The refraetivity indicatrix and optical anisotropy index of minerals are two new conceptions which were adopted by the author of this paper. The refraetivity indicatrix(RI)is the ellipsoid of n-1 instead of n(refractive index), Which is defined by the following equationX2/(Ng-1)2+Y2/(Nm-1)2+Z2/(Np-1)2=1The optical anisotropy indices(OAI value)is derived from the refraetivity indicatrix. The OAI value of crystal in a given direction is equal to the ratio of the radius of RI in the direction to the mean radius of the ellipsoid section of RI, which is perpendicular to the direction. For example, for uniaxial crystals; for biaxial crystals.The refraetivity indicatrix and optical anisotropy index are very useful for the studies, of structural-optical mineralogy. The optical anisotropy indices of the isostructural minerals are discussed in this paper. The conclusions are given as follows:1. The optical anisoptropy indices in the minerals of same isostructural type are almost a constant for apatite-, barite-, beryl-, ossumilite-, cristobalite-type and so on. The standard deviation of OAI values of minerals for each above isostructural type is not exceed 0.01. In the other case the OAI values of minerals are approximately a constant with the standard deviation equal to 0.01-0.04. for example, for the minerals of zircon-, K.PtCl4-, NaCNS-type and so on.2. In the isostructural mineral groups for olivine-, ealcite-, aragonite-type and so on, a isomorphic series of minerals alone has a constant OAI value. For example, NR-OAI=1.040+0.004, for silicate-olivines, but NaOAI=1.010+0.006 for phosphate-olivines.3. The OAI values of some isostructural mineral group vary with their refractive indices or valence of atoms in the minerals. For example, the OAI value of rutite-type structural, compounds varies linearly with their refractive indies, and the OAI value of minerals of wurtzite-type structure(AB tyn inereases with the valence of B atom(Fig. 2).
叶大年. 1985, 等结构矿物的光性异向指数. 地质科学, (1): 59-69.
Yeh Danian. OPTICAL ANISOTROPY INDICES OF ISOSTRUCTURAL MINERALS[J]. Chinese Journal of Geology, 1985, (1): 59-69.