THE FIRST DISCOVERY OF TERRESTRIAL ORTHOPYROXENE WITH SPACE GROUP P2_1ca

Conventionally the space group of the individual member in orthopyroxenes is Pbca. Up to the present it is known that there are only a few orthopyroxene specimens found in lunar rocks and meteorites to possess space group P21ca. Having investigated the hypersthene megacryst from norite pegmatite in Blackhill, Chengde, Hebei Province, the authors affirm that the space group of the host for the megacryst is P21ca. Thus, it is considered that as a low orthopyroxene polymorph of space group P21ca it is firstly discovered on Earth.

P2_1cα斜方辉石可能的拓扑结构

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。