Concentrations of Ni, Ga, Ge and Ir in 106 iron meteorites are reported. Three new groups are defined: IC, IIE and IIIF containing 10, 12 and 5 members, respectively, raising the number of independent groups to 12. Group IC is a cohenite-rich group distantly related to IA. Group IIE consists of those irons previously designated Weekeroo Station type and five others having similar compositions though diverse structures. The IIE irons are compositionally similar to the mesosiderites and pallasites, and the three groups probably formed at similar heliocentric distances. The mixing of the globular IIE silicates with the metal probably occurred during shock events. Group IIIF is a well-defined group of low-Ni and low-Ge irons. The compositions of these groups are summarized as follows:
The largest British earthquake to occur in several hundred years took place in Essex, in the south east of England, in 1884. From contemporary accounts and a recent visit to the area affected by the earthquake, the intensity of the earthquake was assessed. It is estimated that the peak accelerations generated may have been as much as 0.1 g. This would be of consequence to some modern structures, designed without earthquake provisions. 相似文献
The olivine-metal textures of pallasites help elucidate the formation of these meteorites. Large sections of pallasites with angular olivines commonly contain olivine masses 5–30 cm in size, which appear to have been disintegrating into smaller fragments, like those dispersed through the metal. It is suggested that these masses were parts of olivine layers, which were intruded by underlying molten metal. Gravitational separation of olivine and metal was probably prevented by incipient solidification of metal.Pallasites with rounded olivines probably had a fairly similar history, but with additional annealing after metal solidification had started. Grain boundary energy was reduced by decreasing the area of metal-olivine interface and increasing the contact between touching olivines. Because of the vastly different nature of metal and olivine, their mutual interfacial energy is high and the degree of anisotropy of this energy is low. The occurrence of profuse, small faces on rounded olivines is therefore consistent with the ‘metamorphic’ origin which is proposed for rounded olivines. In many pallasites with angularolivine macrostructures, magnifications of 5–50 × reveal similar rounding effects, including aggregates of rounded olivines, and individual crystals which may be faceted.The high metal-troilite ratio of Phillips County suggests that this pallasite formed by mixing of olivine with S-rich liquid metal around 1000°C, near the Fe-FeS eutectic temperature. 相似文献
We investigate the variation of current star formation in galaxies as a function of distance along three supercluster filaments, each joining pairs of rich clusters, in the Pisces-Cetus supercluster, which is part of the two-degree Field Galaxy Redshift Survey (2dFGRS). We find that even though there is a steady decline in the rate of star formation, as well as in the fraction of star-forming galaxies, as one approaches the core of a cluster at an extremity of such a filament, there is an increased activity of star formation in a narrow distance range between 3 and 4 h −170 Mpc , which is 1.5–2 times the virial radius of the clusters involved. This peak in star formation is seen to be entirely due to the dwarf galaxies (−20 < M B ≤−17.5) . The position of the peak does not seem to depend on the velocity dispersion of the nearest cluster, undermining the importance of the gravitational effect of the clusters involved. We find that this enhancement in star formation occurs at the same place for galaxies which belong to groups within these filaments, while group members elsewhere in the 2dFGRS do not show this effect. We conclude that the most likely mechanism for this enhanced star formation is galaxy–galaxy harassment, in the crowded infalling region of rich clusters at the extremities of filaments, which induces a burst of star formation in galaxies, before they have been stripped of their gas in the denser cores of clusters. The effects of strangulation in the cores of clusters, as well as excess star formation in the infalling regions along the filaments, are more pronounced in dwarfs since they more vulnerable to the effects of strangulation and harassment than giant galaxies. 相似文献
The Ragland, New Mexico chondrite was found in 1978, and consists of a single stone of 12.16 kg that broke into three pieces. The stone is moderately weathered and has a pronounced chondritic texture. Bulk composition favors an LL classification, and modal analysis and oxygen isotopic composition are consistent with this. The thermoluminescence sensitivity of 0.056 ± 0.020 normalized to Dhajala, compositional variability of olivine (mean Fa 18.3, σ = 10.1) and low-Ca pyroxene (mean Fs 14.6, σ = 6.7), and Ca concentrations in olivine indicate metamorphic subtype 3.4 ± 0.1. The isotopically heavy oxygen composition, which is characteristic of subtypes 3.0–3.1, may be a primary characteristic and not a result of weathering. Low concentrations of radiogenic 40Ar and planetary 36Ar suggest noble gas loss. 相似文献
