低温热处理刚玉实验中含水矿物包裹体的特征研究

近年来，低温热处理刚玉出现在宝石交易市场，由于其特征容易与天然刚玉混淆，如何鉴定低温热处理刚玉成为宝石实验室的研究热点。在弱氧化氛围，360，610和650 ℃条件下，先后对9粒刚玉进行了热处理实验，并采用显微拉曼定性分析刚玉中的包裹体、显微镜下观察包裹体形貌、显微红外光谱分析含水矿物包裹体中羟基的特征峰等方法，对刚玉低温热处理前后的特征进行了对比研究。热处理实验揭示：600 ℃左右温度、弱氧化氛围已能有效去除刚玉中的蓝色调，并增强红色，可达到热处理改善或改变刚玉颜色的目的。研究结果表明：针铁矿、高岭石、勃姆石等含水矿物包体主要存在于刚玉的开放裂隙中，硬水铝石、磷灰石、云母等含水矿物包体主要存在于刚玉晶体中。针铁矿热处理前红外光谱可显示与羟基相关的3 435 cm-1吸收峰，并伴有以3 185 cm-1为中心的吸收宽带，经360 ℃热处理后相关吸收消失，其颜色由亮黄色变为红色；高岭石热处理前红外光谱在3 620，3 648，3 670和3 698 cm-1附近显示一组与羟基相关的吸收峰，经610 ℃热处理后相关吸收峰消失；勃姆石热处理前红外光谱显示与羟基相关的3 086和3 311 cm-1吸收峰，经610 ℃热处理后相关吸收峰消失。硬水铝石包裹体通常呈针状，热处理前红外光谱显示与羟基相关的1 980和2 110 cm-1吸收峰，经610 ℃热处理后相关吸收峰消失，但仍保持针状晶形假像；磷灰石包体通常呈透明柱状或粒状晶形，由于OH与F相互作用，红外光谱在3 550 cm-1附近显示与羟基相关的吸收峰，610 ℃热处理后相关吸收峰仍然存在，磷灰石包体的形貌未见改变；白云母呈近透明无色片状晶形分布于刚玉中，红外光谱在3 624 cm-1附近显示与云母中羟基相关的吸收峰，650 ℃热处理后这一吸收峰仍然存在，云母的形状未见变化，透明度略微降低。通过实验，证明含水矿物包裹体对于鉴定低温热处理刚玉具有重要作用。

The Characteristics of Hydrous Mineral Inclusions in Low-Temperature Heated Corundum

In recent years,some low-temperature heated corundum has occurred in the gem trade market,which characteristics are similar to nature corundum.Therefore,how to distinguish low-temperature heated corundum has become a research hotspot in gem laboratories.In this paper,nine samples were treated at 360,610 and 650℃in a weak oxidizing environment,Micro-Raman spectrometer is used to determine the mineral inclusions,the microscope is used to observe the change of inclusions,and Micro-Infrared spectrometer is used to analysis of hydrous minerals and so on,to compare the characteristics of corundum before and after low-temperature heat treatment.Heat treatment reveals that around 600℃temperature and weak oxidizing environment,the blue tone decreased and the red tone increased in the corundum,which could achieve the purpose of heat treatment to improve or change the color of the corundum.Hydrous minerals such as goethite,kaolinite and boehmite exist in the open fractures of corundum,diaspore,apatite and mica exist in the corundum crystal.FTIR absorption peaks of hydroxyl(—OH)differ from different minerals.Following is before heat treatment hydrous inclusion FTIR absorption spectra:goethite shows—OH related absorption peak in 3435 cm^-1 and absorption band center in 3185 cm^-1,kaolinite shows a group of absorption peaks in 3620,3648,3670 and 3698 cm^-1,boehmite shows absorption peak in 3086 and 3311 cm^-1,diaspore shows a peak in 1980 and 2110 cm^-1,apatite shows a peak around 3550 cm^-1,and muscovite shows peak around 3624 cm^-1.After 360℃heat treatment,related absorption peaks of—OH in goethite disappeared.After 610℃heat treatment,related absorption peaks of—OH in kaolinite,boehmite and diaspore also disappeared,but the apatite peaks still exist.After 650℃heat treatment,—OH related absorption peak in muscovite still exists.After heat treatment,goethite turns from yellow to red.Although diaspore has been dehydrated and phase changed,it still presents needle-like crystal pseudomorph.The crystal shape of apatite and muscovite has no change,and the transparency of muscovite is slightly reduced.The study provides that hydrous mineral inclusions are crucial to identify low-temperature heated corundum.