Characteristics And Quality Evaluation Of Gem-grade Olivine In Jilin Jiaohe

Olivine is an ancient gemstone,and its characteristic is the color of olive green.Ancient Egypt is the earliest country to use olivine.The history can be traced back to more than a thousand years ago;especially the large grain single-crystal olivines with flawless are highly sought after by high-end consumers in recent years.At the same time,olivine is also an important object in the study of mantle petrology and deep geological processes.China is an important producer of large-grain gem-grade olivine,Jilin jiaohe is the second largest producer of olivine in China.This paper takes Jilin Jiaohe olivine as the research object,based on the microscopic petrology research of host basalt,peridotite xenoliths and olivine peritectic.We mianly used gemological conventional instrument test,infrared and Raman spectroscopy,ultraviolet-visible spectrophotometer to analyse and determine the gemological and mineralogical characteristics of the olivines in Jilin jiaohe.And the electron probe and LA-ICP-MS analysis were the important study methods to focus on the large-grain olivine peritectic and peridotite xenoliths.Through the elements geochemical comparison,we can investigate the elemental characteristics differences and genetic mechanisms of different gem-grade olivines.Based on the characteristics differences in gemology and mineralogy,we proposed the basis and criteria for olivine quality evaluation.The main achievements of this paper are as follows:?1?Field investigation and petrographic studies have shown that according to the output form,Jilin Jiaohe olivines can be divided into two types:peridotite xenoliths and olivine peritectic.The former occurs in the form of a small grain olivine aggregation.The lithology is mainly lherzolite with cataclastic structure and porphyroclast structure.The olivine porphyroclast can be used as large grain gem-grade olivine.The latter refers to large grain single crystal or double crystal olivine produced in alkaline basalt.?2?Gemmological research found that Jilin Jiaohe gem-grade olivine is mainly composed of forsterite,chrysolite and transitional varieties,which is brittle and granular form,perfect crystal form is rare.From the optical properties,they are mainly yellow-green and brown-green,vitreous luster,transparent,refractive index1.653¹.688;From the mechanical property,mohs hardnessis lower than the standard value;relative density?3.33³.36g/cm³?is stable;it can be seen irregular shell-like fractures or cracks and the specific"water lily leaf-like inclusions"under magnified inspection.Under the polarizer,there are four bright and four dark and irregular extinction phenomenon,indicating that the stress of olivine aggregation is strong.Infrared spectroscopy test showed that the spectral differences on different crystal faces were extremely large,indicating that the olivines were in different optical indicatrix directions and crystal growth structures.2350 nm indicated that the CO₂-rich melt-fluid was closely related to the formation of olivines;ultraviolet-visible spectrophotometer confirmed that 457nm,477nm,497nm,639nm are the characteristic absorption spectrum of Jilin Jiaohe gem-grade olivine,and the absorption intensity of different tonality is different.?3?Situ elemental geochemical studies have found that the Ni content of olivine in peridotite xenoliths and olivine peritectic crystals is regularly different.The sufficient octahedral coordination of bivalent cations in the olivine facilitates the isomorphic substitution of Ni and magnesium iron.The olivine in the peridotite xenoliths can be divided into high Ni and low Ni types,?high Ni>0.3%,low Ni<0.1%?,Fo value is between 89.1⁹1.3.The Ni element content is higher,the greener the color,the deeper the output environment,the longer the crystal accumulation time,the higher the density and the higher the hardness.The large grain olivine can be used as the gem-grade olivine.While the low-Ni olivines were produced in a relatively shallow environment with the characteristics of melt metasomatism in late stage,and the density and hardness are relatively low,the color is slightly yellowish and brownish.The olivine peritectic are all high Ni,and the Fo value is higher than 89%;the gem-grade olivine is mostly due to this.The content of Ni element is increased from the crust to the core and enriched in the early mantle stage of olivine.The analysis of trace elements in monoclinic pyroxene in peridotite xenoliths by LA-ICP-MS showed strong deficit K,Nb and Nd,confirming that the primitive mantle was depleted mantle.Combined with spectroscopy,it is shown that the formation of olivine in this area has experienced two times magmatism.?4?Jilin Jiaohe gem-grade olivine is mainly peridotite xenoliths?including peritectic?in alkaline basalt,which is cumulus crystal as residual mineral before magma eruption;it occurs in the deep mantle in the form of layered,which is dominated by lherzolite.The basalt magma formed by mantle fluid metasomatism and fusion,along with the frequent volcanic activities in the Cenozoic period,the peridotite xenoliths and peritectic were carried out of the surface through the deep and large faults to form the present olivine deposit in Jilin Jiaohe.?5?This paper attempts to establish a practical olivine evaluation system and model,using the range of the statistical value to determine the price of olivine,and providing a market reference for the quality evaluation of gem-grade olivine.We also propose relevant improvements suggestions for the development of gem-grade olivine,and discuss the deep geological significance and cultural value of the olivine.