墨西哥黄绿色磷灰石的宝石学和光谱学研究

Gemological and Spectral Characterization of Yellowish Green Apatite From Mexico

墨西哥Durango地区是宝石级氟磷灰石最丰富的产地之一,对该地磷灰石的研究集中于地质年代学和矿物学标准,宝石学和光谱学资料较少。作为一种发光材料,早期学者主要利用激光诱导的光致发光光谱对未知产地的磷灰石进行研究,缺乏三维荧光光谱的研究。采用基础的宝石学测试、 LA-ICP-MS化学成分分析、红外吸收光谱、激光拉曼光谱、显微紫外-可见-近红外吸收光谱以及三维荧光光谱测试,系统性地对收集到的9颗墨西哥Durango磷灰石进行宝石学和谱学研究,旨在丰富墨西哥磷灰石的光谱学研究数据,并为产地鉴别提供科学依据。化学成分研究表明墨西哥磷灰石中稀土元素丰富,含量较高的La、 Ce、 Pr、 Nd和Sm元素,平均含量分别达3 956、 5 430、 472、 1 596和213μg·g^(-1),具有明显的轻稀土元素富集,重稀土元素亏损的特点,δEu平均值为0.29,负Eu异常显著,Ca/P摩尔比接近岩浆磷灰石的标准值1.68,为岩浆作用的产物,且形成岩浆处于中等还原状态。红外吸收光谱指纹区606和575 cm^(-1)吸收峰强度随方向的变化具有明显的规律性,能为晶体定向提供依据,官能团区显示3 482、 3 538和3 556 cm^(-1)结构水相关的吸收峰。紫外-可见-近红外吸收光谱中,由Nd^(3+)导致的528 nm的单峰和578和585 nm的双峰共同造成了黄绿区明显的透过窗,推测黄绿色体色由Nd^(3+)造成;紫外光区298 nm的吸收峰是造成可见光紫区吸收边的原因,推测由Ce^(3+)导致。三维荧光光谱最强荧光峰(λ_(ex)300 nm/λ_(em)356 nm)由Ce^(3+)的电子跃迁造成,红光区603和647 nm的发射峰由Pr^(3+)和Sm^(3+)的电子跃迁造成,与紫外灯下观察到的暗红色荧光现象相对应,推测暗红色荧光由Pr^(3+)和Sm^(3+)造成。该研究中系统的光谱学特征丰富了墨西哥产地磷灰石的谱学数据,并为产地溯源提供了科学依据。

Durango City,Mexico,is one of the richest sources of gem-quality fluorapatite.In previous studies,Mexican apatite was mainly used as a reference material for geochronology and mineralogy,with few gemological and spectroscopic materials.As a kind of natural luminescent material,early scholars mainly used laser-induced photoluminescence spectra to study apatite from unknown origin,but lacked 3D fluorescence spectra.In this paper,nine apatite samples from Durango City,Mexico,were systematically examined by using basic gemological tests,LA-ICP-MS chemical analysis,FTIR spectroscopy,Raman spectroscopy,UV-Vis-NIR absorption spectroscopy and 3D fluorescence spectroscopy,aiming to enrich thedata of spectrum of Mexican apatite and to provide the scientific information for origin determination.Chemical research shows apatite is rich in rare earth elements(REE),with high contents of La,Ce,Pr,Nd,and Sm,with average contents of 3956,5430,472,1596,and 213μg·g^(-1),respectively,which shows the characteristics of obvious enrichment of light REE and deficit of heavy REE.The average value ofδEu is 0.29,with significant Eu negative anomaly,and the Ca/P molar ratio is close to the standard value of 1.68 of magmatic apatite,which indicate that Mexican apatite is the product of magmatism and the forming magma is in a moderately reductive state.FTIR spectroscopy shows that the intensity of the absorption peaks at 606 and 575 cm^(-1)in the fingerprint region has obvious directionalregularity,which can provide a basis for crystal orientation.The functional group region shows absorption peaks of 3482,3538,and 3556 cm^(-1)of structure water.In the UV-Vis-NIR absorption spectrum,the single peak at 528 nm and the double peaks at 578 and 585 nm caused by Nd^(3+)generate the obvious transmission window in the yellow-green region.Therefore,it is speculated that Nd^(3+)causes the yellow-green color of apatite.In the UV region,the absorption peak at 298 nm is responsible for the absorption edge in the visible violet region,presumably caused by Ce^(3+).Studies on the 3D fluorescence spectrum showthe strongest fluorescence peak(λ_(ex)300 nm/λ_(em)356 nm),caused by the electron leap of Ce^(3+).In addition,the emission peaks at 603 and 647 nm in the red region are caused by the electron leap of Pr^(3+)and Sm^(3+),corresponding to the dark red fluorescence phenomenon observed under the UV lamp.Therefore,the dark red fluorescence is presumed to be caused by Pr^(3+)and Sm^(3+).The systematic spectroscopic features in this study enrich the spectroscopic data of apatite of Mexico and provide a scientific basis for origin determination.