Mineralogical Characteristics Of Semitranslucent Green Jadeite Jade And Their Indicative Significance

Semitranslucent green jadeite jade refers to a uniformly deep or vivid green,semitranslucent to opaque jadeite jade.Based on the systematic collection of previous researches,this paper systematically collected representative semitranslucent green jadeite jade samples and control samples,and observed general appearance macroscopic features and conventional gemological properties,examined mineral constituents,textures and structures under gem microscope and polarizing microscope;and researched on chemical composition quantitatively by backscattered electron images and EPMA.Some research progress and new understandings have obtained.Semitranslucent green jadeite jade can be divided into three categories:the first type contains more chromite,and the mineral assemblage is chromite,kosmochlor,chrome-rich jadeite or chrome jadeite（or chrome omphacite）,magnesioriebeckite;The main minerals of type Ⅱ are green jadeite,containing a small amount of magnesioriebeckite,and some samples occasionally contain residues of kosmochlor + chromite assemblage,without obvious oriented structure;the main minerals of type Ⅲ are green or dark green jadeite,containing a small amount of magnesioriebeckite,no residues of kosmochlor + chromite assemblagee,with oriented structure.The specific gravity of the first type of sample is usually close to the upper limit in the national standard range,and certain individual samples can reach 3.63;the specific gravity values of the second and third types of samples are mostly around the lower limit of the national standard.The refractive index of dark green portion in the first type is 1.71,and the green portion is 1.66;the refractive index of the second type and the third type sample is 1.66;the refractive index of the albite strip contained in the individuals samples is 1.53.In the first type of semitranslucent green jadeite jade,in the chromite fracture or on the periphery of chromite,the dark green monoclinic pyroxene has an zonal structure or rhythmic banding structure,boundary with chromite is regular,containing a large number of fluid-melt inclusions,crystallied from fluid-melt system instead of the result of hydrothermal alteration.Type Ⅱ contains kosmochlor or kosmochlor + chromite residues,the formation of which is closely related to Type Ⅰ.Most of the main mineral constituents of Type Ⅱ and Type Ⅲ have two stages of formation.In the early stage,the white or light-colored coarser particles are of cataclastic texture or porphyroclast texture,and the contents of Cr₂O₃,Fe O^* and MgO are lower;The particles formed in the late stage are finer,distributed along the edges of the early stage particles or growing in their intergranular spaces,and the contents of Cr₂O₃,Fe O^* and MgO are significantly increased.The formation process is not accompanied by dynamic recrystallization,so that the particle gap is not reduced,so the green color is bright but not transparent.The color of the second type and the third type is due to the Cr³⁺ substitution caused by the chromium-rich hydrothermal process,that is,the"concentration of Cr³⁺".Hydrothermal action causes Cr³⁺ to form chrome-containing jadeite,which should also be the formation mechanism of high-color jadeite.The residues of chromite+kosmochlor have also been observed in certain samples of white-and-green,green coarse-grained,finer coarse-grained green jadeite and green Maw-sit-sit,indicating that the formation of them is closely related to Type Ⅰ.