Spinel on Sapphire

Recently the authors examined a 4.8 cm tall pale blue sapphire crystal (figure 1) that featured numerous purple spinel crystals up to 8 mm in size on the surface (figure 2), a rather uncommon association. Steve Dubyk of Albuquerque, New Mexico, had acquired this specimen and doubted the accuracy of the accompanying label, indicating aquamarine from Tres Pozos in Baja California, Mexico. Raman spectroscopy confirmed Mr. Dubyk’s suspicion that the specimen was in fact corundum and identified the associated purple crystals as spinel. LA-ICP-MS chemical analysis suggested that the specimen was from Sri Lanka based on the trace elements present, which indicated an average of 130 ppma iron (Fe), 38 ppma titanium (Ti), and 37 ppma magnesium (Mg). The trace element chemistry is also notable, as the magnesium will preferentially charge compensate the titanium, leaving very little excess titanium to pair with iron to produce a blue color. The chemistry measurements were consistent with the very pale blue color observed in this stone. The chemistry of the spinel was also interesting in that its reasonably saturated purple color would indicate it was at least partially caused by chromium. However, the spinel was inert to long-wave UV light, suggesting no chromium was present. LA-ICP-MS testing confirmed the absence of chromium but showed relatively high iron (10,566 ppma) and some cobalt (2.75 ppma average) which together are likely responsible for the purple color (A. Palke and Z. Sun, “What is cobalt spinel? Unraveling the causes of blue color in blue spinel,” Fall 2018 G&G, p. 262). While spinel is uncommon in sapphire, a spinel inclusion has been previously reported in a Sri Lankan yellow sapphire (Winter 2015 Micro-World, p. 444). This sapphire is one of the more unusual examples the authors have encountered.

Purple spinel crystals were intergrown on the outermost layer of the sapphire crystal, which appears to be from Sri Lanka based on trace element chemistry. Photomicrograph by Nathan Renfro; field of view 13.42 mm.

Article republished with permission from GIA

https://www.gia.edu/gems-gemology/fall-2020-microworld-spinel-on-sapphire

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