Safety and Reassurance after The Great East Japan Earthquake

特集　東日本大震

Compositional heterogeneity of fine-grained rims in the Semarkona (LL3) chondrite

The fine-grained opaque matrix and chondrule rims in the Semarkona (LL3) chondrite have been investigated in detail with the scanning electron microscope. The chemical analyses were made by the SEM-EDS technique. Although the rims have similarities in texture and composition to the matrix, they show wider variabilities in chemical composition in comparison with the matrix in Semarkona. Based on both mode of occurrence and Al/Si ratio, the fine-grained opaque rims in Semarkona are classified into three types : type I, type IIa and type IIb. Type-I rims commonly occur as chondrule-rimming materials, having high Al/Si ratios. Type-I rims are composed mostly of smectite, showing a wider variation of the FeO and Al_2O_3 contents than that of the matrix. Type-IIa rims occur as magnetite-bearing chondrule rims, characterized by their low Al/Si ratios. They are composed of mixtures of smectite, fayalitic material and its alteration products. Metal-sulfide-magnetite aggregates have opaque rims (type IIb) with the lowest Al/Si ratios. The SiO_2,Al_2O_3 and FeO contents of type-IIb rims suggest that they consist mainly of fayalitic material and its alteration products. In compositon, it is suggested that \u27fayalitic\u27 material and its alteration products in types-IIa and IIb rims correspond to one of the non-refractory, SiO_2-, FeO-rich components of precursor materials of the chondrules in Semarkona. The \u27fayalitic\u27 material might have formed through intensive fractional condensation of forsterite in the cooling nebula gas

The chemical compositions and textures of matrices and chondrule rims of unequilibrated ordinary chondrites-II. Their constituents and the implications for the formation of matrix olivine

The micron-size constituents of fine-grained matrices and chondrule rims in ten unequilibrated ordinary chondrites, Semarkona (LL3), Krymka (L3), Sharps (H3), Chainpur (LL3), Tieschitz (H3), Mezo-Madaras (L3), ALH-764 (LL3), ALH-77214 (L3), ALH-77216 (L3), Yamato-790448 (LL3), have been investigated in detail. Special emphasis is put on the mode of occurrence and mineral chemistry of micron-sized matrix olivine in six primitive type-3 ordinary chondrites having lower petrologic subtypes (Krymka, Chainpur, Sharps, Tieschitz, ALH-764 and Yamato-790448). Textural evidences concerning occurrence of matrix ferrous olivine observed in matrices of chondrite samples studied here seem to indicate that some of matrix intermediate to Fe-rich olivine would have formed through solid-solid reactions between matrix enstatite, silica-rich spherules and metallic Fe-Ni. It is shown that the MnO content increases remarkably with increasing FeO content, showing a strong positive correlation of Mn with Fe in matrix ferrous olivine. The ratios of MnO to FeO appear to be distributed around solar ratio and lower than those of chondrule olivines. Although the solid-solid reaction hypothesis explains a wide variation of Fa mole % of matrix olivine, it is not clear whether reactions between matrix enstatite, sillica-rich spherules and metallic Fe-Ni can explain the positive correlation of Mn with Fe in matrix ferrous olivine. Direct vapour⟶solid condensation is suggested as an alternative mechanism for the formation of matrix ferrous olivine

Reentrant spin-glass behavior induced by the frustration of Fe-Fe interactions in Laves phase Nb1-xHfxFe2 alloys

We carried out magnetization and specific-heat measurements of metallic compoundsNb1-xHfxFe2 (x = 0 and 0.65), which exhibit reentrant spin-glass freezing. The frequencydependence of ac susceptibility and the bifurcation between zero-field-cooled and field-cooledmagnetization curves suggest the complex coexistence of magnetic phases at low temperatures. A long-time relaxation of magnetization is possibly due to the cluster-type and traditional reentrant spin-glass transition, which may be induced by the frustration of long-range Fe-Fe interactions. We did not observe any discontinuity indicating long-range magnetic phase transition in the data of specific heat vs temperature, but a broad hump characteristic of spin-glass freezing. The difference between the behaviors of two samples proves the existence of two types of reentrant spin-glass in alloys. The moment arrangements are briefly discussed.PACS number(s): 75.50.Lk; 75.30.Kz; 75.50.B