环状硅酸盐的结构光性矿物学研究

A STUDY ON STRUCTURAL-OPTICAL MINERALOGY OF CYCLOSILICATES

自从伍斯特的著名论文发表以来,层状结构的矿物是以负光性为特征,链状结构的矿物是以正光性为特征,这是众所周知的。但是环状结构的硅酸盐矿物,究竟以正光性还是以负光性为特征,并不是几句话可以说清楚的问题。环状硅酸盐矿物的光性与结构之间的关系是十分有趣的,并且有些问题至今仍未解决。

The optical sign and birefringence of a cyclosilicate mineral are not only depended on the kinds of the ring of Si-O tetrahedrons and their orientations, but also on the crystallochemical sites of cations. These cations in cyclosi-licates may be classified into several types such as the large cations of "Mica-type", the octahedron cations of "Benitoite-type", the tetrahedron cations of "Beryl-type", the chain-type octahedron cations, the layer octahedron cations of "Sandwich-type" and so on. The one-dimentionality factor always co-exists with the two-dimentionality factor in a crystal structure. On condition that the one-dimentionality factor overpowers the two-dimentionality factor, the crystal has a positive birefringence, conversely, on condition that the two-dimentionality factor overpowers the one-dimentionality, the crystal has a negative birefringence. In the cyclosilicates the structure elements of Si-O tetrahedron ring are a two-dimentionality factor. The cations of "Mica-type" and "Chain-type" are always a factor of one-dimentionality resulting in a positive birefringence, on the contrary, the cations of "Benitoite type" and "Sandwich type" are always a factor of two-dimentionality resulting in a negative birefringence. The cyclosilicates with tetrahedrons of "Beryl-type" have a structure of three-dimentional framework, and have very low birefringence.