Metallic spheroids approximately 1 mm in diameter and having a dendritic structure appropriate to a rapidly solidified liquid alloy are encountered in conjunction with Canyon Diablo meteorites in the vicinity of the Barringer crater, Arizona. Compositionally and structurally similar particles have been encountered in lunar soils returned from Apollo 11, 12, 14, and 15 expeditions. The present study is devoted entirely to Canyon Diablo spheroids, which are shown by means of electron microprobe techniques to be enriched in Ni and considerably enriched in S and P relative to the metallic phases of Canyon Diablo meteorites. In contrast to earlier theories that considered only selective oxidation, a process of formation is now put forward involving the earlier stage of the shock‐induced melting of sulphide‐rich areas in the meteoritic projectile when it hits the earth. This process is also applicable to metallic spheroids formed on the moon, where oxidation processes will be minimal. Measuring the interarm spacing of the dendrites leads to estimates of 500°–30,000°C/sec for the rates at which these spheroids cooled through the solidification range. The breakup of the Canyon Diablo iron meteorite projectile is described by a series of events involving increasing shock pressures.
Measurements have been made of the lattice spacings of solid solutions of lithium, magnesium, silicon, copper, zinc, germanium and silver in aluminium. The lattice of aluminium is expanded by the solution of magnesium or germanium, and contracted by the solution of lithium, silicon, copper or zinc. No change in lattice spacing can be detected when silver is dissolved in aluminium, although microscopic examination shows that a solid solution is formed, and this is confirmed by the absence of any diffraction lines other than those of the solid solution in aluminium. If the lattice spacing/composition curve for dilute solutions is extrapolated to 100% of solute, the resulting lattice spacing refers to a hypothetical face-centred cubic modification of the solute, and the corresponding closest distance of approach of the atoms is called the apparent atomic diameter (A. A. D.) of the solute when in solution in aluminium. Previous work enables the corresponding A. A. D. values to be deduced for the above solute elements when dissolved in univalent copper, silver or gold, and in divalent magnesium. The differences between the A. A. D. values of a given element when dissolved in various solvents are discussed, and it is suggested that they are controlled by the interplay of four factors: (1) the relative volume per valency electron in crystals of the solvent and solute, (2) the relative radii of the ions of solvent and solute, (3) Brillouin zone effects, and (4) the difference between solvent and solute in the electrochemical series. If this line of approach adopted be correct, it follows that it is only in exceptional circumstances that the so-called Vegard’s law will apply to metallic solid solutions.
SummaryThe Verkhne Dnieprovsk material (BM 51183) in the collection of the British Museum (Nat. Hist.) is chemically group IIE and therefore distinct from Augustinovka. Structurally it is a finest octahedrite, with a kamacite band-width (0.05 mm) and bulk Ni content (Scott and Wasson, 1976) appropriate to a cooling rate of 830 K Myr−1. Subsequent pre-terrestrial shock has produced localized deformation and shock-heating effects, which include mm-size globules of metal-phosphide melt product with a dendritic texture indicating resolidification at c.10 000 K sec−1. It is suggested that the phosphides were already fissured at the time of shockmelting. The structure of Verkhne Dnieprovsk, although lacking silicates, is analogous to that of Kodaikanal.
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