The Study On Gemological And Mineralogical Characteristics Of Epidote From Pakistan

In recent years,epidote has been gradually collected as a rare gem,but the research on it is still limited.Gemological characteristics and coloring mechanism of epidote from Pakistani were analyzed in this paper,by conventional gemological testing methods and modern means of testing such as UV-Vis/NIR,infrared spectroscopy,Raman spectroscop y,scanning electron microscope,electron probe and differential thermal analysis.The epidote crystals from Pakistan are small,with high degree of self^shape,mostly in long or short plate columnar.It shows strong vitreous luster,with high transparency.I t's refractive index is between 1.714 to 1.745,birefringence 0.014 to 0.022,the relative d ensity is 3.36³.41,Mohs hardness is 6.6⁷.0.Ultraviolet fluorescence is none,and chan ges to red under Charles color filter.There are long white needle^like inclusions and a lar ge number of fluid inclusions are seen.With the increase of Fe³⁺content,the color of epi dote deepened and the refractive index and the birefringence increased as well.Pakistani epidote is more of brown tone than green,related to its distinctive plate an d columnar crystals.Strong polychromatism can be seen by the naked eye.The?100??010??001?faces are dark brown,brown and dark green respectively,while the?100?crystal fa ce is more developed than the?001?crystal face so that it is plate^like,making the?100?f ace brown dominant.Combined with the electron probe data and the absorption spectra in the visible light band,it can be seen that the strong polychromatism of epidote is mainly caused by the spin inhibited transition of ⁶A_1g?⁴T_1g,⁶A_1g?⁴T_2g,⁶A_1g?⁴E_g,⁶A_1g?⁴A_1g,and is also influenced by Mn and Ti.The color zoning in the crystal is related to the enric hment of Ti.The main chemical components of epidotes were CaO,SiO₂,FeO and Al₂O₃.The co ntents were 23.018²3.457%,38.313³8.997%,6.063⁸.712%and 25.346²7.637%res pectively,among which the contents of Fe and Al were inversely proportional.Epidote c ontains Mg,K,Mn,Ti,V,Cr,Sr and other trace elements as well,which are not evenly d istributed in the crystal.Among those trace elements,the content of MnO,TiO₂ and SrO are highist,which are between 0.038⁰.208%,0.075⁰.226%and 0.058⁰.280%,respect ively.When the temperature reaches 920?,the stability of crystal structure decreases.There are 24 and 32 groups of characteristic peaks in infrared spectroscopy and Ram an spectra,which mainly reflect the Si-O symmetric stretching vibration of SiO₄ and Si₂O₈ groups,the bending vibration of Si-O-Si,the stretching vibration of M-O and the bendin g vibration of OH.With the increase of Fe content,some of the characteristic peaks move d to the higher wave number segment,while the OH characteristic peaks moved to the lo wer wave number segment.The spectral summit of the same wave value changes with th e change of lattice direction.In addition,the"splitting"at the top of the stretching vibrati on of OH in the Raman spectrum may be related to the substitution of Fe³⁺in the M3 posi tion.