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Colourless Chrysoberyl from Mogok
first published in Facette 22 (February 2016)
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Chrysoberyl is found in various colours, mostly yellow to green and brown due to iron, light green to bluish green (vanadium) or as the colour-changing variety alexandrite (due to chromium).
In rare cases, chrysoberyl is also found in colourless and chemically rather pure crystals. Recently, the authors had the opportunity to study a tabular colourless chrysoberyl reportedly originating from an alluvial placer in the Mogok Gemstone Tract in Myanmar.
The most interesting feature in this sample were small nail-head spicules oriented along the c-axis of the chrysoberyl, similar to the ones observed in synthetic emeralds and occasionally also in natural gems such as sapphire, spinel, tourmaline and quartz (Schmetzer et al., 1999, 2011; Choudhary and Golecha, 2007).
An examination of these nail-head inclusions with Raman micro-spectroscopy revealed the presence of CO2 in a completely enclosed hollow spicule and secondary iron (hydro)oxide in a hollow tube reaching the surface of the specimen. The colourless inclusions at the ends of the enclosed growth tubes were identified as entrapped chrysoberyl inclusions.