芬兰晕彩斜长石晕彩成因探讨——包裹体及其与晕彩的关系

摘要

Macroscopic and microcosmic observations of the plagioclases from plagioclasite of southern Finland indicate that the plagioclases with parallel needle-like inclusions has iridescence, while the plagioclases without inclusions do not have iridescence.The inclusions can be divided into two types based on their degrees of development.Dense-type inclusions are 35～50 stripes/mm with 0.01 ～0.03 mm in length and 0.004 mm in width, while sparse-type inclusions are 25～35 stripes/mm with 0.005～0.015 mm in length and also 0.004 mm in width.The plagioclases with dense-type inclusions are mainly blue while the sparse-type inclusions are orange and red in color.A compositional analysis by means of scanning electron microscope, electron microprobe and laser Raman spectrometry shows that the inclusions are magnetite in composition, and the plagioclases with or without inclusions have the same composition.It is suggested that the composition of the plagioclase and inclusions and the twin of the plagioclases are not the causes for the iridescence of the plagioclases, and the parallel distribution of the needle-like opaque inclusions in the plagioclase seems to be the factor responsible for the iridescent plagioclase.The needle-like parallel inclusions in plagioclases resemble the grating in physics.The iridescence of the plagioclase is probably generated by the diffraction and interference of the light.%对芬兰斜长岩中斜长石的宏观和微观观测显示,无晕彩的斜长石包裹体不发育,有晕彩的斜长石发育定向排列的针状包裹体.依据斜长石中包裹体的发育程度,可将包裹体分为两类:密集型包裹体(35～50条/mm)和稀疏型包裹体(25～35条/mm).发育密集型包裹体的斜长石的晕彩以蓝色为主,发育稀疏型包裹体的斜长石的晕彩以红色或橙红色为主.电子探针、扫描电镜和激光拉曼光谱的分析结果表明,该晕彩斜长石中的所有包裹体在成分上一致,均为磁铁矿,有晕彩的斜长石和无晕彩的斜长石的成分也一致.这表明该斜长岩中的斜长石是否具有晕彩与斜长石的成分、包裹体的成分及斜长石的双晶发育程度无关,而与包裹体的发育程度有关.斜长石和其中定向排列的磁铁矿针状包裹体类似于光学中的光栅效应,斜长石中晕彩的出现可能是光栅衍射和干涉造成的.