夕卡岩中石榴石的双折射性之研究

本文研究了河北省涉县符山铁矿夕卡岩中石榴石的双折射性。该石榴石属钙铝-钙铁榴石系列,有较强的双折射性,重折率最大值约为0.009。加热可使双折射性减弱甚至消失。依形成温度,可将石榴石分为两群,重折射率相差一、两个数量级,形成温度较低者其双折射性强。在每一群中,重折率与分子式中八面体位置 Al 含量成上弯曲线关系,最强双折射大致在 And₆₅ Gro₃₅成分处。X 光单晶衍射照相出现异常,衍射点变成由数个点组成的拉长的线,这些点沿一定方向排列,强度和数目有变化,可能是因为结构中八面体位置上 Fe³⁺/Al 有序引起的。有序引起石榴石对称性降低,变为二轴晶,可以较好地解释石榴石双折射性的成因及其变化规律。本文还讨论了环带与双折射性的关系。

AN INVESTIGATION OF BIRE FRINGENCE OF GARNETS FROM A SKARN DEPOSIT

Some grandite garnets from a skarn deposit show strong birefringence. The birefringent garnets are biaxial with 2V≈50°(+)and the maximum birefraction recorded is about 0.009. Experimental results suggest that birefringence is controlled by the formation temperature, according to which two distinct groups of garnets are divided. The garnets formed at lower temperatures show much stronger birefringence than those formed at higher temperatures. In each group the relation between birefraction and the amount of Al in the octahedral positions is a convex line with the maximum birefraction at a composition around And₆₅ Gro₃₅. X-ray photographs are anomalous in that the diffraction spots become dotted lines. The anomaly is considered to have resulted from the ordering of Fe³⁺ /Al in the octahedral positions, which is also thought to be the cause of birefringence of garnet. Because of the ordering, the symmetry of garnets is reduced so that they become biaxial. The phenomena concerning the birefringence are well explained with this mechanism. The relation between zoning and birefringence is also discussed.