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A combinative technique to recognise and discriminate turquoise stone
Vibrational Spectroscopy 99 (2018) 93–99
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Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
A combinative technique to recognise and discriminate turquoise stone
T
Hamid Sabbaghi School of Mining Engineering, College of Engineering, University of Tehran, North Kargar St., 14395-515, Tehran, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Natural Turquoise Fake turquoise Color Infrared spectroscopy XRF
This study attempts to identify and categorize turquoise stone based on its compounds and major factors that introduces its color change. Turquoise samples which are investigated in this study have been collected from the most important turquoise mines in world. Combination of two different analysis methods assists this study for scrutinizing turquoise constituents and effects of their percent on its color change. The application of infrared spectroscopy and X-Ray Fluorescence (XRF) methods can reveal the true composition of various “turquoise” samples in range of blue color to green color. Whereas the first technique (FTIR) is especially capable in diff ;erentiating between natural samples and fake or simulant stones for prevent of jobbery. Changing percent of four compounds including CuO, Al2O3, P2O5 and Fe2O3, is the most effective factors of varying color in turquoise stones. In conclusion, blue turquoise stone contains great percent of light elements or blue color factors in comparison to green turquoise stone which contains jorum of heavy element.
1. Introduction
materials or adulterating the stone properties by means of physical (e.g. heat and gamma irradiation, mostly [14] or chemical (dyes, enhancers, whiteners, oils and resins) [15–17]) treatments. Many minerals especially gemstones such as: quartz, turquoise, garnet and so on have color range. Therefore accustom to their color range and its factors to avoid of unscrupulous persons jobbery, is so significant. Gemologists usually have limited instruments for recognising gemstones. Their instruments such as: birefringence, refractive index, optical spectrum, dispersion and inclusions all are practical for transparent stones, but in case of non-transparent stones must refer to analytical methods. X-ray fluorescence technique provides viable and effective analytical approaches and is appropriate for examine heavy elements to essential accuracy [18]. Characteristic group vibrations of ions such as phosphate are clearly identified by FTIR even in natural samples which typically are a complex mixture of several inorganic phases [19]. Infrared spectroscopy and X-ray fluorescence techniques has been executed to investigate various turquoise stones and compared the results in term of scopes.
Turquoise is one of the oldest known gems that its apply in jewellery and for personal ornamentation traces back 70 centuries, to ancient Egypt [1]. This stone finds extensive application due to its unique color and also easy lapidary and facet cutting [2]. It is found in only a few places on earth, in dry and infertile regions where acidic, copper-rich groundwater reacts with minerals containing phosphorous and aluminum. This reaction creates a hydrous phosphate of copper and aluminum with general chemical formula of A0-1B6(PO4)4(OH)8.4H2O [3–5]. In this formula, Cu2+ and Fe2+ are mostly considered as substitutes for the A-site and also in some cases, Zn2+ is observed in rare minerals formula of turquoise group [6]. Fe3+ and Al3+ are the most common substitutes for the B-site and also in some cases, Si3+ is observed [7–9]. In the south western part of the USA, turquoise is found in several localities mainly in Nevada and Arizona. Also it is applied typically in jewellery of many native Indian tribes, such as the Pueblo, Navajo, Hopi or Zuni [10]. Other famous places where turquoise is found in larger quantities are Iran, Senegal, China, Mexico, or Australia [7]. Gemstones have always seized an important position in human history [11], mostly due to their attractive colors or other optical properties such as high reflectivity [12], This fascination was so extreme that magical and healing features were assigned to some gems [13] which, when rare, were applied as wealth and a representation of power. As jewelry, such stones can reach extremely high values and for this reason are frequently targeted by unscrupulous persons devoted to acquire large profits by simulating precious stones from several
2. Materials and methods 2.1. Studied samples The studied samples of the natural turquoise include both known and unknown provanence samples which known provanence stones were prepared from six different occurrences: Lavender Pit, Bisbee and Cochise county from Arizona (labeled as A), Nayshabour mine from
E-mail address: [email protected]. https://doi.org/10.1016/j.vibspec.2018.09.002 Received 12 June 2018; Received in revised form 21 August 2018; Accepted 6 September 2018 Available online 07 September 2018 0924-2031/ © 2018 Elsevier B.V. All rights reserved.
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2.2. X-ray fluorescence Prepared natural samples were analyzed using a S8 Tiger XRF machine of Bruker with a Rh anode angled 52° from the sample and a glancing angle of 62˚ toward a Silicon Drift Detector with a 7.5 μm Be window. The parameters were set with a beam intensity of 15 kV and 25 μA with a vacuum of < 15 torr in order to be able to detect light elements such as sodium (Na), magnesium (Mg), aluminum (Al), silicon (Si), calcium (Ca) and phosphorus (P); this mode is also effective for certain heavier elements such as iron (Fe); no filter was used. The measuring time was 90 s. Also results were reported in the form of element wt%. 2.3. Infrared spectroscopy Infrared spectra was recorded by micro diffuse reflectance method (DRIFTS) on a Nicolet Magna 760 FTIR spectrometer (range 400–3500 cm−1, resolution 4 cm−1, 128 scans, 2 level zero-filtering, Happ-Genzel apodization), equipped with Spectra Tech InspectIR micro FTIR accessory. Sample of amount less than 0.050 mg was mixed without using pressure with KBr. Samples were immediately recorded together with the same KBr as a reference. Spectral manipulation such as baseline correction/adjustment and smoothing were performed using the Spectracalc software package GRAMS (Galactic Industries Corporation, NH, USA). Band component analysis was undertaken applying the Jandel ‘Peakfit’ software package that enabled the type of fitting function to be chosen and permits specific parameters to be fixed or changed accordingly. Band fitting was done employing a Lorentzian–Gaussian cross-product function with the minimum number of component bands performed for the fitting process. The Lorentzian–Gaussian ratio was maintained at values more than 0.7 and fitting was undertaken until reproducible results were achieved with squared correlations (r2) more than 0.995.
Fig. 1. Some analyzed turquoise samples in this study: a) Known provanence samples with Gilson turquoise (F2) and mixed simple gravel (F3), b) Unknown provanence samples with dyed howlite (F1) and Viennese turquoise (F4).
Table 1 Introduction of studied samples. Sample. ID
Color
Type
S1, S2, S3,A1, A2, A3,N1, N2, N3, P1, P2 S4, S5, A4, A5, N4, N5, P3 S6, S7, A6, A7, N6, N7, P4 S8, S9, S10, A8, A9, A10, N8, N9, N10, P5, P6 F1 F2
blue
real
greenish blue bluish green green
real real real
bluish green blue
F3 F4
greenish blue bluish green
dyed howlite inorganic polymer (Gilson turquoise) mixed simple gravel inorganic polymer (Viennese turquoise)
3. Results and discussion 3.1. FTIR analysis Infrared spectroscopy has already been employed in the investigation of turquoise [18,20–22], so as a first step IR spectra were registered in order to diff ;erentiate between natural turquoise samples and those that are imitation and as second step were employed to recognize factors of varying color. The most significant bands for that purpose are those corresponding to OH vibrations (3000–3500 cm−1), which are very typical. Full range infrared spectra of the studied stones from Arizona (A), Senegal (S), Nayshabour (N) and unknown provanence (P) are given in Figs. 2 and 3 and their tentative assignments listed in Table 2. These spectra show the position of the bands and their relative intensities. The spectra are subdivided into sections according to the type of vibration is being investigated. In this way the precise position of the bands can be described. Infrared spectra at 3431, 3269, 3075 and 3067 cm−1 (A), 3276, 3072 and 3051 cm−1 (S), 3417, 3288 and 3058 cm−1 (N) and 2968, 3073 and 3290 cm−1 (P) are belonged to the ν OH stretching vibrations of symmetrically distinct hydrogen bonded water molecules Fig. 2a–d. Very weak infrared bands at 2934 cm−1 is probably connected with organic impurities Fig. 2c. Infrared bands at 1646 and 1595 cm-1 (A), 1654 and 1587 cm−1 (S), 1622 cm−1 (N) and 1641, 1650 cm−1 (P) are attributed to the ν2 (δ) H2O bending vibrations of the symmetrically distinct differently hydrogen bonded water molecules (Fig. 3a–d). Infrared bands at 1474 cm−1 (A), 1467 cm−1 (S), 1513 cm−1 (N) and 1518 cm−1 (P) are assigned to overtones or
Iran (labeled as N), Kouroudaiko mine and Faleme river from Senegal (labeled as S), also unknown provanence samplesn (labeled as P) were purchased from reliable merchants by Aflak and SHS Mine Exploration Companies. The mentioned samples were exhibited in Fig. 1 and their specification listed in Table 1. Also four fake samples were kindly supplied by the private collectors (labeled as F) (Fig. 1). F1 and F3 were dyed howlite and dyed simple gravel composite, while F2 and F4 both were famous fully synthetic turquoise (inorganic polymer) which named Gilson turquoise and Viennese turquoise in the world market, respectively. Samples were first decontaminated and dried, then crushed by jaw crusher to reach the dimension of 3–7 mm, Then were powdered employing disc mill to reach the dimension of less than 75 μm. Eventually, samples powder were consumed for making KBr pellets for analysis. It is noteworthy that real samples were unaltered.
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Fig. 2. Full range infrared spectra of the studied real stones with cleared peaks of the ν OH stretching vibrations: a) Arizona samples, b) Senegal samples, c) Nayshabour samples and d) Unknown provenance samples.
Fig. 3. Full range infrared spectra of the studied real stones with cleared peaks of the ν2 (δ) H2O bending vibrations differently hydrogen bonded water molecules: a) Arizona samples, b) Senegal samples, c) Nayshabour samples, and d) Unknown provenance samples.
degenerate ν3 (PO4)3- antisymmetric stretching vibrations (Fig. 4a–d). Infrared bands at 1055 cm−1 (A), 1056 cm−1 (S), 1042 cm−1 (N) and 1035 cm−1 (P) are assigned to the ν1 (PO4)3- symmetric stretching vibrations (Fig. 4a–d), which is considered as phosphate compounds
combination bands (Fig. 3a–d). Infrared bands and shoulders at 1195, 1158, 1103 and 1084 cm−1 (A), 1194, 1143, 1104 and 1082 cm−1 (S), also 1192, 1142, 1110, 1092 cm−1 (N) and 1079, 1198 cm−1 (P) are connected with split triply 95
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Table 2 Tentative assignment of turquoise spectra. Tentative assignment
ν OH of water molecules ν2 (δ) H2O bending overtones of combination bands ν3 (PO4)3− antisymmetric stretching ν1 (PO4)3− symmetric stretching librational modes of H2O δ Al–OH / δ Cu–OH / δ Fe–OH
Wavenumber/cm−1 Arizona
Senegal
Nayshabour
Unknown provenance
3431, 3269, 3075,3067 1646, 1595 1474 1195, 1158, 1103,1084 1055 835, 727, 785 1034, 1011, 990, 964, 899
3276, 3072, 3051 1654, 1587 1467 1194, 1143, 1104, 1082 1056 786, 722, 835 1035, 1002, 948, 897
3417, 3288, 3058, 2934 1622 1513 1192, 1142, 1110, 1092 1042 784, 723, 835 1008, 991,956, 897
2968, 3073, 3290 1641, 1650 1518 1198, 1079 1035 733, 798 894, 949, 979
Fig. 4. Infrared spectrum of turquoise over the 500–1300 cm−1 range with specified peaks of the ν1 (PO4)3- symmetric stretching vibrations: a) Arizona samples, b) Senegal samples, c) Nayshabour samples, and d) Unknown provenance samples.
structure changes the degree of ring deformation, which influences on the band intensity [25]. Whereas increase band intensity connected to heavy metal cation, the intensity of the bands decreases due to nontetrahedral cations [25]. Infrared spectra delineate that band intensity of green natural samples is more than bluish green samples, also infrared spectra of greenish blue samples display more intensity than blue samples and less than bluish green samples. Variation of exhibited intensity is due to increasing heavy elements content such as iron (Fe) and decreasing light elements content like Al and P which reference to Fe2O3, Al2O3 and P2O5 compounds respectively (Fig. 6). The results established that IR spectroscopic studies should be accompanied by other examination to evaluate the effect of heavy metal cations on the turquoise structure. Synthetic turquoise stones and dyed howlite (F1, F2 and F4) can easily be recognized by the lack of significant bands in the OH region, and also lack the bands due to the
especially P2O5 in turquoise samples. Infrared bands at 835 cm−1 (A), 835 cm−1 (S), 835 cm−1 (N), while the infrared bands at 785 and 727 cm−1 (A), 786 and 722 cm−1 (S), 784, 723 cm−1 (N) and 733, 798 cm−1 (P) are assigned to the librational modes of water molecules (Fig. 5a-d). Shoulders at 1034, 1011, 990, 964, 899 cm−1 (A), 1035, 1002, 948, 897 cm−1 (S), 1008, 991,956, 897 cm−1 (N) and 894, 949, 979 cm−1 (P) are attributed to the δ AleOH and δ CueOH and δ FeeOH bending vibrations together (Fig. 6a-d), which in this study, δ AleOH is considered as Al2O3 [23] and aslo δ FeeOH and δ CueOH are considered as Fe2O3 and CuO in natural turquoise samples [6]. While thickness of the samples all be same, hence bands intensity are just depended to compounds concentration of samples [24]. The comparison of the integral intensity of the FTIR bands at 8001100 cm−1 delineates that the intensity has increased systematically (Fig. 6). The incorporation of heavy metal cation into the turquoise
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Fig. 5. Infrared spectrum of turquoise over the 500–1300 cm−1 range with specified peaks of the librational modes of water molecules: a) Arizona samples, b) Senegal samples, c) Nayshabour samples, and d) Unknown provenance samples.
Fig. 6. Infrared spectrum of turquoise over the 500–1300 cm−1 range with cleared peaks of the δ AleOH, δ CueOH and δ FeeOH bending vibrations: a) Arizona samples, b) Senegal samples, c) Nayshabour samples, d) Unknown provenance samples.
stretching vibrations of water at 722 and 786 cm−1 (Fig. 7a–c). Other fake sample (F3) which has been mixed of simple gravel, oil and resin and coated by green and blue polyurethance lacquers in laboratory of
Aflak company, exhibits indicative bands in the CH region (2800–3050 cm−1), and the amide bands at 1719 and 1534 cm−1 [26,27] (Fig. 7d). 97
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Table 3 Several chemical composition of collected turquoise samples (wt.%).
Fig. 7. FTIR spectra of fake turquoises: a) fully synthetic sample (inorganic polymer (Viennese turquoise)) (F4); b) dyed howlite (F1); c) other fully synthetic sample (inorganic polymer (Gilson turquoise)) (F2); d) simple gravel with oil and resin (F3).
3.2. XRF measurements Turquoise stones was investigated using ESR and EPR techniques by [28–30]. According to their obtained results, four compounds (CuO, Al2O3, P2O5 and Fe2O3) are considered as the most effective compounds in turquoise molecular structure. In this research, effect of these compounds percent on the turquoise stone color was investigated by XRF analysis which supports FTIR results. Concentration of several selected compounds of analyzed real stones and XRF result of dyed howlite and mixed simple gravel have been presented in Tables 3 and 4. XRF spectra proceeding of samples has been shown in Fig. 8. XRF results clears that CuO and Fe2O3 are the most important factors in assigning color range of turquoise stone. If CuO > 8%, the sample can be classified in blue category without considering the concentration of other compounds. While, if 6% < CuO < 8%, the turquoise color can be categorized as greenish blue or bluish green based on Al2O3 and P2O5 contents (Table 5). Greenish blue turquoise contains more Al2O3 than bluish green turquoise. As shown in Table 5, greenish blue turquoise contains 27% < Al2O3 < 30%, while bluish green turquoise includes Al2O3 < 27%. In the case of P2O5, they both include P2O5 < 30%, but bluish green turquoise contains less P2O5 than greenish blue. In addition to mentioned reasons for tending turquoise color into green, Fe2O3 concentration is also very significant. Blue turquoise have 6% > Fe2O3, while If 6% < Fe2O3 < 10%, the turquoise color will be greenish blue and if 10% < Fe2O3 < 14%, the color will be bluish green. In fact, investigating the concentration of CuO is first preference in recognising and classifying turquoise stone, and next preference is Fe2O3 percentage. CuO is the most important factor in rising blue color, which has the lowest concentration in green and bluish green turquoise stone (less than 6%) in comparison to the other samples. In green turquoise, Al2O3 < 27% and P2O5 < 30% which is similar to bluish green turquoise. In this case, Fe2O3 as the most important factor of rising
Sample. ID
CuO
Al2O3
P2O5
Fe2O3
ZnO
BaO
H2O*
Provenance
A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 N1 N2 N3 N4 N5 N6 N7 N8 N9 N10 P1 P2 P3 P4 P5 P6
9.71 9.88 8.82 6.63 7.19 6.86 7.49 5.73 5.81 5.59 9.21 8.58 8.32 7.43 7.39 7.16 6.33 5.23 4.91 5.09 9.32 9.58 9.49 6.84 7.62 7.36 6.83 4.64 5.31 4.49 9.12 9.45 6.96 7.38 4.26 4.33
37.81 36.74 36.96 29.84 28.08 26.57 26.78 24.96 24.64 25.88 36.71 37.74 36.79 28.84 28.38 25.87 25.72 24.78 25.54 24.87 35.61 35.64 36.73 28.81 29.51 24.83 25.94 24.75 24.58 25.33 35.26 36.13 29.76 25.69 24.81 24.62
34.15 35.97 35.09 28.26 27.83 26.69 26.43 26.21 25.98 25.86 35.35 36.97 36.19 28.26 29.83 26.58 27.43 26.36 25.98 25.36 33.26 33.36 34.38 29.22 29.73 27.66 27.46 26.26 25.94 25.55 33.41 33.19 28.18 27.88 26.12 26.09
5.06 4.96 4.82 8.85 9.12 12.9 13.5 15.4 15 15.3 5.02 5.14 4.88 8.76 8.88 13 12.6 14.5 14.5 14.3 4.76 4.89 4.92 7.78 8.56 13.9 12.8 14.7 15.4 14.8 4.17 4.82 7.82 13 14.6 14.3
0.16 0.18 0.16 0.15 0.12 0.17 0.15 0.23 0.21 0.25 0.19 0.18 0.18 0.18 0.13 0.19 0.14 0.22 0.24 0.26 0.17 0.17 0.18 0.19 0.14 0.18 0.14 0.23 0.23 0.27 0.19 0.15 0.18 0.16 0.21 0.26
0.62 0.75 0.61 0.63 0.67 0.64 0.61 0.71 0.69 0.73 0.67 0.65 0.69 0.63 0.68 0.66 0.61 0.81 0.79 0.73 0.77 0.61 0.61 0.6 0.69 0.76 0.71 0.71 0.79 0.75 0.66 0.7 0.59 0.69 0.77 0.63
17.42 17.02 17.36 17.65 17.68 17.28 17.15 17.26 17.49 17.57 17.82 17.33 17.35 17.45 17.63 17.48 17.27 17.69 17.48 17.52 17.88 17.98 17.41 17.44 17.71 17.39 17.77 17.66 17.41 17.59 17.45 17.71 17.52 17.43 17.8 17.49
Arizona Arizona Arizona Arizona Arizona Arizona Arizona Arizona Arizona Arizona Senegal Senegal Senegal Senegal Senegal Senegal Senegal Senegal Senegal Senegal Nayshabour Nayshabour Nayshabour Nayshabour Nayshabour Nayshabour Nayshabour Nayshabour Nayshabour Nayshabour Unknown Unknown Unknown Unknown Unknown Unknown
H2O* content was calculated on the basis of charge balance and ideal content of 4 H2O molecules.
green color in turquoise, has a concentration greater than 14% in green turquoise samples. 4. Conclusions Synthetic or fake turquoise stones is very often produced with simulant material and traded by unscrupulous persons in world market instead natural stones, thus it is so important and requirement to have reliable methods for the recognising kinds of gem especially those are in range of colors. Infrared spectroscopy can very well discriminate between natural and synthetic turquoise and in some cases gives also valuable information, e.g. the impregnation with oil and resin. OH vibrations bands are index to discriminate natural from synthetic turquoise stones. Natural turquoise stones display significant bands in OH region, while fake turquoise stones do not exhibit vibration band in OH region clearly, and also lack the bands due to the stretching vibrations of water at 722 and 786 cm−1. Spectra of green turquoise stones exhibit higher relative intensity in comparison to other samples due to more heavy elements content and less light elements content. XRF results confirmed FTIR studies which heavy elements content increases from blue color turquoise to green color turquoise. In fact, CuO content is blue color index and Fe2O3 content is green color index in turquoise stone. Also greenish blue and bluish green sample can be distinguish of each other based on Al2O3 and P2O5 contents after investigation CuO percent.
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References
Table 4 XRF result of F1 and F3. Sample. ID
F1
F3
B2O3 CaO SiO2 Al2O3 MgO Fe2O3 K2O H2O*
45.23 28.45 15.83 0 0 0 0 10.49
0 25.53 28.56 5.41 9.7 2.64 2.36 25.89
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Fig. 8. XRF spectra of some turquoise samples: a) bluish green turquoise sample of Arizona (A7), b) blue turquoise sample of Nayshabour (N2), c) green turquoise sample of Senegal (S10), d) greenish blue turquoise sample of Nayshabour (N4), e) XRF spectra of P3, f) XRF spectra of P4, g) dyed howlite and h) mixed simple gravel (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). Table 5 Classification turquoise samples based on concentration of effective compounds (values are as percent). Sample. ID
Color
CuO
Al2O3
P2O5
Fe2O3
S1, S2, S3,A1, A2, A3,N1, N2, N3, P1, P2 S4, S5, A4, A5, N4, N5, P3 S6, S7, A6, A7, N6, N7, P4 S8, S9, S10, A8, A9, A10, N8, N9, N10, P5, P6
blue
>8
> 30
> 30
<6
greenish blue bluish green green
6-8 6-8 <6
27-30 < 27 < 27
< 30 < 30 < 30
6-10 10-14 > 14
Acknowledgement The author would like to thank the Aflak and SHS Mine Exploration Companies for providing samples, in particular the Research and Development Department, also for providing access to the Nayshabour mine and logistical support and laboratory analysis. The author thanks private collectors for supply fake samples.
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Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
A combinative technique to recognise and discriminate turquoise stone
T
Hamid Sabbaghi School of Mining Engineering, College of Engineering, University of Tehran, North Kargar St., 14395-515, Tehran, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Natural Turquoise Fake turquoise Color Infrared spectroscopy XRF
This study attempts to identify and categorize turquoise stone based on its compounds and major factors that introduces its color change. Turquoise samples which are investigated in this study have been collected from the most important turquoise mines in world. Combination of two different analysis methods assists this study for scrutinizing turquoise constituents and effects of their percent on its color change. The application of infrared spectroscopy and X-Ray Fluorescence (XRF) methods can reveal the true composition of various “turquoise” samples in range of blue color to green color. Whereas the first technique (FTIR) is especially capable in diff ;erentiating between natural samples and fake or simulant stones for prevent of jobbery. Changing percent of four compounds including CuO, Al2O3, P2O5 and Fe2O3, is the most effective factors of varying color in turquoise stones. In conclusion, blue turquoise stone contains great percent of light elements or blue color factors in comparison to green turquoise stone which contains jorum of heavy element.
1. Introduction
materials or adulterating the stone properties by means of physical (e.g. heat and gamma irradiation, mostly [14] or chemical (dyes, enhancers, whiteners, oils and resins) [15–17]) treatments. Many minerals especially gemstones such as: quartz, turquoise, garnet and so on have color range. Therefore accustom to their color range and its factors to avoid of unscrupulous persons jobbery, is so significant. Gemologists usually have limited instruments for recognising gemstones. Their instruments such as: birefringence, refractive index, optical spectrum, dispersion and inclusions all are practical for transparent stones, but in case of non-transparent stones must refer to analytical methods. X-ray fluorescence technique provides viable and effective analytical approaches and is appropriate for examine heavy elements to essential accuracy [18]. Characteristic group vibrations of ions such as phosphate are clearly identified by FTIR even in natural samples which typically are a complex mixture of several inorganic phases [19]. Infrared spectroscopy and X-ray fluorescence techniques has been executed to investigate various turquoise stones and compared the results in term of scopes.
Turquoise is one of the oldest known gems that its apply in jewellery and for personal ornamentation traces back 70 centuries, to ancient Egypt [1]. This stone finds extensive application due to its unique color and also easy lapidary and facet cutting [2]. It is found in only a few places on earth, in dry and infertile regions where acidic, copper-rich groundwater reacts with minerals containing phosphorous and aluminum. This reaction creates a hydrous phosphate of copper and aluminum with general chemical formula of A0-1B6(PO4)4(OH)8.4H2O [3–5]. In this formula, Cu2+ and Fe2+ are mostly considered as substitutes for the A-site and also in some cases, Zn2+ is observed in rare minerals formula of turquoise group [6]. Fe3+ and Al3+ are the most common substitutes for the B-site and also in some cases, Si3+ is observed [7–9]. In the south western part of the USA, turquoise is found in several localities mainly in Nevada and Arizona. Also it is applied typically in jewellery of many native Indian tribes, such as the Pueblo, Navajo, Hopi or Zuni [10]. Other famous places where turquoise is found in larger quantities are Iran, Senegal, China, Mexico, or Australia [7]. Gemstones have always seized an important position in human history [11], mostly due to their attractive colors or other optical properties such as high reflectivity [12], This fascination was so extreme that magical and healing features were assigned to some gems [13] which, when rare, were applied as wealth and a representation of power. As jewelry, such stones can reach extremely high values and for this reason are frequently targeted by unscrupulous persons devoted to acquire large profits by simulating precious stones from several
2. Materials and methods 2.1. Studied samples The studied samples of the natural turquoise include both known and unknown provanence samples which known provanence stones were prepared from six different occurrences: Lavender Pit, Bisbee and Cochise county from Arizona (labeled as A), Nayshabour mine from
E-mail address: [email protected]. https://doi.org/10.1016/j.vibspec.2018.09.002 Received 12 June 2018; Received in revised form 21 August 2018; Accepted 6 September 2018 Available online 07 September 2018 0924-2031/ © 2018 Elsevier B.V. All rights reserved.
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2.2. X-ray fluorescence Prepared natural samples were analyzed using a S8 Tiger XRF machine of Bruker with a Rh anode angled 52° from the sample and a glancing angle of 62˚ toward a Silicon Drift Detector with a 7.5 μm Be window. The parameters were set with a beam intensity of 15 kV and 25 μA with a vacuum of < 15 torr in order to be able to detect light elements such as sodium (Na), magnesium (Mg), aluminum (Al), silicon (Si), calcium (Ca) and phosphorus (P); this mode is also effective for certain heavier elements such as iron (Fe); no filter was used. The measuring time was 90 s. Also results were reported in the form of element wt%. 2.3. Infrared spectroscopy Infrared spectra was recorded by micro diffuse reflectance method (DRIFTS) on a Nicolet Magna 760 FTIR spectrometer (range 400–3500 cm−1, resolution 4 cm−1, 128 scans, 2 level zero-filtering, Happ-Genzel apodization), equipped with Spectra Tech InspectIR micro FTIR accessory. Sample of amount less than 0.050 mg was mixed without using pressure with KBr. Samples were immediately recorded together with the same KBr as a reference. Spectral manipulation such as baseline correction/adjustment and smoothing were performed using the Spectracalc software package GRAMS (Galactic Industries Corporation, NH, USA). Band component analysis was undertaken applying the Jandel ‘Peakfit’ software package that enabled the type of fitting function to be chosen and permits specific parameters to be fixed or changed accordingly. Band fitting was done employing a Lorentzian–Gaussian cross-product function with the minimum number of component bands performed for the fitting process. The Lorentzian–Gaussian ratio was maintained at values more than 0.7 and fitting was undertaken until reproducible results were achieved with squared correlations (r2) more than 0.995.
Fig. 1. Some analyzed turquoise samples in this study: a) Known provanence samples with Gilson turquoise (F2) and mixed simple gravel (F3), b) Unknown provanence samples with dyed howlite (F1) and Viennese turquoise (F4).
Table 1 Introduction of studied samples. Sample. ID
Color
Type
S1, S2, S3,A1, A2, A3,N1, N2, N3, P1, P2 S4, S5, A4, A5, N4, N5, P3 S6, S7, A6, A7, N6, N7, P4 S8, S9, S10, A8, A9, A10, N8, N9, N10, P5, P6 F1 F2
blue
real
greenish blue bluish green green
real real real
bluish green blue
F3 F4
greenish blue bluish green
dyed howlite inorganic polymer (Gilson turquoise) mixed simple gravel inorganic polymer (Viennese turquoise)
3. Results and discussion 3.1. FTIR analysis Infrared spectroscopy has already been employed in the investigation of turquoise [18,20–22], so as a first step IR spectra were registered in order to diff ;erentiate between natural turquoise samples and those that are imitation and as second step were employed to recognize factors of varying color. The most significant bands for that purpose are those corresponding to OH vibrations (3000–3500 cm−1), which are very typical. Full range infrared spectra of the studied stones from Arizona (A), Senegal (S), Nayshabour (N) and unknown provanence (P) are given in Figs. 2 and 3 and their tentative assignments listed in Table 2. These spectra show the position of the bands and their relative intensities. The spectra are subdivided into sections according to the type of vibration is being investigated. In this way the precise position of the bands can be described. Infrared spectra at 3431, 3269, 3075 and 3067 cm−1 (A), 3276, 3072 and 3051 cm−1 (S), 3417, 3288 and 3058 cm−1 (N) and 2968, 3073 and 3290 cm−1 (P) are belonged to the ν OH stretching vibrations of symmetrically distinct hydrogen bonded water molecules Fig. 2a–d. Very weak infrared bands at 2934 cm−1 is probably connected with organic impurities Fig. 2c. Infrared bands at 1646 and 1595 cm-1 (A), 1654 and 1587 cm−1 (S), 1622 cm−1 (N) and 1641, 1650 cm−1 (P) are attributed to the ν2 (δ) H2O bending vibrations of the symmetrically distinct differently hydrogen bonded water molecules (Fig. 3a–d). Infrared bands at 1474 cm−1 (A), 1467 cm−1 (S), 1513 cm−1 (N) and 1518 cm−1 (P) are assigned to overtones or
Iran (labeled as N), Kouroudaiko mine and Faleme river from Senegal (labeled as S), also unknown provanence samplesn (labeled as P) were purchased from reliable merchants by Aflak and SHS Mine Exploration Companies. The mentioned samples were exhibited in Fig. 1 and their specification listed in Table 1. Also four fake samples were kindly supplied by the private collectors (labeled as F) (Fig. 1). F1 and F3 were dyed howlite and dyed simple gravel composite, while F2 and F4 both were famous fully synthetic turquoise (inorganic polymer) which named Gilson turquoise and Viennese turquoise in the world market, respectively. Samples were first decontaminated and dried, then crushed by jaw crusher to reach the dimension of 3–7 mm, Then were powdered employing disc mill to reach the dimension of less than 75 μm. Eventually, samples powder were consumed for making KBr pellets for analysis. It is noteworthy that real samples were unaltered.
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Fig. 2. Full range infrared spectra of the studied real stones with cleared peaks of the ν OH stretching vibrations: a) Arizona samples, b) Senegal samples, c) Nayshabour samples and d) Unknown provenance samples.
Fig. 3. Full range infrared spectra of the studied real stones with cleared peaks of the ν2 (δ) H2O bending vibrations differently hydrogen bonded water molecules: a) Arizona samples, b) Senegal samples, c) Nayshabour samples, and d) Unknown provenance samples.
degenerate ν3 (PO4)3- antisymmetric stretching vibrations (Fig. 4a–d). Infrared bands at 1055 cm−1 (A), 1056 cm−1 (S), 1042 cm−1 (N) and 1035 cm−1 (P) are assigned to the ν1 (PO4)3- symmetric stretching vibrations (Fig. 4a–d), which is considered as phosphate compounds
combination bands (Fig. 3a–d). Infrared bands and shoulders at 1195, 1158, 1103 and 1084 cm−1 (A), 1194, 1143, 1104 and 1082 cm−1 (S), also 1192, 1142, 1110, 1092 cm−1 (N) and 1079, 1198 cm−1 (P) are connected with split triply 95
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Table 2 Tentative assignment of turquoise spectra. Tentative assignment
ν OH of water molecules ν2 (δ) H2O bending overtones of combination bands ν3 (PO4)3− antisymmetric stretching ν1 (PO4)3− symmetric stretching librational modes of H2O δ Al–OH / δ Cu–OH / δ Fe–OH
Wavenumber/cm−1 Arizona
Senegal
Nayshabour
Unknown provenance
3431, 3269, 3075,3067 1646, 1595 1474 1195, 1158, 1103,1084 1055 835, 727, 785 1034, 1011, 990, 964, 899
3276, 3072, 3051 1654, 1587 1467 1194, 1143, 1104, 1082 1056 786, 722, 835 1035, 1002, 948, 897
3417, 3288, 3058, 2934 1622 1513 1192, 1142, 1110, 1092 1042 784, 723, 835 1008, 991,956, 897
2968, 3073, 3290 1641, 1650 1518 1198, 1079 1035 733, 798 894, 949, 979
Fig. 4. Infrared spectrum of turquoise over the 500–1300 cm−1 range with specified peaks of the ν1 (PO4)3- symmetric stretching vibrations: a) Arizona samples, b) Senegal samples, c) Nayshabour samples, and d) Unknown provenance samples.
structure changes the degree of ring deformation, which influences on the band intensity [25]. Whereas increase band intensity connected to heavy metal cation, the intensity of the bands decreases due to nontetrahedral cations [25]. Infrared spectra delineate that band intensity of green natural samples is more than bluish green samples, also infrared spectra of greenish blue samples display more intensity than blue samples and less than bluish green samples. Variation of exhibited intensity is due to increasing heavy elements content such as iron (Fe) and decreasing light elements content like Al and P which reference to Fe2O3, Al2O3 and P2O5 compounds respectively (Fig. 6). The results established that IR spectroscopic studies should be accompanied by other examination to evaluate the effect of heavy metal cations on the turquoise structure. Synthetic turquoise stones and dyed howlite (F1, F2 and F4) can easily be recognized by the lack of significant bands in the OH region, and also lack the bands due to the
especially P2O5 in turquoise samples. Infrared bands at 835 cm−1 (A), 835 cm−1 (S), 835 cm−1 (N), while the infrared bands at 785 and 727 cm−1 (A), 786 and 722 cm−1 (S), 784, 723 cm−1 (N) and 733, 798 cm−1 (P) are assigned to the librational modes of water molecules (Fig. 5a-d). Shoulders at 1034, 1011, 990, 964, 899 cm−1 (A), 1035, 1002, 948, 897 cm−1 (S), 1008, 991,956, 897 cm−1 (N) and 894, 949, 979 cm−1 (P) are attributed to the δ AleOH and δ CueOH and δ FeeOH bending vibrations together (Fig. 6a-d), which in this study, δ AleOH is considered as Al2O3 [23] and aslo δ FeeOH and δ CueOH are considered as Fe2O3 and CuO in natural turquoise samples [6]. While thickness of the samples all be same, hence bands intensity are just depended to compounds concentration of samples [24]. The comparison of the integral intensity of the FTIR bands at 8001100 cm−1 delineates that the intensity has increased systematically (Fig. 6). The incorporation of heavy metal cation into the turquoise
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Fig. 5. Infrared spectrum of turquoise over the 500–1300 cm−1 range with specified peaks of the librational modes of water molecules: a) Arizona samples, b) Senegal samples, c) Nayshabour samples, and d) Unknown provenance samples.
Fig. 6. Infrared spectrum of turquoise over the 500–1300 cm−1 range with cleared peaks of the δ AleOH, δ CueOH and δ FeeOH bending vibrations: a) Arizona samples, b) Senegal samples, c) Nayshabour samples, d) Unknown provenance samples.
stretching vibrations of water at 722 and 786 cm−1 (Fig. 7a–c). Other fake sample (F3) which has been mixed of simple gravel, oil and resin and coated by green and blue polyurethance lacquers in laboratory of
Aflak company, exhibits indicative bands in the CH region (2800–3050 cm−1), and the amide bands at 1719 and 1534 cm−1 [26,27] (Fig. 7d). 97
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Table 3 Several chemical composition of collected turquoise samples (wt.%).
Fig. 7. FTIR spectra of fake turquoises: a) fully synthetic sample (inorganic polymer (Viennese turquoise)) (F4); b) dyed howlite (F1); c) other fully synthetic sample (inorganic polymer (Gilson turquoise)) (F2); d) simple gravel with oil and resin (F3).
3.2. XRF measurements Turquoise stones was investigated using ESR and EPR techniques by [28–30]. According to their obtained results, four compounds (CuO, Al2O3, P2O5 and Fe2O3) are considered as the most effective compounds in turquoise molecular structure. In this research, effect of these compounds percent on the turquoise stone color was investigated by XRF analysis which supports FTIR results. Concentration of several selected compounds of analyzed real stones and XRF result of dyed howlite and mixed simple gravel have been presented in Tables 3 and 4. XRF spectra proceeding of samples has been shown in Fig. 8. XRF results clears that CuO and Fe2O3 are the most important factors in assigning color range of turquoise stone. If CuO > 8%, the sample can be classified in blue category without considering the concentration of other compounds. While, if 6% < CuO < 8%, the turquoise color can be categorized as greenish blue or bluish green based on Al2O3 and P2O5 contents (Table 5). Greenish blue turquoise contains more Al2O3 than bluish green turquoise. As shown in Table 5, greenish blue turquoise contains 27% < Al2O3 < 30%, while bluish green turquoise includes Al2O3 < 27%. In the case of P2O5, they both include P2O5 < 30%, but bluish green turquoise contains less P2O5 than greenish blue. In addition to mentioned reasons for tending turquoise color into green, Fe2O3 concentration is also very significant. Blue turquoise have 6% > Fe2O3, while If 6% < Fe2O3 < 10%, the turquoise color will be greenish blue and if 10% < Fe2O3 < 14%, the color will be bluish green. In fact, investigating the concentration of CuO is first preference in recognising and classifying turquoise stone, and next preference is Fe2O3 percentage. CuO is the most important factor in rising blue color, which has the lowest concentration in green and bluish green turquoise stone (less than 6%) in comparison to the other samples. In green turquoise, Al2O3 < 27% and P2O5 < 30% which is similar to bluish green turquoise. In this case, Fe2O3 as the most important factor of rising
Sample. ID
CuO
Al2O3
P2O5
Fe2O3
ZnO
BaO
H2O*
Provenance
A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 N1 N2 N3 N4 N5 N6 N7 N8 N9 N10 P1 P2 P3 P4 P5 P6
9.71 9.88 8.82 6.63 7.19 6.86 7.49 5.73 5.81 5.59 9.21 8.58 8.32 7.43 7.39 7.16 6.33 5.23 4.91 5.09 9.32 9.58 9.49 6.84 7.62 7.36 6.83 4.64 5.31 4.49 9.12 9.45 6.96 7.38 4.26 4.33
37.81 36.74 36.96 29.84 28.08 26.57 26.78 24.96 24.64 25.88 36.71 37.74 36.79 28.84 28.38 25.87 25.72 24.78 25.54 24.87 35.61 35.64 36.73 28.81 29.51 24.83 25.94 24.75 24.58 25.33 35.26 36.13 29.76 25.69 24.81 24.62
34.15 35.97 35.09 28.26 27.83 26.69 26.43 26.21 25.98 25.86 35.35 36.97 36.19 28.26 29.83 26.58 27.43 26.36 25.98 25.36 33.26 33.36 34.38 29.22 29.73 27.66 27.46 26.26 25.94 25.55 33.41 33.19 28.18 27.88 26.12 26.09
5.06 4.96 4.82 8.85 9.12 12.9 13.5 15.4 15 15.3 5.02 5.14 4.88 8.76 8.88 13 12.6 14.5 14.5 14.3 4.76 4.89 4.92 7.78 8.56 13.9 12.8 14.7 15.4 14.8 4.17 4.82 7.82 13 14.6 14.3
0.16 0.18 0.16 0.15 0.12 0.17 0.15 0.23 0.21 0.25 0.19 0.18 0.18 0.18 0.13 0.19 0.14 0.22 0.24 0.26 0.17 0.17 0.18 0.19 0.14 0.18 0.14 0.23 0.23 0.27 0.19 0.15 0.18 0.16 0.21 0.26
0.62 0.75 0.61 0.63 0.67 0.64 0.61 0.71 0.69 0.73 0.67 0.65 0.69 0.63 0.68 0.66 0.61 0.81 0.79 0.73 0.77 0.61 0.61 0.6 0.69 0.76 0.71 0.71 0.79 0.75 0.66 0.7 0.59 0.69 0.77 0.63
17.42 17.02 17.36 17.65 17.68 17.28 17.15 17.26 17.49 17.57 17.82 17.33 17.35 17.45 17.63 17.48 17.27 17.69 17.48 17.52 17.88 17.98 17.41 17.44 17.71 17.39 17.77 17.66 17.41 17.59 17.45 17.71 17.52 17.43 17.8 17.49
Arizona Arizona Arizona Arizona Arizona Arizona Arizona Arizona Arizona Arizona Senegal Senegal Senegal Senegal Senegal Senegal Senegal Senegal Senegal Senegal Nayshabour Nayshabour Nayshabour Nayshabour Nayshabour Nayshabour Nayshabour Nayshabour Nayshabour Nayshabour Unknown Unknown Unknown Unknown Unknown Unknown
H2O* content was calculated on the basis of charge balance and ideal content of 4 H2O molecules.
green color in turquoise, has a concentration greater than 14% in green turquoise samples. 4. Conclusions Synthetic or fake turquoise stones is very often produced with simulant material and traded by unscrupulous persons in world market instead natural stones, thus it is so important and requirement to have reliable methods for the recognising kinds of gem especially those are in range of colors. Infrared spectroscopy can very well discriminate between natural and synthetic turquoise and in some cases gives also valuable information, e.g. the impregnation with oil and resin. OH vibrations bands are index to discriminate natural from synthetic turquoise stones. Natural turquoise stones display significant bands in OH region, while fake turquoise stones do not exhibit vibration band in OH region clearly, and also lack the bands due to the stretching vibrations of water at 722 and 786 cm−1. Spectra of green turquoise stones exhibit higher relative intensity in comparison to other samples due to more heavy elements content and less light elements content. XRF results confirmed FTIR studies which heavy elements content increases from blue color turquoise to green color turquoise. In fact, CuO content is blue color index and Fe2O3 content is green color index in turquoise stone. Also greenish blue and bluish green sample can be distinguish of each other based on Al2O3 and P2O5 contents after investigation CuO percent.
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References
Table 4 XRF result of F1 and F3. Sample. ID
F1
F3
B2O3 CaO SiO2 Al2O3 MgO Fe2O3 K2O H2O*
45.23 28.45 15.83 0 0 0 0 10.49
0 25.53 28.56 5.41 9.7 2.64 2.36 25.89
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Fig. 8. XRF spectra of some turquoise samples: a) bluish green turquoise sample of Arizona (A7), b) blue turquoise sample of Nayshabour (N2), c) green turquoise sample of Senegal (S10), d) greenish blue turquoise sample of Nayshabour (N4), e) XRF spectra of P3, f) XRF spectra of P4, g) dyed howlite and h) mixed simple gravel (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). Table 5 Classification turquoise samples based on concentration of effective compounds (values are as percent). Sample. ID
Color
CuO
Al2O3
P2O5
Fe2O3
S1, S2, S3,A1, A2, A3,N1, N2, N3, P1, P2 S4, S5, A4, A5, N4, N5, P3 S6, S7, A6, A7, N6, N7, P4 S8, S9, S10, A8, A9, A10, N8, N9, N10, P5, P6
blue
>8
> 30
> 30
<6
greenish blue bluish green green
6-8 6-8 <6
27-30 < 27 < 27
< 30 < 30 < 30
6-10 10-14 > 14
Acknowledgement The author would like to thank the Aflak and SHS Mine Exploration Companies for providing samples, in particular the Research and Development Department, also for providing access to the Nayshabour mine and logistical support and laboratory analysis. The author thanks private collectors for supply fake samples.
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