Contribution of metallurgy to the origin of meteorites Part II - the significance of neumann bands in meteorites
Neumann bands in alpha iron are produced by shock waves, their formation occurring readily at low temperatures but not at all at high temperatures. They represent lamellae of mechanically twinned metal, where twinning of the crystal along the (112) planes more easily accommodates sudden shear than does slip. Reinforcement and interference of reflected shock waves probably account for the serrated outline and frequent convolution of the bands. The bands make up characteristic metallographic markings of hexahedrites and of kamacite in octahedrites. They reveal, therefore, that sometime in history after the Widmanstätten pattern formed (in the case of octahedrites) and at relatively low temperatures, meteorite bodies were subjected to violent impact or explosion. The appearance of acicular phase transformation products both within and without Neumann bands of the Dexter meteorite constitute evidence that the bands were formed previous to collision with the earth. It is proposed that the shock wave to which the parent body was subjected first formed Neumann bands, followed by pressure changes which produced observed transformation products throughout kamacite, including twinned metal. The necessary conditions for transformation include a temporary increase of pressure brought about perhaps by distortion or impact, followed by sudden release of pressure, such as would accompany fragmentation of a large body.