日本彩虹石榴石的矿物学特征研究

Mineralogical characteristics of Japanese rainbow garnet

利用带能谱仪的扫描电镜、电子探针、傅立叶红外光谱仪和激光拉曼光谱仪对日本奈良县吉野郡天川村彩虹石榴石样品进行了矿物结构、化学组成及光谱学特征的研究,分析彩虹效应成因,推测其形成环境。背散射图像可见深浅不一的条带交互排列,能谱半定量分析显示,浅灰色条带元素组成与较纯的钙铁榴石一致,深灰色条带元素组成为含Al的钙铁榴石,两者互层形成薄层结构,这种特殊结构使光发生干涉和衍射作用从而产生彩虹效应。电子探针测试确定日本彩虹石榴石主体成分接近纯钙铁榴石。在彩虹色矿物表面(简称彩虹面),垂直的薄层结构构成衍射光栅,使入射光产生光栅衍射,而平行彩虹面生长的薄层使入射光发生干涉作用,两种作用产生的光波结合形成彩虹色。日本彩虹石榴石的反射红外光谱中可见[SiO4]峰位以及受少量Al-O结构影响的Fe-O结构峰位,其红外光谱特征与含有少量Al的钙铁榴石结构对应。激光拉曼光谱测试发现Al含量高的部分较Al含量低(或无)的部分峰位向高频方向移动2cm^-1,整体谱学特征与钙铁榴石特征一致。日本彩虹石榴石具有钙铁榴石和铝含量较高的钙铁榴石交互排列的结构,据此推测其在富Fe贫Al的环境中形成。

The mineral structure,chemical compositions and spectral characteristics of rainbow garnet samples from Tenkawa,Yoshino area,Nara Prefecture,Japan,were studied by Scanning Electron Microscopy with Energy Dispersive Spectrometer,Electron Microprobe,Fourier Transform Infrared Spectrometer and Laser Raman Spectrometer. In this paper,the authors analyzed the factors responsible for the rainbow effect and interpreted rainbow garnet’s formation environment. The backscattering image shows stripes of different shades,the semi-quantitative analysis of the energy spectrum proves that the elemental composition of light gray stripe is the same as that of pure andradite,and the dark gray stripe’s elemental composition likes that of Al-containing andradite. The two components form a lamellae structure,which causes interference and diffraction of light to produce a rainbow effect. Electron microprobe analysis makes sure that the main component of Japanese rainbow garnet is close to that of pure andradite. On the surface of the iridescent mineral( rainbow face),the vertical lamellae structure forms a diffraction grating,which causes the incident light to undergo grating diffraction. The lamellae of parallel rainbow growth cause interference of incident light. The light waves generated by the above two actions combine to form the iridescent color. The [SiO4]peak position and the Fe-O peak position are affected by a small amount of Al-O structure in the reflection infrared spectrum,which proves that the infrared spectrum characteristics of the Japanese rainbow garnet correspond to those of the andradite structure containing a small amount of Al. The laser Raman spectroscopy test reveals that the peak position of the high Al moves 2 cm^-1 in the high frequency direction compared with that of the less Al content( or none),and the overall spectral characteristics are consistent with the structural features of the andradite. Japanese rainbow garnet is intercalated with andradite and Al-bearing andradite. It is inferred that it was formed in an environment rich in Fe and depleted in Al.