CrystalOptics1晶体光学.ppt

一轴晶正光性光率体主要切面 （二）一轴晶负光性光率体 For optically negative minerals the X indicatrix axis corresponds to the optic axis and the indicatrix is an oblate ellipsoid, i.e. flattened along the optic axis, and nomega nepsilon In each case, for positive and negative minerals the circular section through the indicatrix is perpendicular to the optic axis and has a radius = nomega. The radius of the indicatrix along the optic axis is always nepsilon. Any section through the indicatrix which includes the optic axis is called a principal section, and produces an ellipse with axes nomega and nepsilon. 三、二轴晶光率体 1、基本形态 对于低级晶族矿物，由于其a≠b≠c，相应地其三个结晶轴方向都有所差别，在光学性质上，表现在具有三个不同大小的折射率； 如镁橄榄石（斜方晶系）在X、Y、Z三个方向上的折射率分别为1.715，1.651和1.680，根据其大小次序，分别用Ng、Nm和Np（NgNmNp）表示三个折射率，称为主折射率。 三个主折射率的方向相互垂直，构成了低级晶族矿物折射率的三个光学主轴，故其光率体为一个三轴椭球体。 二轴晶光率体 The indicatrix is a shape called an ellipsoid. Every cross section of an ellipsoid is an ellipse. The longest direction of the ellipsoid is its major axis. The shortest direction, perpendicular to the major axis, is called the minor axis. Perpendicular to the major and minor axes is the intermediate axis. These three axes are called the principal axes. An ellipse with three unequal principal axes has two circular cross sections. The plane containing the major and minor axis cuts the ellipsoid in an ellipse with the maximum and minimum possible radii. Somewhere in between is a radius equal to the intermediate axis. The two circular sections have radius equal to the intermediate axis and intersect along the intermediate axis. They are shown in blue and purple at left. The direction perpendicular to each circular section is the optic axis. For a generic ellipsoid with three unequal axes, there are two optic axes. Such materials are called biaxial. Biaxial minerals are orthorhombic, monoclinic or triclinic and include the pyroxenes, amphiboles, micas, feldspars, and olivine. If the intermediate axis is closer to the major axis in length, the ci