Hexahedrites are a category of iron meteorites
with low nickel content, primarily made up of the nickel-iron alloy kamacite.
Compared to octahedrites, they contain less nickel, with concentrations
typically below 5.8% and seldom dropping under 5.3%. Their name derives from
the cubic (hexahedron) shape of the kamacite crystals.
Unlike octahedrites, hexahedrites lack a
Widmanstätten pattern. Instead, they often exhibit Neumann lines, which
are signs of impact shock that occurred after the initial cooling of the
taenite-kamacite mix.
The mosaic aggregates found in hexahedrites emerge
from rapid cooling and reheating events experienced during their journey
through space and entry into Earth’s atmosphere. These aggregates are
composed of irregularly shaped kamacite grains, oriented in different
directions, giving them a mosaic-like texture.
Their structure forms through a transformation
process: alpha-iron shifts to gamma-iron and then reverts to alpha-iron when
heated above 750°C, followed by swift cooling. These features are
considered secondary structures. |
