Crystallographic Orientation Of Inclusions In Garnet Exhibiting Asterism

There are many kinds of garnet minerals and they are widely distributed.However,only a few garnets exhibit asterism,and there are few researches on garnet exhibiting asterism.In this paper,the basic gemological properties,chemical composition characteristics,spectroscopic characteristics and crystallographic characteristics characteristics of asteriated garnet are tested and analyzed by means of electron probe,infrared spectrum and Raman spectrum,so as to determine the varieties of asteriated garnet and the crystallographic orientation relationships between the acicular inclusions and garnet,and to discuss its formation mechanism and its origin significance.The color of asteriated garnet is mainly brownish red-purplish red.Some only display oblique 4-ray asterism,and a few can exhibit 6-ray asterism and orthogonal 4-ray asterism at the same time,with refractive index>1.78,relative density value in3.91-4.15.The inclusions are abundant,mostly mineral crystals and fractures,mainly oriented acicular inclusions.Electron probe microanalysis shows that all the samples of asteriated garnet belong to almandine,and the acicular inclusions in garnet are rutile,in addition to quartz,ilmenite,apatite,zircon,uraninite.The D peak of infrared spectrum,which is approximately negative correlation with the content of pyrope,is mostly weak,some of which are nearly absent,indicating less pyrope content.The Raman spectrum shows the characteristic absorption.The characteristic peaks of 4-ray asteriated garnet samples are 919cm^-1,559cm^-1,351cm^-1,and 6-ray asteriated garnet samples are 921cm^-1,559cm^-1,351cm^-1.Rutile in the asteriated garnet presents needle,point and other forms.Rutile needles along two<111>_grt directions form two light bands which intersect at 70°/110°exhibiting oblique 4-ray asterism.Rutile needles parallel to the<100>_grt directions forms the third light band,which is located in the middle of the obtuse angle of the other two light bands.Three light bands intersect at 70°/55°/55°exhibiting 6-ray asterism.In theory,there are seven possible sets of rutile needles oriented along four<111>and three<100>directions of garnet.The rutile needles in garnet are mainly oriented along the<103>_rt//<111>_grt and<001>_rt//<001>_grt directions following multiple crystallographic orientation relationships.The rutile needles in 4-ray asteriated garnet mainly follow the direction of<103>_rt//<111>_grt.Several groups of rutile needles oriented in garnet indicate the exsolution genesis of needle-like rutile.The garnet with oriented rutile needles is mainly formed in the rocks with extreme temperature and pressure conditions,which is a reference for judging whether the rocks have undergone ultrahigh temperature or ultrahigh pressure metamorphism.In this paper,the clarity of asterism of garnet is divided into five grades:clear,relatively clear,medium,fuzzy and nearly none.From strong to weak,the brightness of the light bands gradually decreases,and the boundary with garnet gradually blurs.With the weakening of asterism,the shape of rutile needles gradually shortened from long needle to short needle,and its distribution changed from dense uniform to sparse uneven.