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)




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