Gemmological Characteristic and Genetic Mineralogy of Dark Green Jade

Recently, a new kind of jade called "Sajinhuaheiqingyu" appeared in the market. A series of samples have been tested on basic gemmological characteristics and analyzed by using polarizing microscope, X-ray diffraction (XRD), scanning electron microscope (SEM), electron probe micro-analyzer (EPMA) and other test methods for the structure and mineral components of the samples. The samples are dark green and the metallic flake is reflected on the surface. The gemmological results showed that the refractive index of the samples are between 1.60—1.61, smaller than that of normal nephrite, and the relative density is between 3.11—3.21, larger than normal nephrite. The results of XRD, polarizing microscope and SEM showed that the main mineral components of the samples are actinolite (45%), chlorite (45%), pyrite (5%) and other minerals (5%). The actinolite was fibrous, crystalline, fascicular, aggregative and micro directional. Chlorite showed a scale-like crystalline structure, and the peak value of d₀₆₀ is between 1.53-1.54 Ǻ, which indicates a trioctahedral structure. Chlorite and actinolite dominate the matrix components, and pyrite is embedded in the matrix. The actinolite is further named according to the Mg/(Mg+Fe²⁺) value (between 0.88 and 0.89) and the Mg/(Mg+Fe²⁺)-value vs Si-value diagram based on the results of EPMA. According to the classification diagram of Fe-Mg- (Al+□) of chlorite, the points are all plotted within the range of amesite, and within a trioctahedral structure of chlorite, which were consistent with the results of XRD analyses. According to the Fe-Si classification diagram of chlorite, the points dropped all within the range of clinochlore (one of the amesite). The naming models of two kinds of chlorite species showed that the chlorite in sample is rich in Mg and poor in Fe. It is indicated that the genesis of the samples should be related to the serpentinized ultrabasic rocks according to the lower values of the Mg/(Mg+ Fe²⁺)(≈0.9)in actinolite. According to the values of Al/(Al+ Mg+ Fe) and Mg/(Mg+ Fe) in chlorite, the results showed that the original rock series of the samples are likely ultrabasic and the magnesium-rich, but not iron-bearing, in good agreement with the result from actinolite. The correlations between Mg and Si, Mg and Al^IV, Mg and Al^VI, Mg and Fe in chlorite are studied, respectively. The results showed that the linear correlations between Mg and the above main cations were weak, indicating that the sample should have undergone multistage metamorphisms. Temperature estimates from chemical composition of chlorite indicated that the forming temperature should be between 219 ℃—252 ℃, the oxygen fugacity lgf_(O2) should be between -63.3—-59.4, and the sulfur fugacity lgf_(S2) should be between -15.6—-9.1. It is concluded that the forming environment of the sample is a low temperature hydrothermal deposit with relative reduction condition. Since the GB/T 16552-2017 Gems-Nomenclature do not well define the specific content and structure of the main components of nephrite, the high chlorite content in the sample has have a great impact on its main features. Considering the differences in mineral content in different samples, it could be named as nephrite with more than 50% actinolite. The authors suggested that the samples should not be named as nephrite, despite the accrediting institutes have named them as nephrite.