Optical Microscopy of Meteoritic Metal

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Most meteorites contain at least some Fe-Ni metal.  There are some exceptions, such as the suite of Martian meteorites, but many achondrites and even some lunar meteorites contain significant quantities of metal.  These metal-bearing achondrites are fertile subjects for metallographic study.

Upon arriving at Earth's oxygen-rich environment, meteoritic metal becomes susceptible to oxidation.  Metallographic studies should focus on pristine metal that has escaped such alteration, and the best-preserved metal occurs in meteorites that were observed to fall and promptly recovered.  In contrast to observed "falls", meteorite "finds" exhibit excellent to poor metal preservation depending on the terrestrial residence time and the local humidity conditions. 

For irons, the relative degree of terrestrial alteration can be estimated by examining the condition of the heat-altered rim that formed during transit through Earth's atmosphere. In fresh irons this alteration zone is well-preserved whereas in highly-terrestrialized irons the rim is entirely absent.

The degree of weathering of chondrites is quantified based on the overall condition of metal particles.  A weathering scale, W0-W6, is often included in chondrite classifications. In general, it is preferable to limit metallographic studies to chondrites with weathering grades ranging from W0 (most observed falls) to W2 (in which 20-60% of the metal has been replaced by oxides).