分析卡—钠双晶的光学现象

来源期刊：中南大学学报(自然科学版)1981年第3期

论文作者：阮道源

文章页码：106 - 110

关键词：光率体； 卡斯巴双晶； 光学现象； 双晶现象； 双晶轴； 干涉色； 方向关系； 特殊方向； 赤平极射投影； 切片

摘    要：以双晶两光率体剖面及剖面半径与偏光镜方向交角是否相同,即以双晶两单体干涉色与亮度是否相同及这二者综合结果来阐明卡—钠双晶光学现象。并指出前人文献对这些光学现象描述的不周全,及⊥α轴切片上卡—钠双晶与卡斯巴双晶光学现象的相反。用赤平极射投影图示卡—钠双晶等有关双晶⊥(010)晶带的切片上两光率体方向上关系,以阐明卡—钠双晶光学现象,其中包括卡—钠双晶与卡斯巴双晶两特殊方向切片上的光率体关系,及偏近这两个特殊方向的两类切片上光率体的关系,此外还图示卡—钠双晶两光率体之间有以钠长石双晶轴和卡斯巴双晶轴先后转动180°的方向关系。

Abstract: In Carlsbad-albite twins which commonly occur in numerous lamellaethere are Carlsbad twins and Albite twins. The relation of the twins between two lamellae is in fact that of theindicatrixes to the direction. In a thin section with the twins perpendicularto the composition plane, the sections of the two indicatrixes and their direc-tions may indicate this relation when the direction of the composition planeand that of the thin section are known. In a thin section of the plagioclase perpendicular to the composition planewhich is parallel to the longitudinal cross hairs or forms an angle of 45°with it, the combined results of the interference color and the brightness insome grade determine whether the optical phenomena of the twins are promi-nent or whether they appear or not. Stereographic projection may show correctly and clearly a rotation of180° between two indicatrixes in the two lamellae of every twinning law. The parallel twins and the complex twins of plagioclase are not the sameas the normal twins. In thin sections that are perpendicular to the composition plane, the sec-tions of the two indicatrixes in two lamellae are commonly not the same andare asymmetric. In some of these sections, extinction angles of the two lam-ellae are unequal but with the same sign, while in the others, neither theextinction angles nor the signs are the same. There are two sections with par-ticular directions that are in exception. One is parallel to the twinning axiswhere the sections of the indicatrixes are the same and are symmetrical; theother is perpendicular to the twinning axis where the sections of the indica-trixes and their directions are all the same.