无色透明斜长石的基本特征及晕彩成因研究

Study on the Basic Characteristics of Colorless and Transparent Plagioclase and the Cause of Iridescence

无色透明斜长石具有多种晕彩色,发育聚片双晶和解理,外观与月光石相似,且不易鉴别。本文通过偏光显微镜、电子探针、X射线粉晶衍射、扫描电镜、透射电镜等测试,对无色透明斜长石的基本宝石学特征、矿物成分和结构特征进行研究,并根据光的薄膜干涉及光栅衍射原理,对其晕彩效应的成因进行探讨。研究结果表明:标本为钙长石及钠长石的类质同相混晶,端员分子式为Ab51.84An46.04Or2.11,属于中长石;由于钙长石(An)含量较低,因而折射率、相对密度低于标准拉长石,高于月光石,由此可与相似宝石区分;样品包裹体、双晶结构及晶胞参数对晕彩色的影响较小;解理厚度约为1~4μm,不会因发生薄膜干涉而产生晕彩,但可以发生光栅式衍射而对晕彩效应产生作用;透射电镜测试表明,斜长石晕彩的颜色与出溶条纹厚度之间存在联系,晕彩可由内部出溶的层状结构对光形成薄膜干涉导致。

The colorless and transparent plagioclase with varieties of iridescence is developed in polysynthetic twin and cleavage. It looks similar to moonstone and is difficult to identify. In this paper, the basic gemological characteristics, mineral compositions and structural characteristics of colorless and transparent plagioclase are studied by means of polarizing microscope, electron probe, X-ray diffraction, scanning electron microscope and transmission electron microscope;and the cause of iridescence is discussed according to the principle of film interference and grating diffraction. The results show that the sample is a mixture of anorthite and albite, and the terminal molecular formula is Ab51.84An46.04Or2.11, which belongs to andesine. Due to the low content of anorthite(An), the refractive index and relative density are lower than standard labradorite and higher than moonstone, which can be distinguished from similar gemstones. The inclusions, polysynthetic twins and cell parameters have little effect on iridescence;the cleavage thickness is about 1~4μm and iridescence can be caused by grating diffraction instead of film interference. Transmission electron microscopy test shows that there is a relationship between the color of iridescence and the thickness of exsolution stripe and iridescence can be caused by the interference of the inner exsolution layer structure.