The Study On Gemological Characteristics And Luminescent Properties Of Scapolite

Scapolite belongs to silicate.And its subspecies,such as marialite,dipyre,mizzonite and meionite,belong to complete isomorphic substitution.However,the current research mainly focuses on the end member minerals of the scapolite group,such as marialite and meionite.There is a lack of research on gemological and spectral characteristics of dipyre.In this paper,the gemmological characteristics,crystal structure,chemical composition,spectral characteristics and luminescent properties of andalusite group minerals were studied by using conventional gemmological testing,X-ray powder diffraction,electron probe,micro Raman spectroscopy,Fourier transform infrared spectroscopy,ultraviolet visible spectrophotometer,fluorescence spectrometer and other modern testing techniques.The color of the samples are colorless,yellow,purple,purple pink and maroon.The refractive index is 1.538-1.567.The birefringence is 0.005-0.019.And the relative density is2.57-2.75.The refractive index,birefringence and relative density of colorless to yellow samples are higher than purple samples.Under the short wavelength of UV fluorescence,most colorless to yellow samples can see red fluorescence,which from medium to strong.Purple samples can see purplish red fluorescence,which from medium to weak.Maroon samples have strong crimson fluorescence,and pink samples can see strong purplish fluorescence.Under the long wavelength of UV fluorescence,most of the samples have weak purple fluorescence or inert.The chemical compositions of colorless to yellow scapolite samples are mainly Si,Al,Na,Ca and O.And the contents of Na and Ca,Si and Al are negatively correlated.The calculated Me%of colorless to yellow series are 28～35,which indicates that the series belongs to dipyre.The X-ray powder diffraction data show that the diffraction peak of dipyre is similar to that of the end member minerals,such as marialite and meionite,which indicates that the crystal structure of different subspecies of scapolite group has little difference.The Al,Si occupancy of each tetrahedral site is estimated as follows:T（1）（4.33%Al,95.67%Si）;T（2）（81.50%Al,18.5%Si）;T（3）（13.01%Al,86.99%Si）.The infrared spectra characteristic peaks of colorless to yellow scapolite samples are mainly around 1215cm^-1,1035cm^-1,550cm^-1and 457cm^-1.It corresponds to the stretching vibration of Si-O,stretching vibration of Si-Al-（Si）,bending vibration of O-Si（Al）-O and bending vibration of Si-O-Si respectively.The characteristic peaks of Raman spectrum are around 160cm^-1,458cm^-1and 537cm^-1.The splitting of Raman diffraction peak in low frequency region proves the existence of Al ^VI,and the diffraction peak near 770cm^-1proves the existence of^ⅣAl.According to the data of ultraviolet-visible spectrophotometer,the color of colorless to yellow samples is related to the transition of Fe³⁺.The color of purple sample is related to the d-electron transition of Fe³⁺.The color of the pink sample is related to the d-electron transition of Mn.Under the excitation light of 282 nm,the fluorescence center of the sample is obviously red shifted,which is supposed to be caused by different S^2-centers.