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by gagan choudhary
Update on Mexifire Synthetic Fire Opal
Authors: Rajneesh Bhandari and Gagan Choudhary
Introduction
In Choudhary and Bhandari (2008), we described a new synthetic fire opal marketed as “Mexifire.” The article detailed the material’s gemological properties, chemical composition, and infrared spectra, as well as providing a brief outline of the manufacturing process. Since November 2009, this process has been slightly modified: Water content is now controlled in such a manner that the refractive index and specific gravity values are much closer to those of natural fire opal. This article presents the properties of this new version of Mexifire; the previous version is no longer being produced, although some undoubtedly remains in the marketplace.
Figure 1: These new samples of Mexifire synthetic fire opal (3.40–4.40 ct) were manufactured by a modified process and exhibit properties different from those recorded in the original production characterized in 2008.
Materials and Methods
We examined nine faceted ovals (3.40–4.40 ct; figure 1) representative of the new Mexifire production. Standard gemmological tests were performed on all samples. In addition to the nine faceted samples, two slices (one previous version and one new version) with parallel surfaces were made for IR analysis. Both the slices measured 10.06 x 8.06 x 4.75 mm. Qualitative energy-dispersive X-ray fluorescence (EDXRF) chemical analyses of all samples were conducted using a PANalytical Minipal 2 instrument under two different conditions: Elements with a low atomic number (e.g., Si) were measured at 4 kV tube voltage and 0.850 mA tube current, while transition and heavier elements were measured at 15 kV and 0.016 mA. Spectroscopic measurements of all samples in the infrared range (6000–400 cm-1) were performed with a Shimadzu IR Prestige 21 Fourier-transform infrared (FTIR) spectrometer, operating at room temperature with a diffused-reflectance accessory in transmittance mode. We used a standard resolution of 4 cm-1, and recorded 50 scans per sample.
Results and Discussion
Physical and Optical Properties
Figure 2: The turbid zones observed in almost every Mexifire sample remain an identifying criterion. Also note the scattered pinpoints throughout. Magnified 15x
The most significant development with these products was the fact they had higher RI and SG values than the Mexifire synthetics studied previously. All the new samples yielded consistent RI and SG readings of 1.470 and 2.19, respectively, which are closer to those of their natural counterparts (again, see table 1). Simoni et al. (2010) reported RI values for natural fire opal from Bemia, Madagascar, of 1.440−1.460 and SG values of 2.15−2.38. The SG values of these new Mexifire synthetics clearly overlap those of the fire opals from Madagascar, though the RI values, while close to natural fire opals, are sufficiently higher to allow a clear distinction. However, because there are minute variations in the calibration of refractometers of different makes and models, one must be very careful when using RI to separate natural from synthetic fire opals.
Microscopic Features
In addition to the zoned turbidity, these samples displayed fine pinpoints scattered throughout (figure 3), like the earlier product (Choudhary and Bhandari, 2008). These pinpoints were clearly seen with fiber-optic illumination, but they were only weakly visible with darkfield illumination. We could not resolve the exact nature of the pinpoints with the instruments we used. Although similarly scattered flake-like inclusions have been seen previously in natural opals, and Gübelin and Koivula (2005) mentioned tiny grains of pyrite scattered throughout a stone, we did not find any reports of such “pinpoint” inclusions in natural opal.
Figure 3: The exact nature of the scattered pinpoints in figure 2 could not be resolved at 80x magnification
Table 1:
EDXRF Analysis
As in Choudhary and Bhandari (2008), only Si, Fe, and Ca were detected in the Mexifire synthetic fire opals. There were no additional elements. Gaillou et al. (2008) report the presence of Al as a major impurity in natural opals; however, we did not detect any Al in the natural samples we studied for the previous article (Choudhary and Bhandari 2008) or received for identification at the laboratory over the years. In our samples, we recorded the same results for both the natural and the synthetic opals.
FTIR Analysis
The IR spectra of the new Mexifire product were quite different from those of either natural fire opal (studied in the earlier article or at the Gem Testing Laboratory in Jaipur) or the earlier synthetic product (figure 4). All nine samples displayed a weak hump at ~5440 cm-1, a sharp peak with a shoulder ~4520 cm-1, an absorption band in the 4000–3250 cm-1 region, a weak feature at 2652 cm-1, , and complete absorption of wavelengths below 2400 cm-1.
The earlier version of Mexifire had an absorption band in the 5350–5000 cm-1 region; this feature also consisted of a series of sharp peaks, depending on the transmission. A hump was observed in the 4600–4300 cm-1 range, often with small peaks (a feature absent in some natural opals, including fire opal). The detector was saturated by strong absorption below ~4000 cm-1. The absorption at ~5440 cm-1 in the new Mexifire product is attributed to O-H stretching/vibration, the peak at ~4520 cm-1 is due to a combination of O-H stretching and Si-O-H bending, and the absorption band in the 4000–3250 cm-1 region is due to the presence of O-H groups (Yamagishi et al., 1997).
Figure 4: This representative IR spectrum of a slices of the new Mexifire synthetic opals (top) is quite different from that of the previous version (centre) and from natural fire opals (bottom) studied by the authors in the past. Slices of old and new Mexifire opals were measured 4.75 mm thick while the natural samples were in the range of 0.60 to 4.00ct.
These differences in the IR spectra reflect the lower water content of the new type of Mexifire opal. Although some of the differences could also have been due to variations in sample thickness (i.e., thicker samples would have greater absorbance and vice versa), the fact that the samples studied for the previous article were smaller (0.23–3.50 ct) than those in the present study (3.40–4.40 ct) negates this possibility. To confirm this, we cut slices of equal thickness (4.75 mm) from one piece each of old and new Mexifire opal and polished two parallel faces; the IR spectra of both samples were similar to the spectra described above. In case of the slice of new Mexifire opal, a sharp peak at 2262 cm-1 was resolved out of complete absorption below 2400 cm-1 and this absorption was reduced to 2100 cm-1; rest of the features / peaks remained unchanged.
It should be noted, however, that some natural opals from Ethiopia show absorption features similar to those seen in this new Mexifire product (E. Gaillou, pers. comm., 2010). Therefore, it does not appear that IR spectra provide a conclusive means of differentiating these new Mexifire opals from natural opal.
Conclusions
The higher RI and SG values of these new Mexifire synthetic fire opals will make their identification more difficult. However, the microscopic features are unchanged, and the fine pinpoints scattered throughout remain helpful in identifying the synthetic product. IR spectra, when used carefully, can offer some identification criteria, although similar absorption features have been seen in some natural opals from Ethiopia. The changes in the RI and SG values correlate with changes seen in the IR spectra as a result of lower water content. Since the water content in these synthetics can be controlled, we anticipate additional changes in the properties of future product. Work is ongoing to characterize this material as it evolves.
References
- Choudhary G., Bhandari R. (2008) A new type of synthetic fire opal: Mexifire. G&G, Vol. 44, No. 3, pp. 228–233.
- Gaillou E., Delaunay A., Rondeau B., Bouhnik-Le Coz M., Fritsch E., Cornen G., Monnier C. (2008) The geochemistry of opals as evidence of their origin. Ore and Geology Reviews, Vol. 34, pp. 113-126.
- Gübelin E.J., Koivula J.I. (2005) Photoatlas of Inclusions in Gemstones, Vol. 2. Opinio Publishers, Basel, Switzerland.
- O’Donoghue M. (1988) Gemstones. Chapman and Hall, London.
- O’Donoghue M., Ed. (2006) Gems: Their Sources, Description, and Identification, 6th ed. Butterworth-Heinemann, Oxford, UK.
- Simoni M., Caucia F., Adamo I., Galinetto P. (2010) New occurrence of fire opal from Bemia, Madagascar. Gems & Gemology, Vol. 46, No. 2, pp 114-121
- Webster R. (1994) Gems: Their Sources, Descriptions and Identification, 5th ed. Edited by P. G. Read, Butterworth-Heinemann, Oxford, UK.
- Yamagishi H., Nakashima S., Ito Y. (1997) High temperature infrared spectra of hydrous microcrystalline quartz. Physics and Chemistry of Minerals, Vol. 24, pp. 66–74.
Acknowledgments
The authors acknowledge Dr. Eloise Gaillou of the Smithsonian Institution, Washington, DC, for her valuable input on the manuscript.
All photographs and photomicrographs by Gagan Choudhary
Refer Part 1, for details on older material