P2_1cα斜方辉石可能的拓扑结构

THE POSSIBLE TOPOLOGIC STRUCTURE TYPES OF ORTHOPYROXENE WITH SPACE GROUP P2_1cα

P2_1ca空间群的斜方辉石晶体,其可能的拓扑结构中包含了对称上不等同的4种四面体单链和4种八面体配位位置。八面体层按++--的顺序重复排布。四面体链的旋转方式可有16种不同的组合,但其中有12种违反Thompson的均势法则;另4种两两等价,构成以a轴为极轴的两对异极反取向关系的结构对。它们的O链与S链之比分别为3:1和1:3。从晶体化学角度来看,最可能的结构是3:1的结构。此结构中对称上和拓扑学上均不等同的两列Ⅰ形单元的构成为: ·O+O·O—O··O+S·O—S· 或 +O·S-O·S+ +O·O-O·O+两种M_2位置的构型分别为P·P和P·N。互为异极反取向关系的结构对之间可形成以(100)为双晶面的补足双晶,其双晶界面正好等于一个倒转界面。此外,当Mg:Fe原子比为1:1或3:1时,都有可能出现有序结构。

The possible topologic structure types of orthopyroxene with space groupP2_1 ca comprise four kinds of tetrahedral chains and four kinds of octahedralsites, all of which are unequivalent in symmetry. In these structure types, theskew of octahedral layers has a sequence of ++--. There are sixteen possiblecombination forms for the rotation type of tetrahedral chain. Twelve of themviolate Thompson's parity rule and the remainder constitutes two pairs. Ineach pair, the two polar forms show a relationship of anti-orientation fortheir polat α-axes. Thus, there are only two possible different topologicstructure types for P2_1ca orthopyroxene. The ratios of O-rotated and S-rota- ted tetrahedral chains for these two structure types are 3: 1 and 1: 3,respectively. In view of the crystallochemical principle, the most likely formis the one with a ratio of 3: 1, and its constitutions of two stacks of I-beam, which are unequivalent both in symmetry and topology, are:·O+O·O-O·+O·s-O·S+or·O+S·O-S·+O·O-O·O+and the configurations of the two types of M_2 sites are P·P and P·N, respec-tively. A complementary twinning on (100) would be formed between theanti-oriented structure pair, and their twin boundary is exactly equivalent tothe inversion boundary. Moreover, it is possible that the ordered structurewould appear when the atom ratio of Mg: Fe is equal to 3: 1 as well as 1: 1.