HEXAHEDRITE
Hexahedrites are one of three broad categories of iron meteorite (the others are
octahedrites and ataxites). Hexahedrites are uncommon irons dominated by kamacite,
an iron-rich Fe-Ni metal alloy. The common variety of iron meteorite, the
octahedrite, is dominated by both kamacite and taenite (a nickel-rich Fe-Ni
metal alloy). Hexahedrites are samples from the core of a differentiated
asteroid/dwarf planet.
The two photos below are the front & back of a
cut, polished, and nitric acid-etched slice of hexahedrite. Unlike
octahedrites, hexahedrites lack WidmanstŠtten structure. Almost all the
metal you're seeing is kamacite (~5.4 weight-percent nickel here). The
variously oriented sets of fine lines are Neumann lines, representing
shock twinning planes of kamacite. The blackish-colored needles and small
patches are called rhabdites. These rhabdites are composed of the mineral
schreibersite, an iron-nickel-cobalt phosphide - (Fe,Ni,Co)3P.
Hexahedrite cut & polished & nitric acid-etched slice (1.7 cm
across). The silver metal is kamacite; the dark needles & small
patches are schreibersite.
This hexahedrite sample from the Richland
(Fredericksburg) Meteorite of Texas, USA. The Richland Meteorite was
found in Navarro County, east-central Texas. The Fredericksburg Meteorite
apparently was found in Gillespie County, central Texas. Chemical analysis
has demonstrated that they are identical.
Hexahedrite cut & polished & nitric acid-etched slice (flip side of above
specimen, 1.7 cm across).
Hexahedrite cut & polished & nitric-acid etched slice (1.7 cm across).
If anyone knows what these scattered small shiny grains are (taenite?
tetrataenite? antitaenite?) that show up under low-angle illumination,
let me know.
(More info. on the
Richland Meteorite)