jade, jadeite, fei cui, Burma, gems

Microscopic examination of 102 specimens of Myanmar fei cui documents variations in mineral composition, deformation, alteration, and their effects on gem appearance. Samples were selected to represent differences in colour, grain size, appearance, and categories recognized by Hong Kong dealers. Four petrographic types are described: essentially monomineralic material, crushed material, material altered to tremolite–actinolite, and kosmochlor-rich material, termed ureyite in the paper. The first consists predominantly of jadeite crystals with minor diopside and albite, has a granoblastic texture, and shows little evidence of alteration; specimens are generally white, occasionally lavender or pale green. Crushed material displays uneven grain sizes and mortar textures, with larger crystals surrounded by fine fragments generated by deformation. Increasing crushing produces textures ranging from coarse grains with narrow crushed margins to fine-grained aggregates containing isolated remnants of larger crystals. Electron-microprobe analyses distinguish chromium-free, colourless coarse jadeite crystals from chromium-bearing, green finer crystals, explaining some green patterns on white backgrounds. Alteration to tremolite and actinolite begins along jadeite cleavage planes and grain boundaries and progresses to extensive replacement by fibrous amphiboles. Slightly altered specimens contain approximately 15–20% amphibole, moderately altered material approximately 50–60%, and strongly altered material approximately 80–98%. Advanced alteration produces fibrous textures with residual jadeite and may be accompanied by muscovite, chlorite, and quartz microveins. Tremolite-rich specimens are white to grey, whereas actinolite-rich specimens are dark green and may display an oily polished lustre and greater transparency. The relatively rare kosmochlor-rich specimens are compact, opaque, dark to bright green aggregates containing approximately 90–95% kosmochlor, with chromite and other accessory minerals. Kosmochlor occurs in granular, short-prismatic, and fibrous forms and shows strong pleochroism. Intergrowths interpreted as exsolution lamellae of jadeite in kosmochlor suggest greater mutual solubility at higher temperature and pressure, followed by separation as conditions changed. Undulose extinction and replacement textures indicate deformation and alteration. At least two amphibole generations are recognized: a coarsely fibrous generation replacing kosmochlor and a finer generation forming cross-cutting or enclosing veinlets. No measured optical properties indicate aegirine, consistent with the chemical evidence. The study distinguishes solid-solution relationships between jadeite and kosmochlor from amphibole replacement of either mineral and concludes that mineralogical and textural differences influence colour, lustre, transparency, specific gravity, and gem quality.