Heated Purplish Pink Sapphire from Ilakaka (Madagascar) with Colored Monazite Inclusions
The Laboratoire Français de Gemmologie (LFG) received a 3.02 ct purplish pink sapphire (figure 1) for identification. Under the microscope, zircon and monazite inclusions (figure 2) along with tubes (figure 3) were observed. This inclusion scene was characteristic of similarly colored sapphires from Ilakaka, Madagascar (W. Wang et al., “The effects of heat treatment on zircon inclusions in Madagascar sapphires,” Summer 2006 G&G, pp. 134–150; S. Saeseaw et al., “Low-temperature heat treatment of pink sapphires from Ilakaka, Madagascar,” Winter 2020 G&G, pp. 448–457). No indication of heating was observed. Moreover, the monazite inclusions were orange-brown. It was previously observed that monazite inclusions in pink sapphires turn colorless when heated above 600°C (see again Saeseaw et al., 2020).
Using Raman spectroscopy, the full width at half maximum (FWHM) of the band around 1010 cm–1 due to anti-symmetric stretching vibration SiO4 in the zircon structure and the band around 975 cm–1 due to PO4 stretching vibration in the monazite structure can decrease during heating of pink sapphires from Ilakaka (Wang et al., 2006; Saeseaw et al., 2020). The zircon and monazite inclusions were analyzed using a Raman Renishaw inVia spectrometer, with a 514 nm diode-pumped solid-state (DPSS) laser, with about 10 mW laser power on the sample, 50× long working distance objective lens, confocal mode (100 microns entrance slit), 1800 lines/mm grating, and about 1.5 cm–1 spectral resolution. The FWHM of the most intense band of seven monazite inclusions at around 975 cm–1 ranged from 12.7 to 14.2 cm–1, and those of 10 zircon inclusions at around 1010 cm–1 ranged from 6.4 to 7.8 cm–1. Both appeared to be sharper and less variable compared to FWHM in unheated pink sapphires, which present zircon inclusions with FWHM of the main band from 7.1 to 21.7 cm–1 and monazite inclusions with FWHM of the main band from 14.2 to 18.4 cm–1 (Saeseaw et al., 2020; M. Krzemnicki et al., “Zircon inclusions in unheated pink sapphires from Ilakaka, Madagascar: A Raman spectroscopic study,” Proceedings of the Online International Gemmological Conference, 2021, pp. 21–23; S. Karampelas et al., “Détection du traitement thermique à basse température des corindons,” Revue de Gemmologie a.f.g., No. 217, 2022, pp. 4–5).
Unpolarized Fourier-transform infrared spectra in different directions were collected using a Thermo Fisher Magna-IR560 with 4 cm–1 spectral resolution and 500 scans using a diffuse reflectance accessory as beam condenser. The sample presented a band at 3309 cm–1 and a less intense band at 3232 cm–1, which in pink sapphire is considered an indication of heating (Saeseaw et al., 2020). These bands are linked to different forms of titanium associated with hydroxyl defects in corundum (E. Balan, “Theoretical infrared spectra of OH defects in corundum (α-Al2O3),” European Journal of Mineralogy, Vol. 32, No. 5, 2020, pp. 457–467).
In line with previous experiments showing that monazite inclusions can keep their color after heating to 600°C (Saeseaw et al., 2020), the presence of colored monazite inclusions alone cannot serve as evidence that a pink sapphire from Ilakaka is unheated. On the other hand, the presence of colorless monazite inclusions in pink sapphires from Ilakaka could be considered an indication of heat treatment.
