Small Spherical Grains Research Articles

Silicate darkening in the Kobe CK chondrite: Evidence for shock metamorphism at high temperature

Abstract— The Kobe CK4 chondrite, like most metamorphosed CK chondrites, exhibits pronounced silicate darkening of matrix and chondrule mesostases. Our petrographic and scanning electron microscopic study reveals that the matrix of Kobe consists mostly of intermixtures of two types of fine‐grained olivine. One forms subhedral to anhedral normal crystals. The other fills interstices of the subhedral to anhedral olivine crystals, exhibiting a complex network of veinlets. The latter type of olivine contains high densities of small spherical vesicles (<0.05‐3 μm in diameter) and grains (<0.05‐5 μm) of magnetite and pentlandite as well as round to anhedral grains (1–10 μm) of plagioclase, low‐Ca pyroxene, diopside and chlorapatite. The vesicular olivine is particularly abundant in regions of matrix that exhibit a relatively high degree of darkening and commonly fills chondrule mesostases. The vesicular olivine is clearly the principal cause of the silicate darkening in Kobe. The internal texture of the vesicular olivine closely resembles those of local melts produced from the matrices of experimentally and naturally shocked carbonaceous chondrites. The occurrence and texture of the vesicular olivine suggest that it resulted from recrystallization of partially melted matrix olivine by shock.Kobe exhibits light shock effects in olivine that are consistent with shock stage S2 that is too low to explain the occurrence of olivine melting. We suggest that the vesicular olivine in Kobe was produced by shock metamorphism at a relatively mild shock pressure (<25 GPa) and a high temperature (>600 °C). Thus, it is probable that the shock effects in olivine, manifest as fracturing and deformation, were relatively minor, but heating was strong enough to cause partial melting of matrix olivine.

Magnetic resonance in spherical Co-Ni and Fe-Co-Ni particles

Magnetic resonance is studied in absence of external field on composite materials made up with spherical and monodisperse fine Co--Ni and Fe--Co--Ni particles. The particle size range varies over one order of magnitude from 25 to 250 nm. In the frequency range studied [0.1--18 GHz] several resonance bands are generally observed attributed to nonuniform resonance modes. The resonance frequencies are found to depend on the magnetic particle size, only weakly for the lowest frequency mode, in a more pronounced manner for the following modes. A theoretical model based on a discrete treatment of the resonant effect in independent small spherical or cylindrical grains is proposed. It allows to compute the resonance frequencies, the spin-wave profiles, and the spin-wave intensities. The respective influences of particle geometry, surface pinning, crystalline anisotropy, and exchange parameter values are presented. According to this model it is found that the size dependence of the resonance frequencies is mainly related to the surface pinning and to the exchange parameter value. In all cases the size dependence of the lowest frequency mode is found weaker than that of the following modes and for large particles the frequency of the first mode is directly related to the crystalline anisotropy. The particle shape effect on resonance frequencies is weak whereas the spin-wave profiles of the first modes are found to depend on the particle geometry. This model enables us to describe the general shape of the experimental spectra and to infer the magnetocrystalline anisotropy constants ${(K}_{1})$ from the experimental data, which are found in good agreement with bulk values. Moreover it shows that the weak size dependence of the lowest frequency mode is due to a weak pinning at the particle surface. Nevertheless, the effect of particle size on higher spin-wave modes requires us to account for the magnetic interactions between grains.

Microscopic characterization of microcrystalline silicon thin films

Microcrystalline silicon (μc-Si) thin films were prepared by closed chamber-chemical vapour deposition with thicknesses between 5 and 360 nm. The cycle includes an a-Si:H layer deposition and a hydrogen-radical treatment step. The layer contains high crystalline fractions at high deposition rates. The films were characterized by field effect-scanning electron microscopy, transmission electron microscopy and atomic force microscopy. Layers with a thickness up to 50 nm consist of an amorphous matrix with small spherical grains. Above 50 nm layer thickness the crystallites have a columnar morphology.

Cross-sectional transmission electron microscopy of thin film polycrystalline semiconductors by conventional microtomy

Conventional ultramicrotomy with a glass knife is shown to be a valuable technique for studying thin film polycrystalline semiconductor materials in cross-section. High quality sections of CuInSe 2 (CIS) polycrystalline thin films and of nanocrystalline PbSe films deposited on mica, suitable for electron microscopic investigations, can be obtained in large quantities within a very short time. Detailed cross-sectional transmission electron microscopy (XTEM) studies performed on CIS films revealed columnar grains of tetragonal chalcopyrite structure that nucleate at the CIS-mica interface. The average grain diameter is about 30–60 nm and increases with increasing distance from the substrate. Intergranular voids predominantly located near the interface were commonly observed. Microtwins on (112) planes were found in many of the grains. No second phases were observed in these samples. XTEM on nanocrystalline PbSe layers on mica revealed films made up of very small spherical grains (4–5 nm in diameter) of cubic PbSe dispersed in an amorphous matrix and uniformly distributed throughout the thickness of the layer.

A Corrective Addendum on Previously Reported Small Spheres in the Meteorites Zagami and Murchison

We have reported the finding of small spherical grains (diameters 30-50 μm) in the meteorites Zagami and Murchison [1]. The spheres were analysed by X-ray diffraction. The results obtained were found to fit reasonably well with the results expected from (Ca, RE)F2 and CaS. In the present corrective letter we compare our results from X-ray diffraction analysis with the results of microprobe studies. Surprisingly, the analysis by microprobe disclosed the absence of both Ca and S in the spheres, and the presence of Ce, La, Nd, Gd, and Fe. We are thus forced to conclude that our preliminary interpretation of the X-ray films may not be correct. We present experimental results showing that it is difficult to reconcile the results obtained by microprobe analysis with the X-ray data.

Interstellar Grains as Small Particles

IN many studies of interstellar grains it has been customary to assume that the properties of these particles can be described by the properties of macroscopic systems with the same composition. Indeed it has been the practice to assume that the particulate nature of interstellar dust is manifested only by the way in which such dust scatters and extinguishes light. Detailed considerations of the properties of interstellar grains as small particles lead, however, to the observation that grains cannot be considered in many instances to behave simply as sub-volumes of a macroscopic solid. Here I show that several pertinent parameters describing interstellar grains cannot be predicted from the bulk properties of an assumed grain material. My discussion is limited to small (r<10−5 cm) spherical grains of refractory materials.

Machinable Glass‐Ceramics Based on Tetrasilicic Mica

Tetrasilicic‐mica glass‐ceramics are formed from glasses in the simple quaternary system K2O‐MgF2‐MgO‐SiO2. The mica phase crystallizes as small (400‐Å) spherical grains at #625°C and recrystallizes into larger booklets at 940°C. The mica platelets grow by a dissolution and reprecipitation mechanism, thereby increasing the aspect ratio during growth. Two regions of strength dependence on grain size are found. When the mica platelets are very small, the strength increases as the 1/5 power of the mica‐plate diameter. Above a critical plate diameter, the strength decreases, being inversely proportional to the first power of the mica‐plate diameter. In the second region, the strength appears to be controlled by the interfacial shear stresses developed as a result of the thermal expansion mismatch between the mica platelets and the residual glass.