| Zircon inclusions in unheated pink sapphires from Ilakaka, Madagascar: A Raman spectroscopic study
M.S. Krzemnicki, P. Lefèvre, W. Zhou, H.A.O. Wang
Photos © M.S. Krzemnicki and SSEF, except where indicated otherwise
“Hot”pinksapphirefrom Ilakaka(Madagascar)andsmallzirconinclusionsinsuchapinksapphire.
• Pink to purple sapphires from Ilakaka (Madagascar) play an important role in the gemstone trade.
• They often contain numerous zircon (and monazite) inclusions.
• These zircons are often aggregated and clustered.
Figures:Xu& Krzemnicki . JournalofRamanSpectroscopy2021,pp.1 11.DOI10.1102/jrs.6092
• Zircon inclusions are found in corundum from many different geological settings and geographic origins.
• They and may provide crucial information, both, about heat treatment and origin.
• Raman spectroscopy to analyse the crystallinity (or metamictisation) of zircon inclusions mainly based on the position and FWHM of the υ3 Raman peak (SiO4 anti-symmetrical stretching) at about 1010 cm 1 (Nasdala et al. 1995, Zhang et al. 2000, Wang et al. 2006).
Zhang et al. 2000 Nasdala et al. 2001
• The position and shape of the Raman peak υ3 of zircon is mainly influenced by radiation damage (metamictisation) or annealing.
• The aim of this ongoing research project is to study and document the variability of Raman spectra of zircon inclusions in unheated pink to purple sapphires from Ilakaka.
• Specifically also in the light of how evident it is to separate unheated stones from Ilakaka from heat treated ones based on criteria derived from Raman spectra.
• More than 35 unheated pink to purple sapphires from Ilakaka (SSEF research collection), either rough or cut.
• More than 100 analyses on zircon inclusions within these samples.
• Focus on Raman peak υ3 (SiO4 anti-symmetrical stretching mode)
• FWHM by fitting into a software-integrated Gaussian-Lorentzian function after baseline correction (Wire software).
zircons at surface
υ1 versus υ3 scatterplot
υ1
υ3
Raman shift (cm 1)
υ3 versus FWHM(υ3) scatterplot
zircons at surface
• Lines connect analyses of several zircon inclusions in same sample.
FWHM: 7.86 cm 1
FWHM: 13.69 cm 1 FWHM: 10.49 cm 1
FWHM: 8.43 cm 1
FWHM: 7.52 cm 1
FWHM: 8.95 cm 1
FWHM (υ3) of three different zircon inclusions in single unheated pink sapphire from Ilakaka (sample 118667_7)
Measured on the same zircon inclusion at slightly different positions (sample 118622_3)
• Comparison of FWHM (υ3) ranges observed in the studied unheated pink to purple sapphires from Ilakaka (Madagascar).
Conclusions
part 1:
• Zircon inclusions in unheated pink to purple sapphires from Ilakaka (Madagascar) show a rather large variability of υ3 peak positions and shapes (FWHM).
• The FHWH may vary considerably in different zircon inclusions in a single pink to purple sapphire from Ilakaka.
• The FWHM may even vary to some extent in a single zircon inclusion.
• The measured FWHM range from about 7.5 to >20 cm 1 in our unheated samples from Ilakaka (Madagascar), with a majority of FWHM between 7.5 to 10 cm 1 .
• The observed peak positions and FWHM values are very similar to those of unheated blue sapphires from Madagascar (from Ilakaka and other deposits, see also Xu & Krzemnicki, 2021).
υ3 versus FWHM(υ3) scatterplot
Pink spots: unheated
Yellow spots: heated
• Largely overlapping FWHM (υ3) ranges of unheated and heated pink sapphires from Ilakaka (Madagascar).
Conclusions part 2:
• In gem testing, the FWHM (υ3) of zircon inclusions in pink sapphires from Ilakaka (Madagascar) is often taken into consideration to separate unheated stones from heated ones (Wang et al. 2006, Wanthanachaisaeng et al. 2006 , Nasdala et al. 2008, Krzemnicki 2010, Saeseaw et al . 2020).
• The present data reveals that the FWHM (υ3) is a rather soft criterion with considerable overlap of values for unheated stones and heated stones.
• The observed range of FWHM and peak positions in unheated pink sapphires from Ilakaka may indicate (among others) variation in trace element chemistry, (heterogeneous) radiation damage, diverse detrital zircon provenance.
• Further research on selected samples using SEM, CL, GemTOF,...
