Arlene,
Someone just explained this not too long ago. If I recall: cooling is a
factor, but mainly it's the amount of nickel in the meteorite that
determines the pattern. The slower it cooled, the larger the interlaced
crystals of taenite and kamacite, both iron nickel alloys. The width of
these bands is the basis for the structural classification system of irons.
The hexahedrites have the lowest nickel content and show a fine pattern of
lines called Neumann bands. The octahedrites, ranging from coarsest to
finest have more and more nickel until you get to the plessitics which in
place of a Widmanstatten pattern, have lines of kamacite needles in a
matrix of high nickel alloy called tetrataenite. Irons with the most
nickel are the ataxites, which have no pattern at all. As to why the
meteorites have varying amounts of nickel is beyond me.
Hope this helps,
Phil Whitmer
Very interesting--thank you for that explanation--that explains all my
silicated irons....now
I have a question regarding the bandwith of the Widmanstatten pattern on
various irons--what exactly determines if it's a "fine", "medium" or
"course" octahedrite....is it the length of time it took to cool in space?
I have a fairly large collection of irons and I'd like to have a clearer
understanding of what determines the pattern.....thanks....Arlene Schlazer
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