Pignatelli I., Giuliani Gaston, Morlot C., Long P. V. (2019). The texture and chemical composition of trapiche ruby from Khoan Thong, Luc Yen Mining District, Northern Vietnam. Journal of Gemmology, 36 (8), p. 726-746. ISSN 1355-4565.
Titre du document
The texture and chemical composition of trapiche ruby from Khoan Thong, Luc Yen Mining District, Northern Vietnam
Pignatelli I., Giuliani Gaston, Morlot C., Long P. V.
Journal of Gemmology, 2019,
36 (8), p. 726-746 ISSN 1355-4565
The trapiche texture and chemical composition of two rubies from the Khoan Thong placer in northern Vietnam were examined by X-ray computed tomography, scanning electron microscopy and electron microprobe analysis. Their texture is similar to that of some Burmese trapiche rubies, with inclusion-rich sector boundaries intersecting at a small central point and a significant concentration of elongated tube-like voids in growth sectors. The most common inclusions are anorthite and margarite; the latter formed by destabilisation of both ruby and anorthite during retrograde metamorphism. The chemical composition of the samples plots in the field of Vietnamese alluvial and primary marble-hosted deposits, although differences in Fe2O3, TiO2 and Cr2O3 contents may distinguish trapiche rubies from various mining areas in Vietnam. Their composition differs from that of Burmese trapiche rubies by a higher Fe2O3/TiO2 ratio and a lower Cr2O3 content. Vietnamese trapiche rubies formed in the same geological environment as non-trapiche ones, under metamorphic conditions prevailing for the Himalayan orogenesis. In this geological context, localised fluid pressure variations and hydraulic fracturing caused changes in the driving force of crystallisation, favouring the development of the trapiche texture.