MINERALOGY AND GENESIS OF GEM-QUALITY SERPENTINE JADE FROM KOREA

A new gem-quality serpentine jade deposit was recently discovered in the Booyeo area, about 250km southwest of Seoul, Republic of Korea, and is being mined by the Korea Green Jade Mining Company. After the serpentine jade has been fashioned as rings, beads, cabochons, pendants, earings, it is traded on the domestic market under the name 'Booyeo precious serpentine'. Serpentine jade forms along both contacts of a grossular garnet vein intruding the age-unknown serpentinite. It is almost transparent or semi-transparent and deeply green with an oily luster. It is very tough and measured about 4-5 on Mohs' scale of hardness. Its specific gravity and refractive index are determined to be 2.57 and 1.56, respectively. X-ray powder diffraction analysis indicate that the serpentine jade material is antigorite. The major chemical compositions(wt.%) are SiO₂(42.49), MgO(39.08), Fe₂O₃(3.85), and H₂O(11.87). Important trace elements(ppm) are Cr(2188) and Ni(1110). It contains numerous black, opaque, and fine grains which are identified, by X-ray Gandolfi camara method and SEM, as magnetite. Under the microscope, there is significant difference, in mineral texture, between gem-quality serpentine and serpentinite. DTA/TGA thermogram gives peaks at 343, 755, 830.1^oC. The endothermic reaction at 755^oC denotes expulsion of structural water, and the strong exothermic reaction at 830.1^oC is related to the formation of olivine. Heat treatment of serpentine jade at 840^oC for 40 days is found to turn antigorite to olivine structure. An IR spectrum shows absorptions at 3670, 1190, 1070, 980, and 610cm^-1. The strong peak at 3650cm^-1 is due to OH stretching, and those at 1190, 1170, and 980cm^-1 are due to SiO stretching. The 610cm^-1 band resulted from OH bending vibration. The sharpness of OH absorptions at 3650 and 610cm^-1 may indicate this precious serpentine jade of trioctahedral 1:1 layer silicate structure has a relatively ordered structure.