Nobuo Morimoto

A transmission electron microscope study of pyroxene chondrules in equilibrated L-group chondrites

Abstract The fine structures and chemical compositions of pyroxenes in L-group chondrites of types 3–4, 4–5, and 6 have been studied by analytical electron microscopy. The pyroxenes in the chondrules of L3-4 and L4-5 are different from those of L3 in that, (1) Ca-free regions of pyroxenes are clinopyroxene in L3 and are orthopyroxene in L3-4 and L4-5, and (2) the differences of Ca content among the different zones of pyroxene laths are less prominent in L3-4 and L4-5 than L3, while Fe content varies in L3-4 and L4-5 and is constant in L3. These differences suggest that the cooling rate of L3 was greater than those of L3-4 and L4-5. In the L6 chondrite textures due to the original thermal history of the chondrite have apparently been wiped out by later shock deformations. In the chondrules of all the L3, 3–4 and 4–5 chondrites, the Ca-rich region of pyroxenes shows spinodal decomposition textures with similar periodicities. This texture indicates that very little diffusion of Ca was possible after spinodal decomposition in all pyroxenes of the L chondrites. Thus the equilibrated chondrites can not have experienced a significant reheating event, but cooled more slowly than L3. This favours the “autometamorphism” model.

Electron microscopy of clinoenstatite from a boninite and a chondrite

Clinoenstatite crystals from a boninite and the Yamato-74191 chondrite have been studied with an analytical electron microscope. (100) twins and cracks perpendicular and parallel to the c axis are characteristic of their submicroscopic textures. The frequency in appearance along the c axis and widths of the cracks have been explained by the dimensional change of the c axis in the direct transformation of protoenstatite to clinoenstatite and by the cooling rate around the transformation temperature. The cracks in the crystals from the boninite are filled with fibrous crystals of talc, while those from the chondrite are open or filled with glass in which fine crystals of plagioclase are common.

Cooling history of pyroxene chondrules in the Yamato-74191 chondrite (L3)—an electron microscopic study

Abstract Fine textures of clinopyroxene in an excentroradial pyroxene chondrule (EPC) and a comb-like pyroxene chondrule (CPC) in the Yamato-74191 chondrite (L3) have been studied by analytical electron microscopy. Both pyroxenes consist of three regions different in composition and texture; core, mantle and marginal regions, though the pyroxenes of the CPC are more Fe-rich than those of the EPC. The core region is the most Mg-rich with no Ca component and commonly shows polysynthetic (100) twins. The mantle region is slightly calcic, and the marginal region shows a rapid increase of Ca outward. The polysynthetic twins, cracks and subgrain boundaries in the core in the EPC and CPC must have formed during the transition from proto-type to clino-type pyroxenes. The exsolution textures in the mantle and marginal regions indicate initial crystallization of pigeonite- C followed by decomposition into pigeonite- P and augite. The decomposition must have taken place by nucleation growth in the mantle region and by spinodal decomposition in the marginal region. The periodicity of 15–20 nm in the spinodal decomposition textures indicates that the cooling rate of the pyroxenes, when passing through about 1000°C, was of the order of a few tens to several degrees centigrade per hour. The cooling history of the chondrules has been explained by a monotonous cooling controlled by the cooling rate of the surrounding medium.

The Modulated Structure of the Intermediate Plagioclases and Its Change with Composition

Since the discovery of satellite reflections in the intermediate plagioclase feldspars of composition An~25 – An~75, the so-called e-plagioclases, many models have been presented for these modulated structures. Recent X-ray diffraction studies of the modulated structure have indicated coexistence of density and shift modulations of atoms in the structure. The fundamental difference between the models is in the treatment of the second harmonic of the density modulation of the (Na, Ca) atoms. The model in which the second harmonic is considered negligible gives alternating arrangement of Na and Ca in consecutive subcells. On the other hand, the model in which the second harmonic is considered important, gives alternating anorthite-like and albitelike bands running perpendicular to the t vector. Although it is not easy to decide which model is correct only from the intensities of the satellite reflections, crystal-chemical considerations of the resultant structures and electron microscopy of the antiphase domain boundaries (APBs) support the second model. In the second model, the modulated structure of the e-plagioclases consists of albite-like and anorthite-like bands, both of which change their orientation and width with composition.

The modulated structures of feldspars

The modulated structures of plagioclase feldspars have been extensively studied by the X‐ray diffraction method and high resolution electron microscopy. The results on bytownite (An73) and labradorite (An52) indicate that the modulated structure of the e‐plagioclase consist of the periodic antiphase domains of the anorthite‐like structure separated by the antiphase boundaries which form bands of albite‐like structure. Orientation and width of the antiphase domains change with composition of the e‐plagioclase. This results in the movement of the e satellites for the e‐plagioclase.The modulated structures in the e‐plagioclase are considered to appear as a possible precursor to unmixing to two stable phases of anorthite and albite in the process of ordering of substituting atoms in the high‐temperature form. The periodicity of the antiphase boundaries observed in the e‐plagioclase becomes irregular as the Na content of plagioclass becomes less than An75, resulting in the b‐type antiphase boundaries.

Fine-grained aggregates in L3 chondrites

Abstract The textures and chemical compositions of the constituent minerals of the fine-grained aggregates (FGAs) of L3 chondrites were studied by the backscattered electron image technique, electron probe microanalysis, and transmission electron microscopy. Plagioclase and glass in the interstices between fine grains of olivine and pyroxene indicate that the FGAs once partly melted. Compositional zoning and decomposition texture of pyroxenes are similar to those observed in chondrules, indicating a common cooling history of the FGAs and chondrules. Therefore, the mechanism that caused melting of the FGAs is considered to be the same as for chondrules. Bulk compositions of the FGAs are within the range of those of chondrules, so some chondrules probably were produced by complete melting of the same precursor materials as those of the FGAs. The precursor materials must have included fine olivine and other grains that probably are condensates.

Wave distribution of vacancies in the NC‐type pyrhotite, Fe1−xS

The diffraction pattern of the NC‐type pyrrhotite is characteriszed by its ’’non‐integral’’ satellite reflections. To explain the extinction rules of the satellites, a structure model with the wave distribution of vacancies has been derived by considerations on the basis of the four‐dimensional formulation of the structure factor. The model is confirmed by the image contrast of a high resolution electron micrograph.