Hexahedrites
Hexahedrites are the irons with the lowest nickel content (4-6% by weight). They consist of large crystals of kamacite and contain no taenite. Kamacite and taenite are both Fe-Ni alloys, but they differ by the relative amounts of iron and nickel, and (consequently) have a different crystal structure; kamacite forms a body-centered cubic lattice whereas taenite forms a face-centered one. Because of the virtual absence of taenite in hexahedrites, polished surfaces of these meteorites are featureless except for the occasional presence of fine striations known as Neumann lines. Neumann lines are formed by shock deformation of the metallic kamacite crystals during violent impacts.
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