摘要
在肇东陨石低钙辉石中观察到各种堆垛缺陷,包括斜方-单斜辉石相转变、畴结构、2.7nm长周期结构以及自然界极为罕见的奇数倍八面体层堆垛缺陷——1.35nm等。高分辨像所揭示的1.35nm的新特征是可以重复两次。文中提出1.35nm的结构是“A+B+A-”型。1.35nm片晶仅见于3,4型球粒陨石,因而它可作为由原顽火辉石淬冷形成的、末遭受后期热变质的典型结构。奇数倍0.9nm宽度单斜辉石片晶在吉林陨石(H5)、安龙陨石(H5)及东台陨石(LL6)中均有出现,它表明这些陨石的辉石都曾由原顽火辉石转变而成。 肇东陨石低钙辉石中相当频繁的相变以及高能量的奇数倍0.45nm层堆垛缺陷的存在,可能表明该陨石形成于退变质过程。
It has been recognized that the ultramicrostructures of pyroxenes can play an important role in thermal equilibration during cooling. UnderHRTEM observations of pyroxenes in the Zhaodong chondrite, very interesting ultramicrostructures including the phase transformation of clino→orthopyroxene, 2.7nm long-period structure and odd-layered stacking defects have been found.Abundant odd-layered stacking sequences in the Zhaodong chondrite, which were rarely observed in nature, are completely, coherent with (100) of pyroxenes and occurred as isolated blocks, in orthopyroxenes or as two repeti tions in highly disordered region of clinopyroxenes. The. new feature of 1.35 nm blocks in pyroxenes exposed by high resolution images is repeated for two times. Based on the lattice images, the 'A+B+A-' stacking sequence of Si-O chains and octahedral layers proposed here would never be confused with Opx+Cpx no matter how many times it will repeats. According to the presence only in type-3 and -4 chondrites, the odd-layered stacking of octahedra is considered to be produc ed from the conversion of protopyroxenes by quenching and could be used as a typical ultramicrostructure without suffering later thermal metamorphism.The preservations of original ultramicrostructures of 1.35nm blocks and highly disordered region with 2.7nm structures in pyroxenes which formed before or during accrection imply that the Zhaodong chondrite was produced by a complex accretion process. In this complex process, some material accreting on a hot but cooling parent body could be rapidly heated to an extremely high temperature or ejected, quenched and accreted again. Thus some material accreting early might begin to attain equilibrium, while that accreting later could still maintain the original ultramicrostructures and textures. The facts mentioned above imply that the Zhaodong chondrite is considered to be formed by the retrograde process without suffering later thermal metamorphism.
出处
《矿物学报》
CAS
CSCD
北大核心
1992年第2期102-110,T002,共10页
Acta Mineralogica Sinica
基金
国家自然科学基金
关键词
陨石
低钙
辉
石
超微结构
chondrite
low-Ca pyroxene
ultramicrostructure of pyroxene
transmission electron microscopy
