Gemological And Mineralogical Studies On Green Jadeite In Different Tone From Guatemala

Guatemalan jadeite has a long history,and many Guatemalan jadeite appear on the market today,with a variety of colors and species diversity.This paper study gemological and mineralogical characteristics on 8 green jadeite in different tone from Guatemala through the XRD,EPMA,Cathodeluminescence and other tests,focusing on the phase composition and structural characteristics,and especially the formation stage of jadeite with different phase composition was speculated.The mineral composition of the green jadeite from Guatemala in different tone is different.Greyish green,yellow green,vivid green and blue green jadeites are mainly composed of jadeite;Dark green jadeite is mainly composed of omphacite;Black green jadeites is mainly composed of jadeite with a small amount of omphacite;Mottled green jadeites is mainly composed of omphacite with a small amount of jadeite.Nearly all the omphacite is P2/n type.The main mineral composition of jadeite with dense green areas is sodium amphibole,which also contains chromite,kosmochlor and chrome jadeite.The samples of jadeite phase are mainly columnar/granular metacrystal texture and recovery structure.The samples of omphacite phase show granular and fibrous metacrystal texture.The mixed phase samples show metasomatic residual structure,columnar/fibrous metacrystal texture,and the residual"phenocryst"is jadeite;Dot-G sample mainly shows the mylonite texture of sodium amphibole,and the dense green kosmochlor presents the vein metasomatic black chromite.In addition,there are jadeite with granular mosaic structure in Dot-G sample.The color of Guatemalan jadeite is related to Fe²⁺and Cr³⁺elements.Fe²⁺can make the color of Guatemalan green jadeite darker or bluer.Cr³⁺is the main element to produce green tone.According to the phase composition,green jadeite from Guatemala can be divided into 2 types:Jadeite-omphacite jade,Jadeite-sodium amphibole jade,the genesis of them are different.Jadeite-omphacite jade is mainly formed by the crystallization?Jad I?of the early jadeite fluid,along with the second-stage fluid metasomatism/crystallization?Jad II?.In the later stage,the fluid that is rich in Ca–Mg–Fe components can replace earlyJad I/Jad II,or it can coexist with Jad II,and metamorphism occurs to produce omphacite jade.The jadeite–omphacite jade can be formed when the omphacite fluid with incomplete metasomatism,with uneven texture and reduced transparency.The formation of Jadeite-sodium amphibole jade is mainly that the fluid rich in sodium flows through the original rock containing chromite.Then through the high-pressure metasomatism,it forms the kosmochlor-chrome jadeite,Na-Ca amphibole-Na amphibole.In the later stage,the metamorphism promote the mineral particles fine-grained or directional.