黄绿色羟基磷铝锂石的谱学特征及其颜色成因

Spectroscopic Characteristics and Color Genesis of Yellowish-Green Montebrasite

磷铝锂石为含F和OH的连续固溶体矿物,属于一种稀有宝石品种,然而国内对其鲜有研究报道,尤其对快速无损估计F含量从而确定宝石名称以及宝石颜色成因研究较少。对市场上收集的黄绿色宝石级磷铝锂石分别进行常规宝石学、激光剥蚀电感耦合等离子体质谱仪(LA-ICP-MS)、红外光谱(FTIR)、拉曼光谱(Raman)、紫外-可见吸收光谱(UV-Vis)测试与分析。常规宝石学测试结果显示较高的折射率和较低的相对密度的特征均指示研究样品属于羟磷铝锂石端元。研究样品在400~1500 cm^(-1)的红外光谱指纹区表现为430 cm^(-1)的吸收谱带,归属Li—O键的弯曲振动;494、546、604和664 cm^(-1)的吸收为PO43-的弯曲振动和AlO6键的伸缩振动共同作用所致;~832 cm^(-1)为OH的弯曲振动(δOH);~1032、~1090、~1211 cm^(-1)为PO43-的伸缩振动。~3386 cm^(-1)左右,是OH的伸缩振动(νOH)。拉曼光谱显示样品具有305、430、489、648、806、893、1013、1058、1107、1193和3367 cm^(-1)等典型拉曼峰。在3367 cm^(-1)左右的相对较宽的峰是典型的OH伸缩振动,1000~1200 cm^(-1)之间的一组峰是PO43-的非对称伸缩振动(V3)。在600 cm^(-1)以下的拉曼光谱峰,为PO43-的非对称伸缩振动和Al—O键的伸缩振动所致。通过利用折射率高值指数、红外和拉曼光谱指数对磷铝锂石F含量进行了综合估算,折射率高值指数nγ、拉曼光谱中3367 cm^(-1)左右的峰值和1058 cm^(-1)左右的峰值之差以及3367 cm^(-1)附近拉曼峰的半高宽(FWHM)指数是氟磷铝锂石-羟磷铝锂石系列中F含量的良好指标,可被用于快速和间接地半定量估计磷铝锂石刻面宝石的F含量。激光剥蚀电感耦合等离子体质谱LA-ICP-MS结果显示,研究样品的主要元素组成为P2O5、Al_(2)O_(3)和Li_(2)O外,还含有微量的Fe,与羟磷铝锂石的化学成分相符。Fe在磷铝锂石中以类质同象取代Al,O^(2-)—Fe^(3+)之间电价转移,Fe^(3+)—Fe^(2+)价键间电荷转移和Fe^(3+)的d—d轨道的自旋禁阻跃迁同时对蓝紫区和橙红区的吸收,与紫外-可见光谱测试结果反映出蓝紫区(300~420 nm)和黄橙区(590 nm附近)的吸收较为一致,可能是导致羟磷铝锂石呈黄绿色调的主要原因。

The amblygonite-montebrasite series are a kind of rare mineral gemstone which shows a complete solid solution containing end members amblygonite(LiAlPO 4F)and montebrasite(LiAlPO 4OH).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 400~1500 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(ν4)of PO 3-4 and the stretching vibration of the AlO 6 bond.The absorptions at~1032,~1090 and~1211 cm^(-1)were owing to the stretching vibration(ν3)of PO 3-4.The absorptions at~832 and 3386 cm^(-1)were linked to the bending vibration of OH(δOH)and stretching vibration of OH(νOH),respectively.Raman spectra of the samples showed typical Raman peaks at 305,430,489,648,806,893,1013,1058,1107,1193,3367 cm^(-1),respectively.The wide peak at~3367 cm^(-1)was the typical OH stretching vibration,and a group of peaks between 1000 and 1200 cm^(-1)were the asymmetric stretching vibration of PO 3-4(ν3).The Raman peaks below 600 cm^(-1)were more complex,relating to the asymmetric stretching vibration of PO 3-4(ν4)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 3367 cm^(-1)and of 1058 cm^(-1)in Raman spectrum,and the FWMH index of the Raman peak near 3367 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 P_(2)O_(5),Al_(2)O_(3)and Li_(2)O,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 O^(2)-—Fe^(3+),the intervalence charge-transfer transition between Fe^(3+)—Fe^(2+),and spin forbidden transition of Fe^(3+)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.