A Rare Inclusion Assemblage in a Ruby from Mogok, Myanmar: Lazurite, Sulphur and Bystrite
A 2.22 ct oval-cut ruby was recently submitted to Guild Gem Laboratories for identification (Figure 6). Fourier-transform infrared (FTIR) analysis and RIs of 1.762-1.770 confirmed the stone's identity as ruby. Trace-element analysis by energy-dispersive X-ray fluorescence revealed traces of V (120 ppmw) and Fe (60 ppmw). The chemical composition and the inclusion characteristics (described below) identified the ruby as originating from Mogok, Myanmar. However, the particular inclusion assemblage in this stone proved quite interesting.
Observation of the ruby with a gemmological microscope revealed a network of short rutile needles in one area along the girdle of the stone (Figure 7a), consistent with a Mogok origin. Also present were well-formed, colourless, transparent calcite crystals, which were partially coated by a thin, semi-transparent, bright blue layer (Figure 7b). Raman analysis revealed this blue layer to be lazurite (Figure 8). Lazurite has been reported previously in ruby and spinel as isolated crystals (Renfro & Koivula 2019; Aung 2020), but this is the first time we have seen lazurite occurring as a 'sugar coating' on calcite inclusions. Also present in the calcite crystals were some dot-like, dark yellow inclusions, which were identified as sulphur (Figure 7b). In addition, the ruby contained a platy, hexagonal, orangey yellow crystal, for which Raman spectroscopy yielded peaks at 451, 506 and 869 cm-1 (Figure 9). The inclusion was identified as bystrite by comparison to a reference spectrum for this rare mineral in the RRUFF database (rruff.info; sample R060642). Sulphur inclusions were also detected within the bystrite crystal (i.e. Raman peak at 221 cm-1).
Bystrite is a tectosilicate with the chemical formula (Na,K)7Ca(Al6Si6O24)(S5)Cl that belongs to the cancrinite group. It has been reported as associated with gem-quality lazurite and calcite in the Lake Baikal area of Russia (Chukanov et al. 2020, 2023). Until now, no other location has been reported for bystrite.
Sulphur is integral to the composition of lazurite and bystrite. The co-occurrence of sulphur and these minerals indicates a locally S-bearing environment for the formation of ruby in the Mogok area. To our knowledge, this is the first time bystrite and sulphur have been observed together in ruby. This combination of inclusions-along with lazurite and calcite-contributes to our understanding of the formation environment of Mogok ruby.
Yujie Gao, Ziyun Zhang (zziyun@guildgemlab.com)and Xueying SunGuild Gem LaboratoriesShenzhenChina
References
Aung, T. 2020. G&G Micro-World: Uncommon inclusion of lazurite in ruby from Mogok, Myanmar. Gems & Gemology, 56(4), 529.
Chukanov, N.V., Sapozhnikov, A.N., Shendrik, R.Y., Vigasina, M.F. & Steudel, R. 2020. Spectroscopic and crystal-chemical features of sodalite-group minerals from gem lazurite deposits. Minerals, 10(11), article 1042, https://doi.org/10.3390/min10111042.
Chukanov, N.V., Sapozhnikov, A.N., Kaneva, E.V., Varlamov, D.A. & Vigasina, M.F. 2023. Bystrite, Na7Ca(Al6Si6O24)S52-Cl-: Formula redefinition and relationships with other four-layer cancrinite-group minerals. Mineralogical Magazine, 87(3), 455-464, https://doi.org/10.1180/mgm.2023.29.
Renfro, N. & Koivula, J.I. 2019. G&G Micro-World: Lazurite in spinel. Gems & Gemology, 55(1), 113.
