set of three identical shorter, and the other set of three identical longer M III–S (As–S and Sb–S) bond distances [8–10]. Consequently, the shorter As–S and Sb–S distances form the intra-layers which are mutually interconnected by the longer contacts of As S type (377.5 and 383.1 in ellisite and proustite, respectively) and Sb S type (360.0 and 375.9 pm in stibioellisite and pyrargyrite, respectively) (Table 1). The shorter As–S distances in arsenic containing synthetic minerals are very similar to each other (222.5 and 225.4 pm in ellisite and proustite, respectively). Similarly, the shorter Sb–S distances in antimony containing analogues stibioellisite and pyrargyrite are also very close to each other (243.1 and 246.3 pm, respectively) (see Table 1). All As–S and Sb–S distances in the structures of As2S3 [11] and Sb2S3 [12], respectively, are non-equivalent (Table 1). (d) Six sulfur atoms surround each MI atom in ellisite (Tl3AsS3) and pyrargyrite (Ag3SbS3), while in stibioellisite (Tl3SbS3) and proustite (Ag3AsS3), the MI atoms are surrounded with five sulfur atoms. Thus, there are three shorter and three longer MI–S bonds in ellisite and pyrargyrite and three shorter and two longer MI–S in stibioellisite and proustite. Contrary to the two sets of two identical Tl–S distances in ellisite and stibioellisite, there are not identical Ag–S bonds in both silver containing minerals, proustite and pyrargyrite. It is to be noted that the coordination of the MI atoms is different—Tl atoms are surrounded by three S atoms in a trigonal-pyramidal arrangement, contrary to the essentially digonal coordination of Ag atoms by S atoms.
Table 1 Some more important structural parameters in the studied As and Sb containing mineralsa Space group As minerals Orpiment As2S3
P21/n (C52h )
Proustite Ag3AsS3
R3c (C63v )
Ellisite Tl3AsS3
R3m (C53v )
Sb minerals Stibnite Sb2S3
r(MI–S) (pm) MI ¼ Tl or Ag
(1) (1) (1) (2) (2)
244.3, 288.6, 342.1 304.9, 344.3,
R3c (C63v )
Stibioellisite Tl3SbS3
R3m (C53v )
(1) (1) (1) (2) (1)
243.2, 288.4, 354.8, 303.0, 308.4,
Structural type
(6) 224–231 (1) 244.8, (1) 318.4,
(3) 225.4, (3) 383.1
Layer-like structure; As–S–As–S– chains of AsS3 pyramids Three-dimensional network of regular AsS3 pyramids with essentially two-coordinated Ag atoms
(1) 309.8, (1) 389.5
(3) 222.5, (3) 377.5
Three-dimensional network of regular AsS3 pyramids with Tl-polyhedra through As–S–Tl bridges
(8) (6) (3) (3)
Crumpled sheet-type struc-ture; infinite (Sb4S6)n chains of SbS3 and squared pyramids Three-dimensional network of regular SbS3 pyramids with essentially two-coordinated Ag atoms
Pnma (D16 2h )
Pyrargyrite Ag3SbS3
r(MIII–S) (pm) MIII ¼ As or Sb
(1) 245.0, (1) 314.6, (1) 366.8 (2) 346.1
245–285, 311–364 246.3, 375.9
(3) 243.1, (3) 360.0
Three-dimensional network of regular SbS3 pyramids with Tl-polyhedra through Sb–S–Tl bridges
a The values given in Table 1 are newly calculated on the basis of the published unit cell parameters and atomic coordinates for sulfosalts given in refs. [8–10].
P. Makreski et al. / Vibrational Spectroscopy 35 (2004) 59–65
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In accordance with the mode predictions made by factor group analysis [6A1 ðIRÞ þ 7A2 þ 13EðIRÞ] [22], six modes of A1 symmetry and 13 modes of E symmetry for E||c and for E?c polarisation, respectively, are observed in the infrared reflectivity spectra of proustite [13]. Due to the common structural properties, the same types and number of spectral modes are expected in the infrared reflectivity spectra of ellisite, pyrargyrite and stibioellisite. It is well known that, for this type of minerals with complex structure, the selection of a unique polyatomic vibrational group to whose the observed infrared bands of
the studied polycrystalline sample could be attributed is not a straightforward task. In spite of that, in our previous work [2], the interpretation of the infrared spectra of these synthetic minerals (in the region from 500 to 180 cm1) was based on the existence of the MIIIS3 pyramids as main vibrational units. In general, one would expect that the common crystal structures of these minerals (Table 1) could be associated with the similarity in their vibrational (infrared and Raman) spectra. This is indeed the case, both with the far infrared (Fig. 1) and also with the Raman spectra (Fig. 2). As it has already been proposed [2], the vibrational bands in the higher frequency region (above 280 cm1), are mainly due to the vibrations involving the identical MIII–S bonds, whereas in the low-frequency range (bellow 130 cm1) the vibrations including MI atom could be expected. It has been confirmed by the detailed analysis of the far infrared spectra
Fig. 1. The FT far IR spectra of the studied arsenic and antimony containing minerals.
Fig. 2. The FT Raman spectra of the studied arsenic and antimony containing minerals.
4. Results and discussion 4.1. Interpretation of the FT IR spectra of Ag3AsS3, Tl3AsS3, Ag3SbS3 and Tl3SbS3
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Table 2 The frequencies (in cm1) and the assignment of the bands in the FT IR spectra of the studied synthetic minerals Assignation
Proustite, Ag3AsS3
n(MIII–S3) n(MIII–S3) d(MIII–S3) n(MI–S) n(MI–S) n(MI–S) d(S–MI–S) d(S–MI–S)
362 335 264 134 118 103 90 62
d(S–MI–S)
sha vs s vw vw w sh vw
34 m 24 w
Ellisite, Tl3AsS3
Pyrargyrite, Stibioellisite, Ag3SbS3 Tl3SbS3
353 325 203 109
315 sh 304 vs 248 s
sh vs vs w
78 s 52 sh
100 vw 90 vw 59 vw
324 295 183 114 94 70 47
sh vs vs w w m sh
34 m 26 sh
a vs: very strong; s: strong; m: medium; w: weak; vw: very weak; sh: shoulder.
shown in Fig. 1. Namely, one weak (expressed as a shoulder) and one strong band are registered in the infrared spectra of both As containing minerals proustite and ellisite in the region above 280 cm1. The corresponding bands at somewhat lower frequency (about 30 cm1) have been also observed in the spectra of the Sb containing minerals pyrargyrite and stibioellisite (Fig. 1 and Table 2). The lower frequency of the bands in the spectra of the Sb containing minerals could be taken as an additional evidence that they are due to the MIII–S3 stretching modes. On the other hand, the frequency closeness between the n(As–S3) modes and also between the n(Sb–S3) modes in the infrared spectra of the threethioarsenates(III) and threethioantimonates(III), respectively, agrees well with the presence of very similar As–S distances in the structure of the former pair as well as between the Sb–S distances in the latter pair of the synthetic minerals (Table 2). It should be mentioned, however, that all n(MIII–S3) modes in the spectra of the Ag containing synthetic minerals are observed at higher frequency compared to the corresponding n(MIII–S3) modes in the spectra of the Tl containing minerals. It could be taken as an indication that As–S and Sb–S stretching vibrations are not pure being partly influenced by the contribution of other types of vibrational modes. Instead of two, even seven bands (for orpiment) and five bands (for stibnite) are assigned in the corresponding infrared spectra in the above discussed region of the stretching MIII–S modes [6] (See Fig. 1). It is due, however, to the presence of six and eight different As–S (224–231 pm) and Sb–S (245–285 pm) distances in the structures of orpiment [11] and stibnite [12], respectively, compared to the structurally equivalent As–S as well as Sb–S distances in the structures of As containing proustite (Ag3AsS3) [9] and ellisite (Tl3AsS3) [8] and Sb containing pyrargyrite (Ag3SbS3) [9] and stibioellisite (Tl3SbS3) [10] (Table 1). Additional six longer Sb–S distances (311–364 pm) are registered in the structures of stibnite [12].
As seen from Fig. 1 and Table 2, going towards the lower frequencies, the strong band at 264 and 248 cm1 in the Ag containing minerals proustite (Ag3AsS3) and pyrargyrite (Ag3SbS3) as well as a very strong band at 203 and 183 cm1 in the Tl containing minerals ellisite (Tl3AsS3) and stibioellisite (Tl3SbS3) appears. As it was previously mentioned [2], the frequency of these bands depends on the nature of the MI atom, being higher (for about 60 cm1) in the infrared spectra of the Ag compounds compared to their Tl analogues. On the other hand, the substitution of Sb by As atom in the silver pair of minerals as well as in the thallium pair of minerals causes blue shifting for about 20 cm1 (Table 2). The above mentioned band frequency dependence on the MIII atom (As and Sb) as well as on the MI atom (Ag and Tl) should be taken as a strong evidence that in addition to the d(S–MIII–S) normal modes, the n(MI–S) coordinates also participate in the potential energy distribution. Thus, the mentioned bands should be basically prescribed to S–As–S or S–Sb–S bending modes mixed with the considerable contribution of the MI–S stretchings, especially pronounced in the case of the Ag compounds. Namely, the more pronounced Ag–S stretching character is reasonable if the stronger Ag–S bonds (compared to the corresponding Tl– S bonds) are assumed (in an approximation of MI–S diatomic oscillators). This assumption is confirmed by following arguments: (i) the smaller difference in electronegativity between Ag and S atoms (compared to Tl and S atoms) implicates higher covalent character of Ag–S bonds; (ii) the shortest observed Ag–S distances (244.3 and 243.2 pm for Ag3AsS3 and Ag3SbS3, respectively) are smaller than the sum of the atomic radii (247 pm) and also smaller than the sum of the effective ionic radii (245 pm) [23] of silver and sulfur, establishing the mostly covalent bond character. On the other hand, experimentally determined Tl–S distances (304.9 and 303.0 for Tl3AsS3 and Tl3SbS3, respectively) are significantly longer than the sum of the corresponding atomic (273 pm) radii but smaller than the sum of the effective ionic radii (334 pm) [23] of thallium and sulfur. The minerals containing the same MI atom in the structure exhibit very common spectral features in the infrared region below 130 cm1, relating the origin of these bands to the MI–S vibrations. Thus, the weak band at 114 cm1 and the strong one at 82 cm1 in the IR spectrum of ellisite, Tl3AsS3 attributed to the Tl–S vibrations [15], in our spectrum are found at somewhat lower frequencies, 109 and 78 cm1, respectively. The very weak band with the same origin observed by Povarennykh and Gerasimenko [15] at 166 cm1 has not been registered in our spectrum, whereas the low frequency band appeared as a shoulder at 52 cm1 in our spectrum, has not been mentioned in the previous work [15]. The same set of two bands at 114 and 70 cm1 are observed in the spectrum of stibioellisite, Tl3SbS3, accompanied by an additional weak band at 94 cm1 (Fig. 1 and Table 2). Similarly to the spectrum of ellisite, these three bands are attributed to the n(Tl–S) modes. The corresponding Ag–S stretching vibrations in the spectra of the Ag
P. Makreski et al. / Vibrational Spectroscopy 35 (2004) 59–65
containing synthetic minerals are prescribed to the bands at 134, 118 and 103 cm1 in proustite and at 100 cm1 in pyrargyrite (no separated bands have been observed around 135 and 120 cm1 in the latter spectrum—the broadening of the lower-frequency side of the intense band at 248 cm1 appears instead) (Fig. 1). The bands (shoulders) at 52 and 47 cm1 in the infrared spectra of ellisite and stibioellisite, respectively, are prescribed to the bending S–Tl–S modes. The corresponding S– Ag–S bending vibrations in the spectra of the analogous Ag compounds proustite and pyrargyrite appear at about 40 cm1 higher frequency (90 cm1). The remaining two bands in the spectrum of proustite (62 and 34 cm1) and pyrargyrite (59 and 34 cm1) are probably due to the d(S– Tl–S) modes (see forthcoming discussion concerning the Raman spectra). The bands below 100 cm1 in the spectra of both Ag containing minerals are observed at very close (almost identical) frequencies (Table 2). Taking this fact into consideration, it seems that Ag–S modes are less influenced by the contribution of the MIII–S coordinates. In other words, these bands are probably mainly due to the pure Ag–S vibrations which additionally justifies the above supported presumption about the existence of stronger Ag–S force constant in the structures of Ag containing synthetic minerals compared to the Tl containing analogues. The evident larger number of the bands due to Ag–S stretchings and S–Ag–S bendings (Fig. 1 and Table 2) corresponds to the larger number of non-equivalent Ag–S distances in the Ag compounds compared to the corresponding Tl–S distances in the Tl analogues (Table 1), especially in the case of pyrargyrite vs. stibioellisite. 4.2. Interpretation of the FT Raman spectra of Ag3AsS3, Tl3AsS3, Ag3SbS3 and Tl3SbS3 The Raman spectra of the studied synthetic minerals accompanied with the spectra of the natural minerals orpiment and stibnite are shown in Fig. 2, whereas the observed band intensities and frequencies are given in Table 3. According to the group theory the first-order Raman spectrum of the investigated minerals in C3v phase should consist of 6A1 and 13E modes, being selected by the X(ZZ)Y and X(ZX)Y scattering geometry, respectively [19]. The previously discussed structural similarities/differences in the studied minerals served as a basic starting point for the interpretation of their Raman spectra. Namely, having in mind that the MIII–S3 pyramids are the main vibrational units, both M3IAsS3 compounds (proustite and ellisite—consisted of AsS3 pyramids) exhibit very similar Raman spectra, whereas in the case of M3ISbS3 type minerals (pyrargyrite and stibioellisite), where SbS3 pyramids are present in the structure, the similarity is even more pronounced and the spectra seems to be almost identical (Fig. 2). In order to facilitate the general assignment of the bands attributed to the MIII–S stretching, S–MIII–S bending as well
63
Table 3 The frequencies (in cm1) and the assignment of the bands in the FT Raman spectra of the studied synthetic minerals Assignation
Proustite, Ag3AsS3
Ellisite, Tl3AsS3
n(MIII–S3) n(MIII–S3) n(MIII–S3) n(MIII–S3) d(MIII–S3) d(MIII–S3) n(MI–S) n(MI–S) d(S–MI–S)
– 360 334 265 217 179 128 117 103
390 371 304 – 201 178 – 115 –
a
vs s vw vw m vw vw vw
wa vs w w m vw
Pyrargyrite, Ag3SbS3
Stibioellisite, Tl3SbS3
448 370 325 250 – 187 137 116 107
447 369 324 250 – 186 142 116 104
m w w vs s vw w vw
m w vw vs s w w vw
vs: very strong; s: strong; m: medium; w: weak; vw: very weak.
as MI–S stretching modes, the recently obtained results of the Raman study of orpiment and stibnite (As2S3 and Sb2S3) [6], where the AsS3 and SbS3 pyramids are also the main constituents of their structures, were included in the present study (Fig. 2). The results of the Raman spectral study of Ag containing minerals proustite and pyrargyrite published by Ewen et al. [16–19] were also taken into account. According to the literature data for the Raman spectrum of orpiment [6,24–26], two groupings of the bands are proposed and observed in the 600–100 cm1 spectral region: (i) set of the internal-mode bands from 130 to 200 cm1; (ii) set of the internal-mode bands between 300 and 400 cm1. The Raman spectra of the studied As containing minerals proustite and ellisite in the 600–100 cm1 region are closely related to the corresponding spectrum of orpiment. The similarities in the spectra are influenced by the common layer structures among the studied minerals. Namely, the As–S intra-layer distances are significantly shorter compared to the corresponding ones between the layers (225.4 pm versus 383.1 pm in the proustite and 222.5 pm versus 377.5 pm in the ellisite structure). The corresponding shortest intra-layer As–S distance and smallest inter-layer As S contacts in the structure of orpiment are 224 and 356 pm, respectively [11]. Thus, as in orpiment, the layer symmetry is dominant and determines the symmetry and the activity of the normal modes. However, the strongest Raman band in the spectra of proustite (360 cm1) and ellisite (371 cm1) is attributed to the symmetric stretching of the pyramidal As–S3 groups. Its blue frequency shifting for 11 cm1 in the spectrum of ellisite compared to proustite spectrum (Fig. 2 and Table 3) is due to the shortening of the As–S bonds from 225.4 pm in the structure of proustite [9] to 222.5 pm in the structure of ellisite [8] (Table 1). The both remaining highfrequency bands in the spectrum of proustite (334, 265 cm1) and ellisite (390 and 304 cm1) are also due to the stretching As–S3 Raman active modes. The band at 179 cm1 in proustite spectrum and at 178 cm1 in the spectrum of ellisite is probably mainly due to the d(As– S3) modes which could be mixed with the n(MI–S) modes.
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The decreasing of the number of bands due to the n(As–S3) modes in the Raman spectra of proustite (two) and ellisite (three) compared to five observed bands in the corresponding spectrum of orpiment in the 400–300 cm1 spectral region (Fig. 2 and Table 3) is in consistence with the existence of only one type of As–S distances in the structure of former two minerals, compared to the six different As–S distances in the structure of the latter mineral (Table 1). The evident differences between the Raman spectrum of orpiment, on the one hand, and the Raman spectra of proustite and ellisite, on the other hand, in the spectral region below 160 cm1 could be taken as a serious suggestion that the remaining three weak bands in the Raman spectrum of proustite (128, 117 and 103 cm1) as well as the weak one in the spectrum of ellisite (115 cm1) are due to the modes where MI atoms (Ag or Tl) are included. Thus, they are attributed to the MI–S stretching (first two in proustite and the unique one in ellisite) and to the S–MI– S bending vibrations (the lowest frequency one in proustite) (Table 3). Similarly to the Raman spectra of the As containing samples, the strongest bands in the corresponding spectra of stibnite (247 cm1) [6], pyrargyrite (250 cm1) and stibioellisite (250 cm1) are assigned to the stretching Sb–S vibrations (Table 3). The remaining three higher frequency weak or medium bands in the spectra of the pyrargyrite (448, 370 and 325 cm1) and stibioellisite (447, 369 and 324 cm1) also seems not to be affected from the nature of the MI atom, being therefore attributed to the n(MIII–S) modes. Two analogue higher-frequency bands with the similar frequencies (443 and 366 cm1) are also observed in the Raman spectrum of stibnite (Fig. 2) being previously attributed to the n(Sb–S) modes [6]. It confirms the presumption that the SbS3 and squared pyramids (as the main structural constituents, see Table 1) could be considered as a rather isolated vibrators, justifying the assignation of these bands as mainly pure n(Sb–S) vibrations. However, it should be noted that, very often, different polarisation effects as well as the extent of grinding of the studied samples [27] could influence the spectral view and consequently, the comparison of the Raman spectra is not always a straightforward task. It is well illustrated in the case of our Raman spectrum of stibnite [6] which, due to the above mentioned influences, is somewhat different from the reported Raman spectrum of the single crystal of stibnite [28]. Therefore, in this case, it is not recommendable to compare the number of the bands in the Raman spectrum of stibnite with the number of the observed bands in the corresponding spectra of pyrargyrite and stibioellisite, and to relate them with their structural characteristics. The similarity between the Raman spectra of pyrargyrite and stibioellisite also continues in the lower frequency region (200–100 cm1) where additional four bands appear in each spectrum. The highest frequency one (at 187 cm1, in pyrargyrite and at 186 cm1 in stibioellisite) are strong, whereas the remaining three (at 137, 116 and 107 cm1, in
pyrargyrite and at 142, 116 and 104 cm1, in stibioellisite) are weak (Fig. 2 and Table 3). Only two strong bands, however, appear in this region in the Raman spectrum of stibnite at 184 and 140 cm1 (Fig. 2) [6]. The former one (184 cm1) exhibits practically the same frequency as well as intensity with the highest frequency bands in the spectra of pyrargyrite (187 cm1) and stibioellisite (186 cm1). This could be taken as a strong evidence that their origin is still related to the presence of Sb–S3 pyramids in the structure and consequently could be assigned as mainly due to the d(Sb–S3) modes which could be eventually mixed with the n(MI–S) modes. Similar spectral behaviour has been observed in the case of the Raman spectrum of orpiment for the band at 178 cm1 [6] which appears at the same frequency as the corresponding band in the spectrum of proustite (179 cm1) and ellisite (178 cm1) (Fig. 2 and Table 3). Contrary to our assignation, in the phase transition study of single pyrargyrite crystal, based on its X(ZZ)Y and X(ZX)Y Raman polarizability spectra at various temperatures this band was prescribed solely to the Ag–S stretchings [19,29]. The above mentioned differences between the Raman spectrum of stibnite, on the one hand, and the Raman spectra of pyrargyrite and stibioellisite, on the other hand, in the spectral region below 140 cm1 strongly indicate that the remaining three weak bands in the Raman spectrum of pyrargyrite (137, 116 and 107 cm1) as well as in the spectrum of stibioellisite (142, 116 and 104 cm1) are related to the modes where MI atoms (Ag or Tl) are included and therefore are assigned as being due to the MI–S stretching (first two) and S–MI–S bending vibrations (the lowest frequency ones) (Table 3). The bands with the same origin in the Raman spectrum of pyrargyrite are also registered below 100 cm1 [17–19] but the limited performances of our instrument makes their registration impossible. References [1] M. Trajkovska, B. Soptrajanov, G. Jovanovski, T. Stafilov, J. Mol. Struct. 267 (1992) 191. [2] B. Soptrajanov, M. Trajkovska, I. Grzetic, G. Jovanovski, T. Stafilov, N. Jb. Miner. Abh. 166 (1993) 83. [3] M. Trajkovska, B. Soptrajanov, T. Stafilov, G. Jovanovski, Geol. Maced. 7 (1993) 55. [4] B. Soptrajanov, M. Trajkovska, G. Jovanovski, T. Stafilov, N. Jb. Miner. Abh. 167 (1994) 329. [5] B. Soptrajanov, M. Trajkovska, T. Stafilov, G. Jovanovski, I. Grzetic, Spectrosc. Lett. 30 (1997) 79. [6] B. Minceva-Sukarova, G. Jovanovski, P. Makreski, B. Soptrajanov, W. Griffith, R. Willis, I. Grzetic, J. Mol. Struct. 651–653 (2003) 181. [7] B. Boev, V. Bermanec, T. Serafimovski, S. Lepitkova, S. Mikulcic, M. Soufek, G. Jovanovski, T. Stafilov, M. Najdoski, Geol. Maced. 15–16 (2001–2002) 1. [8] M. Gostojic, Z. Kristallorg. 151 (1980) 249. [9] P. Engel, W. Nowacki, N. Jb. Miner. Mh. 1966 (1966) 181. [10] N. Rey, J.C. Jumas, J. Olivier-Fourcade, E. Philippot, Acta Crystallogr. 40C (1984) 1655. [11] D.J.E. Mullen, W. Nowacki, Z. Kristallogr. 136 (1972) 48. [12] P. Bayliss, W. Nowacki, Z. Kristallogr. 135 (1972) 308.
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