Study On Gemmological And Mineralogical Characteristics Of Chrysocolla Chalcedony And Its Application In Origin Traceability

Chrysocolla chalcedony is the most valuable and rarest quartzose jade so far.It's from the oxidation zone of porphyry copper deposit,and colored by the minute chrysocolla.Because of the attractive color and fine texture,chrysocolla chalcedony is beloved by many people.One of the famous origins of good-quality chrysocolla chalcedony is Taiwan,China.But with the diminishing of production,other sources are gradually entering into Chinese market in recent years.The material from Taiwan usually has higher price,but the ones from other origins and even some imitations are really confusing for customers.Up to now,the study about chrysocolla chalcedony is limited,how to identify it and separate the geologic localities efficiently are urgent problems.This study is about the gemological,mineralogical and trace-element characteristics of chrysocolla chalcedony samples from five commercially important localities—Taiwan,Indonesia,USA,and Mexico.Then,this study based on the important features to trace the geographical origins and discuss the mineralizing process.The results of basic gemological tests showed all the samples'color is uneven and ranges from blue to bluish green;transparency-translucent to opaque;specific gravity-varies in a large range;fluorescence—inert to the UV light;internal features-inhomogeneous.But there are some differences:the Indonesian samples have better transparency and their color usually with some green hue,and Peruvian samples are greener,Mexican samples sometimes have blue and yellowish green color at one sample;Several Taiwanese samples present obvious yellowish green fluorescence under short wave UV light.By using polarizing microscope,EPMA and scanning electron microscope,the microstructure and micromorphology of chrysocolla chalcedony were observed:(1)In thin sections,the quartz exists as fine crystallites,relatively coarser crystals and radiating fibers(i.e.chalcedony),while chrysocolla sometimes is not readily differentiable,but sometimes shows yellowish green,colorful,or brown anomalous interference colors;(2)backscattered-electron images showed obviously that the chrysocolla aggregates is usually present as irregular patches or fibrous rims and clusters;(3)SEM-EDS investigation revealed the intimately intergrowth pattern of chrysocolla and quartz,in the area with more chrysocolla,flaky and fibrous crystals are common,while in the area with less chrysocolla,euhedral quartz crystals could be seen.Through the tests of X-ray powder diffraction,FTIR spectra,Raman spectra,UV-Vis-NIR absorption spectra,and Thermogravimetric analysis,the mineral composition of chrysocolla chalcedony samples were investigated:(1)the spectra of XRD,FTIR and Raman indicated that?-quartz is dominant in all samples,but chrysocolla peaks are not often seen,only if there is certain content of chrysocolla,mixed spectra are presented;(2)the XRD pattern also showed some weak peaks of impurity minerals;Using typical“five fingers”diffraction peaks of quartz to calculate the crystallinity of quartz in our samples,then obtained the averaging crystallinity index is around 6,and the crystallinity of quartz in Mexican samples is the highest,averaging crystallinity index up to 7.89;(3)Raman spectra generally presented the 501 cm^–1 peak of moganite,combining the?-quartz main peak 464 cm^–1 could infer the content of moganite,and its distribution following the regulation—fine crystalline quartz area>short fibrous chalcedony>long fibrous chalcedony>coarser crystalline quartz area;(4)Due to the poor crystallinity and tiny grains of chrysocolla,it's easier to be detected by UV-Vis-NIR absorption spectra.The main absorption band of chrysocolla chalcedony range from 700 to 720nm,which generated by Cu²⁺,the transmission window in visible area decided the overall color,three narrower bands in near infrared area are caused by water;(5)TGA experiment showed the water in chrysocolla chalcedony varies from 1.05%to 11.98%,and more chrysocolla content leads to more total water;(6)by using Raman spectra to identify the impurity minerals from the surrounding rock,showed obvious differences in the samples—there're a series of copper mineral mixture(native copper,cuprite and tenorite)in Indonesian samples;conichalcite,nearly perfect calcite crystals,and andradite are diagnostic minerals in Mexican samples;minerals associated with propylitic alteration(albite,epidote,chamosite and hematite)and coexist with radiating malachite aggregates are common in Morenci mine samples from USA;special black vernadite-like mineral is representative in Inspiration/Ray mine samples form USA;but the similar black mineral in Taiwanese samples is manganite;lots of goethite and quartz mixture usually exists in Peruvian and part of Taiwanese samples.Chemical composition,including major elements and trace elements of chrysocolla chalcedony are determined by EPMA and LA-ICP-MS.The results indicated that:(1)Si and Cu are the main components of chrysocolla chalcedony samples,while Si,Cu,As,Ca for some Mexican samples;(2)obviously high U values typify the Taiwanese samples,while especially high Mo values in Inspiration/Ray mine samples form USA;a little higher U,Mo,Sb,W,Pb,Bi,Zn,Sr values are indicative of Mexican samples;Fisher-LDA analysis is useful to separate samples from Indonesia,Morenci mine of USA and Peru.In conclusion,the characteristics of microstructure and main mineral composition in chrysocolla samples are similar,but the features of impurity minerals and chemical composition are significantly different,and the gemological properties also have some differences.The identification of origins could base on the major differences above,if impurity minerals are visible with an optical microscope,Raman analysis should be used to identify them;Material without observable impurity minerals should undergo chemical composition methods.What is noteworthy is that applying a combination of the approaches described above will help give a clearer determination of a sample's origin.In addition,according to the characteristics of microstructure and mineral composition in chrysocolla samples could inferred the mineralizing process preliminarily:first,the copper and silica enrich solution and gel is formed when the post-magmatic hydrothermal fluid filled the fault,fracture and breccia in porphyry copper deposit;then chrysocolla intergrow with quartz intimately,and the crystallization sequence is(1)colloidal/small fibrous chrysocolla co-precipitated with initial crystalline quartz;(2)long fibrous chrysocolla and fine crystalline quartz are formed;(3)followed by the growth of chalcedony;(4)the crystallization of coarser crystalline quartz.When the initial environmental condition is stable,chrysocolla could occur as long fibers or radiating spherical aggregates directly.Finally,this study devised a useful flow chart of identification and origin traceability for chrysocolla chalcedony,which can provide a reference for the daily identification,and solve the problem of confusing origin and price in jewelry market.