Petrology And Mineralogy Of Mangui Meteorite From Xishuanbanna,Yunan

The object of study in this paper is a meteorite that landed in Xishuangbanna Dai autonomous prefecture,Yunnan province,China,which has been named Mangui meteorite by The International Meteorite Nomenclature Committee.Among the meteorites collected in the world,the meteorite falls are relatively rare.Due to the new landing,the weathering of the earth is relatively light,therefore,the fall meteorites have been a hot topic of meteorite research.This paper uses optical microscope,Electron Probe Micro-analyzer?EPMA?,Scanning Electron Microscope and other scientific instruments to study Mangui meteorite.The petrological observation indicates that the meteorite experienced strong recrystallization,but retained the residual chondrule texture,the outlines of chondrules had become blurred,the matrix had almost all recrystallization.The secondary feldspar grains in the meteorite were large and widely distributed between the chondrules and other silicate minerals.The main mineral assemblages of the meteorite are olivine,low-Ca pyroxene,plagioclase,kamacite,taenite and troilite.According to EPMA analyses,the average composition of olivine,Fa value is 24.3,the average composition of low-calcium pyroxene,Fs=20.5,Wo=1.4;the relative standard deviation?PMD?of Fa and Fs are all less than 5;the average content of Ni in taenite is22.3 wt.%,and the average content of Ni in kamacite is 4.93 wt.%.Several shock veins were found in the thin sections,in which the melted spherules of troilite aggregate were found,the silicate minerals in the meteorite were seriously broken,and the local Mosaic extinction phenomenon was found.According to the comprehensive evaluation,this meteorite is an ordinary chondrite with a chemical group of L,a rock type of type 6,a degree of weathering of W₁,a degree of impact of S₄.In addition,the texture characteristics of the fusion crust of Mangui meteorite were analyzed.The meteorite has a complete fusion crust.The fusion crust has a three-layer structure,the outermost layer is vitreous layer,the middle part is partial melting layer,the inner layer is widely distributed with dendritic troilite veinlets.According to the observation and analysis,the troilite veinlets were mainly distributed in olivine.According to the comparison of different mineral expansion coefficients,the author believes that the cause of the troilite veinlets is that olivine is easy to form brittle fracture,and the troilite at high temperature is permeated and filled into the olivine fractures.In addition,a comparative study was conducted on olivine and feldspar in the partial melted layer and inside the meteorite.Based on the analyses,it was concluded that the contents of FeO,TiO₂ and MnO in olivine in fusion crust decreased to different degrees,while the contents of FeO,TiO₂,Cr₂O₃ and MnO in pyroxene in fusion crust decreased to different degrees.This shows that the partial melting of olivine and pyroxene in fusion crust has taken place with high temperature reduction.By measuring the Ni content and width of taenite in the center of taenite that coexists with kamacite,it can be concluded that the cooling rate of meteorite parent is more than 100?/Ma.The recrystallization of chondrules in Mangui meteorite was studied based on the rock type,and the recrystallization process of chondrules was analyzed according to the different chondrules.As the basis of the study of meteoritics,especially the detailed study of the fall meteorites,ordinary chondrites are likely to provide us with some new understanding,which is of great significance for understanding the history and evolution of the early solar system.