Black Nephrite Jade from Guangxi, Southern China

ephrite is an essentially monomineralic rock composed primarily of tremolite-actinolite [Ca (Mg,Fe) Si O (OH) ] amphiboles 2 5 8 22 2 in a felted microcrystalline habit (Leake et al., 1997), making it greasy, tough, and easy to carve.Nephrite mainly forms in two settings (Harlow and Sorenson, 2005;Harlow et al., 2014).The first is dolomite replacement by silicic fluids commonly associated with granitic plutonism (i.e., D-type).Important deposits occur at Xinjiang, China (Liu et al., 2011a(Liu et al., , 2015)); Chuncheon, South Korea (Yui and Kwon, 2002); and Cowell, South Australia (Flint and Dubowski, 1990).The other type of setting is serpentinite replacement by Ca-metasomatism at contacts with more silicic rock (i.e., S-type).Related deposits include Siberia, Russia (Prokhor, 1991); South Island, New Zealand (Wilkins et al., 2003); British Columbia, Canada (Leaming,1998); and Fengtien, Taiwan (Yui et al., 2014).Nephrite found in the southern Chinese region of Guizhou forms in a different setting, within the metasomatic zone between diabase and limestone bearing siliceous rock (Yang, 2013).In 2011, a new nephrite deposit was discovered in southern China at Yantan Town, Dahua County, in the Guangxi Zhuang Autonomous Region (Li et al., 2011).The Guangxi deposit is separated from the Guizhou deposit by the Hongshui River.Nephrite from Guangxi comes in a wide variety of colors such as white, green, brown, and black, and some varieties exhibit either a banded structure or a dendritic pattern (Wang et al., 2012;Yin et al., 2014).Among them, the nephrite with a homogeneous, pure black color reminiscent of Chinese calligraphic ink is the most prized by jade carvers and traders (figure 1).Several scholars have studied the mineralogy, spectroscopy, and genesis of black nephrite from Guangxi (Wang et al., 2014;Mo and Mao, 2016;Peng et al., 2017).The results suggest that it consists mainly of actinolite or ferro-actinolite and features a high iron content.Wang et al. (2014) proposed that the Guangxi deposit is a skarn-type occurrence and conducted preliminary analysis of its formation process.But few studies have discussed how high iron content influences its gemological properties or vibrational spectra.Moreover, detailed data and systematic research on its formation mechanism are still limited.This study employed standard gemological testing, polarized microscopy, scanning electron microscopy, electron microprobe analysis, infrared spectroscopy, Raman spectroscopy, and ultraviolet/