Gemmological Characteristics and Origin of Brown Olivine from Southeast Vietnam
Le Ngoc Nang, Pham Minh Tien, Pham Minh, Ho Nguyen Tri Man and Pham Trung Hieu
ABSTRACT: Olivine from Southeast Vietnam is notable for its dark brown colour, rather than the yellowish green appearance exhibited by typical gem-quality peridot. Discovered in 2020, this material occurs as xenocrysts weathered from the Xuan Loc alkali basalt. It is transparent to opaque, with a vitreous lustre. It has relatively high RIs (nα = 1.680–1.685 and nγ = 1.702–1.715) and relatively low birefringence (0.022–0.035), as well as elevated SG values (3.39–3.55). Inclusions consist of distinctive curly fibres of serpentine, and a hazy appearance of some stones is caused by amphibole particles. Raman and FTIR spectroscopy show features characteristic of olivine, and both powder X-ray diffraction and EDXRF chemical analysis indicate the olivine is forsterite. Based on geothermometer calculations from trace-element contents, the olivine crystallised from garnet peridotite in the temperature range of approximately 1400–1460°C and a pressure of 50 kbar. Although this brown olivine has limited usefulness as a gem material, its unusual properties and inclusions may draw the attention of gem collectors.