Spectroscopic Characteristics and Color Genesis of Yellowish-Green Montebrasite
The amblygonite-montebrasite series are a kind of rare mineral gemstone which shows a complete solid solution containing end members amblygonite(LiAlPO4F)and montebrasite(LiAlPO4OH).However,little research has been paid attention to amblygonite-montebrasite in China,especially on the rapid and non-destructive estimation of F contents to determine the variety and the origin of color.In this study,conventional gemology,Laser Ablation Inductively Coupled Plasma Mass Spectrometry(LA-ICP-MS),Infrared spectroscopy(IR),Raman spectroscopy(Raman),UV-VIS absorption spectroscopy(UV-VIS)were employed to test and analyze the gemstone-grade yellowish-green amblygonite-montebrasite series collected in the market.The results of conventional gemological analyses showed that the high refractive index and low relative density of the amblygonite-montebrasite series indicated that the studied samples belong to the endmember of montebrasite.The infrared spectra of the studied samples in the fingerprint region of 400～1 500 cm-1 showed an absorption band of 430 cm-1,belonging to the bending vibration of the Li—O bond.The absorptions of 494,546,604 and 664 cm-1 were caused by the joint actions of the bending vibration(v4)of PO43-and the stretching vibration of the AlO6 bond.The absorptions at～1 032,～1 090 and～1 211 cm-1 were owing to the stretching vibration(v3)of PO43-.The absorptions at～832 and 3 386 cm-1 were linked to the bending vibration of OH(δOH)and stretching vibration of OH(vOH),respectively.Raman spectra of the samples showed typical Raman peaks at 305,430,489,648,806,893,1 013,1 058,1 107,1 193,3 367 cm-1,respectively.The wide peak at～3 367 cm-1 was the typical OH stretching vibration,and a group of peaks between 1 000 and 1 200 cm-1 were the asymmetric stretching vibration of PO 43-(v3).The Raman peaks below 600 cm-1 were more complex,relating to the asymmetric stretching vibration of PO43-(v4)and the stretching vibration of the Al—O bond.Integrated estimations of F contents by using high-value refractive index,infrared and Raman spectral index indicated that the high refractive index nγ,the difference between the peak values of 3 367 cm-1 and of 1 058 cm-1 in Raman spectrum,and the FWM H index of the Raman peak near 3 367 cm-1 were good indicators of F contents in the series of amblygonite-montebrasite.It can quickly and indirectly semi-quantify the F contents of faceted amblygonite-montebrasite gemstones.The results of LA-ICP-MS showed that the main element compositions of the studied samples were P2O5,Al2O3 and Li2O,with a trace amount of Fe,consistent with the chemical composition of amblygonite-montebrasite.The Fe replaced the Al in the octahedral structure of amblygonite-montebrasite.The charge transfer between O2--Fe3+,the intervalence charge-transfer transition between Fe3+-Fe2+,and spin forbidden transition of Fe3+d—d orbit caused the absorptions of the blue-violet and the orange-red regions were consistent with the results of UV-VIS spectroscopy with observed absorptions in the blue-violet region(300～420 nm)and the yellow-orange region(near 590 nm),which may be the main reason for the color genesis of amblygonite-montebrasite showing yellowish-green tone.
万方数据
维普
