ABSTRACT
The microstructural characteristics of naturally deformed omphacite from the high-pressure (HP) and ultrahigh-pressure (UHP) eclogites in the Qinling-Dabie Mountains, central China, have been studied using electron diffraction analysts and high resolution image observation techniques. It has been found that the principal structural form of omphacite has an ordered structure P2, cell parameters: a=0.96nm, b=0.88nm, c=0.53nm and p=106°, and there is also a local secondary ordered structure P2,n and disordered structure C2/c. Therefore, omphacite in eclogite from the Qinling-Dabie Mountains formed at low temperature and high or ultrahigh pressure. The structures of the crystal domiains in omphacite have been investigated, and it has been found that 1/2(a+b) or 1/2<110> antiphase domains have a C2/c structure and (200) microtwin domains in the P2/n structure. The size of the antiphase domains can be regarded as an important indicator of metamophic temperature and time. The uhrastructures of omphacite, including dislocations, faults and distortions etc. have been observed. These heterogeneous textures are possibly related to the physical, chemical conditons in which the mineral crystallized and the metamorphic deformation conditions.
