Pink to Purple Sapphires from Ilakaka, Madagascar: Insights to Separate Unheated from Heated Samples

Abstract

The present study is focused on the analysis of zircon inclusions found in pink to purple sapphires from Ilakaka (Madagascar) with an optical microscope, Fourier-transform infrared (FTIR), and micro-Raman spectroscopy in order to update previous knowledge and find insights to separate heated from unheated samples. In total, 157 zircon inclusions in 15 unheated samples and 74 zircon inclusions in 6 heated samples are analysed using micro-Raman spectroscopy with standardised parameters. The full width at half maximum (FWHM) of the main Raman band due to anti-symmetric stretching vibration ν3 of the SiO4 tetrahedron in the zircon structure has been carefully measured. In the unheated samples, it ranges from 6.26 to 21.73 cm−1 with an average of 10.74 cm−1, a median of 10.04 cm−1, and a standard deviation of 2.84 cm−1. On the other hand, it is lower in the heated samples, ranging from 4.83 to 14.97 cm−1 with an average of 7.23 cm−1, median of 7.06 cm−1, and standard deviation of 1.63 cm−1. In our unheated samples, the FWHM was rarely below 7 cm−1. In our heated samples, the FWHM was rarely above 12 cm−1 but mostly below 8 cm−1, with a variation restricted to less than 3 cm−1 in the same sample. The present work will hopefully further contribute to more accurately identifying the low-temperature heat treatment of pink sapphires from Ilakaka, Madagascar.

Dehydration of Diaspore and Goethite during Low-Temperature Heating as Criterion to Separate Unheated from Heated Rubies and Sapphires

“…In addition, tiny rutile (TiO 2 ) needles commonly present in corundum as so-called "silk" may start to (partially) dissolve at temperatures > 1400 • C. In contrast to this, a heat treatment process at about 800 • C to 1200 • C is generally known in the trade as "low-temperature" heating. It usually results in a slight (but desirable) to distinct (e.g., Fe-rich metamorphic sapphires) shift of color and may leave inclusions visually unaltered [6,7]. As such, the detection of heat treatment in corundum today relies not only on "classic" microscopic observations but also on infrared spectroscopy (FTIR) and Raman micro-spectroscopy.…”

“…The presence and arrangement of OH − -Ti 4+ related bands in the mid-infrared spectral range [8][9][10][11][12][13][14] of corundum of metamorphic origin (not applicable for basaltic sapphires) and the peak width (FWHM) of the main Raman peak (at about 1010 cm −1 ) of tiny zircon inclusions in corundum (mainly pink sapphires from Ilakaka, Madagascar) are considered important criteria to separate unheated corundum from heated stones [6,15,16]. However, both mentioned analytical approaches have certain limitations, i.e., hydroxide-related bands in infrared spectra are not always present in heated rubies and sapphires [17,18], and the FWHM of the main Raman peak of zircon inclusions in unheated and heated corundum may show considerable overlap [6,16]. Additionally, the Raman bandwidth of zircon inclusions is strongly dependent on the concentration of radioactive trace elements, crystallinity (metamictisation), and finally, geological and geographic origin where the corundum formed [19][20][21].…”

Editorial for the Special Issue “Gem Characterisation”

“…Treatments to improve the colour, appearance and/or durability of gems have been used for several years; over recent decades, these have increased and become more sophisticated [10][11][12][13]. In the present issue, four studies on the treatment of gems are published [14][15][16][17]. Zhou et al [14] present the results of the method they developed for the hightemperature copper diffusion process for the surface recolouring of faceted labradorites.…”

“…Detection of this treatment is not easy as the temperature is relatively low and induces little change in internal features, but a combination of UV-Vis-NIR spectroscopy and trace element chemistry provided by LA-ICP-MS might give evidence of such treatment. Two papers on the detection of the low-temperature heat treatment (i.e., below 1200 • C) of corundum by studying inclusions are also published in this Special Issue [16,17]. Krzemnicki et al [16] present a study on rubies and sapphires containing diaspore and goethite inclusions.…”

“…Based on their experiments and in agreement with the literature, the dehydration of diaspore in corundum occurs between 525 and 550 • C, whereas goethite transforms to hematite between 300 and 325 • C. As both diaspore and goethite might be present as inclusions in gem-quality corundum, these dehydration reactions and phase transformations can be considered important criteria to separate unheated from heated stones. Karampelas et al [17] focus on the analysis of zircon inclusions found in pink to purple sapphires from Ilakaka (Madagascar). It is found that by using Raman spectroscopy, the full width at half maximum (FWHM) of the main Raman band of zircon inclusions in the unheated samples from Ilakaka (Madagascar) is slightly larger than in heated samples.…”