青金石中的结构有序化和调制及其与光性异常间的关系

青金石结构有无序和有序之别。无序青金石为光性均质体，但它的有序化及由此引起的结构调制，则有可能使青金石产生光性异常。使结构仍保持立方对称的有序化并不改变它光性上的均质性，而其他形式的有序化以及结构调制，如果它们是多方向的 且在空间取向上具有立方对称的特征，则晶体仍是光性均质体，否则便转变为光性非均质体 。此外，还提出了根据选区电子衍射花样对结构调制方向进行三维判识的可能性；报道了在所研究青金石中发现的沿［110］方向n=4的有公度调制的有序结构，以及所观察到的其他有序结构和无公度调制结构。

Structural Ordering and Modulation in Lazurite and Its Relationship with Optical Anomaly

In general, lazurites are sweepingly described as symmetrically cubic and optically isotropic. It was observed that, however, some lazurites are of optical anomaly, i.e. they are optically anisotropic. By means of the TEM investigation on lazurite from Baikal, Russia, a diversified variance in lazurite structure is observed, and combining with these diversified structures, the cause of optical anomaly of lazurite is discussed in this paper as follows: (1) The disordered structure of lazurite has a space group of P 4 3n, and consequently it is optically isotropic; (2) The structural ordering, which results from that the centers and corners of cubic cells of lazurite are orderly occupied by two different clusters respectively, would lead to an ordered superstrcture. The latter keeps cubic symmetry (but space group becomes P 23) and original cell size. Therefore it is still optically isotropic; (3) In the lazurite studied, the authors discoveried an ordered superstructure which is one dimentionally commensurate modulated along 110] direction with a repeated period of 4d 110 . This kind of structural modulation makes crystal to become optically anisotropic, because its symmetry is non cubic; (4) In lazurite the structural modulation resulting from partial ordering is usually incommensurate. It takes place frequently along one or several directions of <110>, <100> and/or <112>etc. Such modulated structure is also generally symmetrically non cubic and optically anisotropic; (5) However, whether the modulation is commensurate or incommensurate, the structure will be symmetrically modulated cubic and optically isotropic if the modulation concurred along all the symmetrically equivalent directions. Based on a way proposed by the present authors, the existence of such modulated cubic structure in the lazurite studied has been confirmed.