加拿大具“蛙卵”状色带铬钙铝榴石(沙弗莱石)的谱学特征  

Spectra Characterization of Cr-Grossular(Tsavorite) With“Frogspawn”Color Zoning From Canada

作　　者：毛晓天 陈畅[2] 尹作为[1] 王梓民 MAO Xiao-tian;CHEN Chang;YIN Zuo-wei;WANG Zi-min(Gemmological Institute,China University of Geosciences(Wuhan),Wuhan 430074,China;Yunnan Land and ResourcesVocational College,Kunming 652501,China)

机构地区：[1]中国地质大学(武汉)珠宝学院,湖北武汉430074 [2]云南国土资源职业学院,云南昆明652501

出　　处：《光谱学与光谱分析》2023年第2期520-525,共6页Spectroscopy and Spectral Analysis

基　　金：国家自然科学基金项目(41372224);湖北省珠宝工程技术研究中心科研项目(CIGTXM-03-202001)资助。

摘　　要：产于加拿大的宝石级铬钙铝榴石(又称沙弗莱石),常具有特殊的绿色团状色带,其黑色或深绿色的核心往往被边缘的翠绿色色带包围,形似“蛙卵”。为了探究加拿大具“蛙卵”状色带沙弗莱石的宝石学性质以及谱学特征,通过常规宝石学测试、LA-ICP-MS、紫外-可见-近红外光谱、拉曼光谱对该区样品进行了系统研究,旨在探明色带的颜色成因以及不同色区成分和谱学的变化。化学成分分析表明,加拿大沙弗莱石均以钙铝榴石成分为主(核部:Grossular>55.64 mol%;边缘:Grossular>83.90 mol%),但不同颜色区域的Cr_(2)O_(3)含量存在明显差异。在色带中心的深色核心区域呈现较高的钙铬榴石成分(Uvarovite平均为21.49 mol%),其中黑色的核心还具有较高的Ti含量(TiO_(2)>1.9 Wt%)。除此之外,样品还均含少量的Fe以及微量的V,Mg和Mn。紫外-可见-近红外光谱研究表明,Cr为主要的致绿色元素、Fe为致黄色元素。位于蓝紫区435 nm左右和红区603 nm左右的吸收带主要是由于Cr^(3+)所致。700 nm左右出现的双峰可作为区别V^(3+)的Cr^(3+)特征峰。Fe^(3+)在370 nm左右产生吸收峰,同时对蓝紫区435 nm处的吸收带也有贡献,1220 nm处宽缓的吸收带是由Fe_(2)+所致。通过分析Fe_(2)O_(3)与Cr_(2)O_(3)+V_(2)O_(3)含量比值能够有效地区分不同绿色调的钙铝榴石。当Fe_(2)O_(3)与Cr_(2)O_(3)+V_(2)O_(3)的比值小于或在1.61附近时,常显示较为纯正的翠绿色色调;当Fe_(2)O_(3)与Cr_(2)O_(3)+V_(2)O_(3)的比值在2.71附近时,常显示黄绿色色调;当Fe_(2)O_(3)与Cr_(2)O_(3)+V_(2)O_(3)的比值在4.38附近时,常显示绿黄色色调。拉曼光谱分析表示,样品中黄绿色至翠绿色区域均显示出典型的钙铝榴石谱。800~1100 cm^(-1)之间的拉曼峰主要是由[SiO_(4)]四面体的伸缩振动引起的;400~700 cm^(-1)之间的拉曼峰主要是由[SiO_(4)]四面体的弯曲振动引起的;400 cm^(-1)以下的拉曼峰主要是由晶格�The gem Cr-grossular(also called tsavorite)from Canada often has a special green color zoning.The black or dark green core is often surrounded by an emerald green edge,which looks like a“frogspawn”.To study the gemological and spectral characteristics of the Canadian tsavorite with“frogspawn”color zoning,the samples from this area were systematically studied by standard gemological methods,as well as LA-ICP-MS,UV-Vis-NIR spectrum and Raman spectrum,which reveal the causes of the color zoning and the changes of composition and spectroscopy in different color zones.The chemical composition analysis shows that the Canadian tsavorite is mainly composed of grossular(Core:Gro>55.64 mol%;rim:Gro>83.90 mol%),but the content of Cr_(2)O_(3)varies significantly in different color zones.The deep-colored core in the center shows high uvarovite content(Uva average 21.49 mol%).The black core also has high Ti content(TiO_(2)>1.9 Wt%).In addition,the samples also contain a small amount of Fe and a trace amount of V,Mg and Mn.UV-Vis-NIR spectra show that Cr^(3+)is the main ion causing green color and Fe mainly induces yellow color in samples.The absorption bands at about 435nm in the blue violet region and 603 nm in the red region are mainly due to Cr^(3+).The double peak at about 700 nm of Cr^(3+)can be used as the characteristic peak to distinguish the presence of V^(3+).Fe^(3+)causes an absorption peak at about 370 nm and contributes to the absorption band at 435 nm in the blue-violet region.Fe_(2)+causes the broad and weak absorption band at 1200 nm.By analyzing the ratio of Fe_(2)O_(3)to Cr_(2)O_(3)+V_(2)O_(3),color tones of grossular can be effectively distinguished.When the ratio is less than or near 1.61,it often shows pure emerald green;When the ratio is near 2.71,it often shows yellowish-green;When the ratio is around 4.38,it often shows greenish-yellow.Raman spectrum analysis shows that the sample’s yellowish-green to emerald green regions are typical grossular.The Raman peaks between 800 and 1100 cm^(-1)are m

关 键 词：铬钙铝榴石 沙弗莱石 色带 颜色成因 紫外-可见-近红外光谱 拉曼光谱 

分 类 号：P575.4[天文地球—矿物学]