Abstract
Gems & Gemology, Spring 2013, Vol. 49, No. 1
Petrochemical Characteristics and Genesic Significance of Luodian Jade from Guizhou
Yang Lin et al., Journal of Mineralogy and Petrology, Vol. 32, No. 2, 2012, pp. 12-19
Luodian jade is a new nephrite variety discovered in 2009 at Luodian County in China’s Guizhou province. The nephrite formed within the metasomatic zone between the Varisian gabbro and calc-silicate. Geochemistry and petrology studies of both the white and greenish white varieties indicate 98% and 95% tremolite, respectively. Tremolite has an ideal chemical formula of Ca2(Mg, Fe)5[Si4O11]2(OH)2, with Mg and Fe substituting for each other to form a solid solution series. Al can also substitute for a small amount of Mg and Fe, while Na, K, and Mn can substitute for Ca and Mg. Chondrite-normalized REE (rare earth element) patterns of the nephrite and the host rock are very similar with enrichment of LREE (light rare earth elements). The concentration of REE is lower in the nephrite than that in the host rock. The La/Yb ratio is also much higher in nephrite, which indicates a higher differentiation of REE in nephrite. Compared to the greenish and greenish white nephrite, the white variety is more evolved. This tells us that the nephrite deposit formed in two stages. Ca/Mg is higher in the greenish and greenish white varieties, meaning they were formed when Mg was unsaturated in the fluid; the white variety formed when Ca and Mg were both saturated. The substitution of trace-element V and Cr into the crystal lattice is closely related to the saturation of Mg in fluid. V and Cr are also the chromophores of the greenish and greenish white varieties. Nephrite formation is caused by metasomatism during the interaction with thermal fluid brought by the intrusive gabbro and the limestone. Limestone provided Ca and Mg, while the fluid brought Si, K, Na, and Al.
