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Thursday, 04 October 2007 16:05 |
IDENTIFICATION Separation from Natural Emeralds. Malossi synthetic emeralds have a number of characteristics that, in combination, allow them to be separated from natural emeralds: 1. Microscopic features: Irregular growth structures (observed in almost all Malossi synthetic emeralds), natural seed plates (used to initiate growth), and phenakite-like crystals (hinting at the occurrence of an incongruent beryl precipitation) provide evidence of hydrothermal synthesis. 2. Chemical composition: The presence of Cl, combined with the absence of any significant amounts of Fe; Na, and Mg, provides a useful tool for the separation from Fe-alkali-bearing natural emeralds. In the case of Fe-Na-Mg-poor natural samples (such as Colombian stones), a Cl content >0.2 wt.% can be used to identify the Malossi synthetics, although due to possible compositional overlap, chemical analysis alone is not a reliable proof of synthesis. 3. Spectroscopic measurements: Mid-infrared bands in the 3100-2500 cm'1 range (related to Cl) and a band at 3295 cm"1 with an associated shoulder at 3232 cm"1, are further diagnostic features of synthetic origin. In summary, Malossi synthetic emeralds are readily separated from most natural Fe- and/or alkali-bearing emeralds, whereas a combination of the diagnostic features discussed above is required to distinguish them from Fe- and alkali-poor natural emeralds. Separation from Other Synthetic Emeralds. Malossi, like all other hydrothermal synthetic emeralds, are readily separated from flux synthetic emeralds because the latter (1) have lower refractive index (horn 1.556), birefringence (from 0.003), and specific gravity (from 2.64) values,- (2) contain typical flux inclusions; and (3) do not exhibit water-related bands in the mid- (between 4000 and 3400 cm-1) and near- (9000-5000 cm"1) IR spectra. Malossi synthetic emeralds, which are Cr- and Cl-bearing, differ from the Russian, Lechleitner, and Biron hydrothermal synthetic emeralds studied to date on the basis of chemical composition. Russian and Lechleitner synthetics have Cr, Fe, Cu, and Ni, while Biron has V in addition to Cr and Cl. These differences can be seen in their gemological and spectroscopic properties. The separation of Malossi from Chinese synthetic emeralds may be possible based on either a larger amount of Cl in the earlier-generation Chinese material or the presence of Na in the later-generation Chinese synthetics. Also, according to information given by Chinese gemolo-gists at the Fall 2004 International Gemological Congress in Wuhan (China), the production of Chinese hydrothermal synthetic emeralds has been discontinued (K. Schmetzer, pers. comm., 2005). The chemical separation of Malossi from Linde Regency hydrothermal synthetic emeralds is less straightforward and further research is needed. CONCLUSIONS A new type of hydrothermal synthetic emerald is now being produced in the Czech Republic with Italian technology. These Malossi synthetic emeralds have been commercially available since December 2004 (figure 13). This material belongs to the group of Cl-bearing, alkali-free hydrothermal synthetic emeralds, with Cr3+ as the only chro-mophore. Water is present as type I molecules. Malossi synthetic emerald can be distinguished from its natural counterpart on the basis of microscopic features (in particular, irregular growth structures, seed plates, and/or phenakite-like crystals), as well as by the presence of Cl combined with the absence of significant Fe, Na, or Mg. In addition, mid-infrared spectroscopy reveals diagnostic bands in the 3100-2500 cm"1 range and at 3295 cm"1 (with a shoulder at 3232 cm"1). Malossi hydrothermal synthetic emerald can be easily discriminated from its flux synthetic counterparts, primarily on the basis of the absence of water molecules in the latter. The separation from Russian, Lechleitner, Chinese, and Biron hydrothermal synthetic emeralds can be made on the basis of chemical composition. The discrimination from Linde-Regency hydrothermal synthetic emeralds is more ambiguous, and further research is needed.
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Written by Administrator
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Thursday, 04 October 2007 16:03 |
Spectroscopy. The results of UV-Vis-NIR and IR spectroscopy are summarized in table 3, including a comparison to natural and other synthetic emeralds. Mid-infrared spectra (4000-2000 cm"1) in diffuse reflectance mode are shown in figure 9. A series of intense peaks between 4000 and 3400 cm-1 in all the synthetic emeralds we studied is related to their high water contents (Stockton, 1987; Schmetzer et al., 1997). Such features are characteristic of both natural and hydrothermal synthetic emeralds, but they are not found in flux synthetic samples (Stockton, 1987). Bands in the range 3100-2500 cm-1, commonly used to identify hydrothermal synthetic emeralds (Schmetzer et al., 1997; Mashkovtsev and Smirnov, 2004), were observed in our Malossi samples, as well as in those from Biron and Linde-Regency (see also Stockton, 1987; Mashkovtsev and Solntsev, 2002; Mashkovtsev and Smirnov, 2004). Schmetzer et al. (1997) found these bands in Chinese samples as well. However, Russian and Lechleitner synthetic emeralds are transparent over the same energy range (Stockton, 1987; Koivula et al., 1996; Mashkovtsev and Solntsev, 2002; Mashkovtsev and Smirnov, 2004; see also figure 9). Schmetzer et al. (1997) attributed these bands to Cl, in agreement with more recent results by Mashkovtsev and Solntsev (2002) and Mashkovtsev and Smirnov (2004), who specifically cited HCl molecules in the hexagonal channels of the beryl structure. This interpretation is consistent with the chemical compositions we determined for Malossi, Biron, and Linde-Regency synthetics and with the producer's statement that Malossi synthetic emeralds are grown in a solution of HCl. An additional band at 3295 cm"1, with a shoulder at 3232 cm"1, occurred in both the Malossi and Linde-Regency products (Stockton, 1987; Mashkovtsev and Solntsev, 2002; Mashkovtsev and Smirnov, 2004; see also figure 9). Mashkovtsev and Solntsev (2002) and Mashkovtsev and Smirnov (2004) attributed this feature to the vibrational stretching mode of the N-H bond (for details, see also references cited in these two articles), which is consistent with the known use of ammonium halides in the solutions employed for emerald synthesis (Nassau, 1980). The "type" of water molecules in Malossi synthetic emeralds can be determined by (destructive) mid-infrared spectroscopy in transmission mode (see box A in Schmetzer et al., 1997, for the advantages of transmission IR spectroscopy). In the diagnostic range of 3800-3500 cm-1, we recorded a single sharp absorption band at 3700 cm-1 (figure 10), which indicates that H20 molecules in Malossi stones are type I (i.e., their H-H vector is parallel to the c-axis in alkali-free beryl samples; Wood and Nassau, 1967, 1968; Charoy et al., 1996; Kolesov and Geiger, 2000; Gatta et al., in press). All this is in keeping with the absence of any significant alkali content in Malossi material, which agrees with results reported by Kolesov and Geiger (2000), who observed the same single mode at 3700 cm-1 in other hydrous synthetic beryl crystals. However, relatively recent spectroscopic and neutron diffraction studies (Artioli et al., 1995; Charoy et al., 1996; Kolesov and Geiger, 2000; Gatta et al., in press) suggest that there are some uncertainties about the vibrational behavior and orientation of H20 molecules in various beryl samples. Nonpolarized near-infrared spectra (9000-4000 cm"1) in diffuse reflectance mode of our Malossi, Russian, Biron, and Linde-Regency synthetic emeralds are displayed in figure 11. All samples show combination bands and overtones of water molecules (Wickersheim and Buchanan, 1959; Wood and Nassau, 1967, 1968; Farmer, 1974). These features are also typical of natural emeralds (see references above), whereas they are always lacking in flux synthetic emeralds. Russian hydrothermal synthetic emeralds exhibit a broad band at 8475 cm-1 (see also Koivula et al, 1996; Mashkovtsev and Smirnov, 2004) related to an optical transition involving Cu2+ ions (Mash-kovtsev and Smirnov, 2004) that is commonly absent in hydrothermal specimens from other producers. Nonpolarized UV-Vis-NIR absorption spectra of our Malossi, Russian, Biron, and Linde-Regency hydrothermal synthetic emeralds (figure 12) confirm the presence of Cr3"^ through the occurrence of two broad bands at 430 and 600 nm; peaks at 476, 637, 646, and 662 nm; and a doublet at 681-684 nm (see Wood and Nassau, 1968; Rossman, 1988; Schmetzer, 1988, 1990), similar to natural and flux synthetic emeralds. Given that the absorption peaks of Cr3* and V3+ are very close to one another (see references above and Burns, 1993), it is not possible to reliably discriminate the patterns of Malossi and Linde-Regency synthetic stones (Cr-bearing only) from those of Biron synthetic samples (Cr- and V-bearing). However, Russian synthetic emeralds show differences from the other hydrothermal synthetics: a broad band at about 750 nm, which Schmetzer (1988, 1990) related to Cu2+, as well as an absorption at 373 nm, which he associated with Fe3+. In natural iron-bearing emeralds, absorption bands for Fe3+, Fe2+, and Fe2+/Fe3+ may also be present (Schmetzer, 1988; again, see table 3).
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Written by Administrator
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Thursday, 04 October 2007 15:59 |
RESULTS AND DISCUSSION Gemological Testing. The standard gemological properties obtained on the 30 faceted Malossi samples are summarized in table 1. All the samples were transparent, with a bluish green color. They exhibited strong dichroism in yellowish green and bluish green. Their R.I. and S.G values: (1) overlapped those of their natural counterparts, especially low-alkali emeralds from various geographic localities (such as Colombia and Brazil; Schrader, 1983); (2) were similar to those we measured in Biron and Linde-Regency synthetics, and to those reported for Lech-leitner and Chinese synthetic emeralds (Flanigen et al., 1967; Kane and Liddicoat, 1985; Schmetzer, 1990; Webster, 1994; Schmetzer et al., 1997; Sechos, 1997; Chen at al., 2001); but (3) were lower than those of our Russian synthetic samples (in agreement with Schmetzer, 1988; Webster, 1994; Koivula et al., 1996; Sechos, 1997). Most flux-grown synthetic emeralds from various manufacturers have R.I., birefringence, and S.G. values that are lower than those observed in the Malossi samples (for comparison, see Flanigen et al., 1967; Schrader, 1983; Kennedy, 1986; Graziani et al., 1987). The pleochro-ism and Chelsea filter reaction of the Malossi samples were not diagnostic of synthetic origin. The various synthetics showed significant differences in their fluorescence to UV radiation: Malossi synthetic emeralds belonged to a group exhibiting red UV fluorescence that includes Linde-Regency and Chinese products, whereas Russian and Biron synthetic emeralds are inert to long- and short-wave UV radiation. The fluorescence of Malossi synthetic emeralds might hint at a synthetic origin, although a few high-Cr and low-Fe Colombian emeralds also have red UV fluorescence (Graziani et al., 1987). The Malossi synthetic emeralds showed a variety of internal features when viewed with a gemological microscope. Growth patterns of various forms (straight, parallel, uniform, angular, and intersecting), often associated with color zoning, were widespread in some of the crystals and cut stones (e.g., figure 2). Irregular growth structures (figure 3), similar to those observed in other hydrothermal synthetic emeralds, were seen in almost all the samples, providing evidence of hydrothermal synthesis. Six of the faceted Malossi synthetic emeralds contained seed plates ure 4; this seed plate had ne = 1.568, n0 = 1.573, and a birefringence of 0.005). In some cases, irregular growth zoning was seen in the synthetic overgrowth adjacent to the seed plates. The presence of a seed plate is proof of synthetic origin. "Fingerprints" and two-phase (liquid and gas) inclusions were observed in most of the Malossi samples (again, see figure 3 and figure 5). In some cases, these inclusions were similar to those observed in natural emeralds, in contrast to flux-grown synthetics, in which any fingerprint-like inclusions consist of fractures that are healed by flux filling. Fractures were also common in the Malossi synthetic emeralds, but they do not provide any evidence of synthetic origin. Two Malossi samples contained small cone-shaped growth tubes, filled with a fluid, similar to those that were recently documented in a natural emerald (Choudhary, 2005). Prismatic, transparent, and colorless crystals—alone or in aggregates—were observed in four Malossi samples (figure 6). On the basis of their morphology, birefringence, and refractive index (higher than that of emerald), such crystalline inclusions are probably phenakite (Be2Si04), which is somewhat common in hydrothermal synthetic emeralds (Flanigen et al, 1967) and also may provide evidence that the host emerald is synthetic (Kane and Liddicoat, 1985). X-ray powder diffraction of an incrustation on the surface of one Malossi synthetic emerald crystal revealed the presence of phenakite and beryl, hinting at the occurrence of an incongruent precipitation of beryl (Nassau, 1980; Sinkankas, 1981). We did not observe the lamellar metallic inclusions that are sometimes present in other synthetic emeralds (e.g., gold, which is frequently found in Biron samples; Kane and Liddicoat, 1985). |
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Last Updated on Thursday, 04 October 2007 16:01 |
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Written by Administrator
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Thursday, 04 October 2007 16:01 |
Chemical Composition. Quantitative chemical analyses of eight Malossi synthetic emeralds (samples A to H) and three other hydrothermal synthetic emeralds (one each from Russian, Biron, and Linde-Regency production) are summarized in table 2. Chromium was the only chromophore found in the Malossi samples. The following elements were below the detection limits of the electron micro-probe: Na, Mg, V, Fe (in all but one sample), Ni, and Cu. Cl, probably from the growth solution (Nassau, 1980; Stockton, 1984; Kane and Liddicoat, 1985; see also Growth Technique section), was inhomoge-neously distributed within the samples and between different specimens, as shown in figure 7. The Cl content ranged up to 0.93 wt.%, with a mean value of0.10wt.%. Figure 8 and table 2 compare the chemical properties of Malossi synthetic emeralds to those of representative samples from other hydrothermal producers. The chemical composition of Malossi synthetic emeralds is distinctively different from Russian and Biron synthetics. In agreement with the results of Schmetzer (1988), Mashkovtsev and Solntsev (2002), and Mashkovtsev and Smimov (2004), our Russian synthetic sample contained Cr, Fe, Ni, and Cu, but neither Cl nor V was detected. Although not tested for this study, Lechleitner synthetic emeralds reportedly have a similar composition (Hanni, 1982; Schmetzer, 1990). In our Biron sample, V and Cr (acting as chromophores) were found along with Cl, which is consistent with previously published results (Stockton, 1984; Kane and Liddicoat, 1985; Mashkovtsev and Solntsev, 2002; and Mashkovtsev and Smirnov, 2004). The Linde-Regency synthetic emerald was characterized by the presence of Cr and Cl (see also Hanni, 1982; Stockton, 1984), similar to the Malossi material. However, the Cl content in the Malossi samples was generally less than 0.12 wt.%, as shown in figure 7, whereas the Cl in our Linde-Regency sample was never below 0.12 wt.%, in keeping with the results of Hanni (1982), who found a Cl content of 0.3-0.4 wt.% in Linde synthetic emeralds. Cr and Cl also were recorded in the two different generations of Chinese hydrothermal synthetic emeralds examined by Schmetzer et al. (1997) and Chen et al. (2001). Schmetzer et al. (1997) indicated an average Cl content of =0.68 wt.%, in an earlier Chinese synthetic production, whereas Chen et al. (2001) reported Cl «0.15 wt.%, in the later generation, in addition to a significant NaaO content (>1 wt.%). The earlier Chinese production contains more Cl than the Malossi material; the later Chinese synthetic emerald is distinguishable from Malossi synthetics by the presence of Na. As previously reported by Hanni (1982), Schrader (1983), and Stockton (1984), chemical composition can be of great importance in separating synthetic and natural emeralds. In the case of Malossi synthetic emerald, the presence of chlorine—which typically is not found in significant amounts in natural emerald—can be an important indicator. Yu et al. (2000) reported Cl in some natural emeralds, typically at low concentrations, although some Colombian and Zambian samples contained up to 0.19 wt.% Cl. Thus, a Cl content above 0.2 wt.% provides a strong indication of hydrothermal synthetic origin. The Fe-free Malossi synthetic emeralds (except sample B, with a trace of Fe) were similar in composition to some Fe-poor natural emeralds from certain localities (such as Colombia), but they are easily distinguishable from Fe-rich natural emeralds (such as Brazilian, Zambian, and Austrian stones: see Hanni, 1982; Schrader, 1983; Stockton, 1984; Yu et al, 2000). The absence of any significant Na and Mg in Malossi synthetic emeralds (^0.01 and a0.03 wt.% oxide, respectively) can be used to separate these stones from alkali-rich natural emeralds (Hanni, 1982; Schrader, 1983). Electron-microprobe analyses of a seed plate in a Malossi sample (again, see figure 4) revealed an appreciable iron content (0.40 wt.% Fe203), whereas Cr, V, and Cl were below the detection limits. This composition, combined with the R.I. values of the seed plate, is consistent with the producer's claim that natural yellow beryl is used for the seed material (compare to Sinkankas, 1981; Aliprandi and Guidi, 1987; Webster, 1994).
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Written by Administrator
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Thursday, 04 October 2007 15:58 |
MATERIALS AND METHODS For this study, we examined 30 emerald-cut gems and 5 rough samples of the new synthetic emerald, which were provided by A. Malossi (see, e.g., figure 1). The faceted samples weighed 1.34-7.89 ct, and the rough specimens ranged from 28.40 to 141.65 ct (30.0-69.9 x 10.8-22.5 x 7.1-14.8 mm). Representative faceted samples of hydrothermal synthetic emeralds from other commercial sources (all from the collection of the Italian Gemological Institute) were studied for comparison: Russian (5), Biron (5), and Linde-Regency (1). In addition, literature comparisons were made to other synthetic emeralds produced by the hydrothermal technique (Chinese, Lechleitner), as well as to flux synthetics and natural emeralds. All the faceted samples were examined by standard gemological methods to determine their optical properties (refractive indices, birefringence, and pleochroism), specific gravity, UV fluorescence, and microscopic features. Preliminary qualitative and semiquantitative chemical analyses of 11 faceted synthetic specimens (8 Malossi, 1 Russian, 1 Biron, and 1 Linde-Regency) were obtained by a Cambridge Stereo-scan 360 scanning electron microscope, equipped with an Oxford Isis 300 energy-dispersive X-ray spectrometer, for the following elements: Si, Al, V, Cr, Fe, Ni, Cu, Na, Mg, and Cl. Quantitative chemical data (for the same elements) were obtained from these same 11 samples using an Applied Research Laboratories electron micro-probe fitted with five wavelength-dispersive spectrometers and a Tracor Northern energy-dispersive spectrometer. Room-temperature nonpolarized spectroscopy in the visible (460-750 nm), near-infrared (13000-4000 cm-1), and mid-infrared (4000-400 cm"1) regions was carried out on all Malossi, Russian, Biron, and Linde-Regency samples. We used a Nicolet NEXUS FTIR-Vis spectrometer, equipped with a diffuse reflectance accessory (DRIFT), which had a resolution of 4 and 8 cm-1 in the infrared and visible ranges, respectively. Mid-infrared spectroscopy (4000-400 cm-1) was also carried out in transmission mode using KBr compressed pellets with a 1:100 ratio of sample:KBr. Since this is a destructive technique, we restricted these IR measurements to portions of two rough specimens only. Additional UV-Vis-NIR reflectance spectra were recorded by an Avantes BV (Eerbeek, the Netherlands) apparatus equipped with halogen and deuterium lamps and a CCD spectrometer with four gratings (200-400 nm, 400-700 nm, 700-900 nm, and 900-1100 nm), a 10 ^im sht, and a spectral resolution of 0.5 nm. A polytetrafluoroethylene disk (reflectance about 98% in the 400-1500 nm range) was used as a reference sample. X-ray powder diffraction was also used to investigate an incrustation on the surface of one Malossi synthetic emerald crystal. Measurements were performed at room temperature, by means of a Bragg-Brentano parafocusing X-ray powder diffractometer Philips X'Pert, in the 6-6 mode, with CuKa radiation (X = 1.5418 A), over the range of 5° to 75° 28.
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