Gary, and all...
Yes iron meteorites do have fusion crust. And the crust is more
complex than what is seen in stones.
In iron meteorites the blue-black surface is only part of the fusion
crust. It is the outer rind of magnetite (iron oxide produced by
direct exposure to the atmosphere during the bolide phase), but
underneath it is fused un-oxidized metal. This underlying fused metal
is very thin where it has been completely melted and flowing. Flow
lines often appear in this melted metal and in freshly recovered irons
the blue-black magnetite covers it. But some Sikhotes that have been
recovered recently have been buffed of outer brown rust and show flow
lines that are the fused metal that was below the original blue black
magnetite.
This melted un-oxidized metal I read somewhere is called "A1"
(Alteration 1) zone.
Now just below this is another portion of the iron meteorite fusion
crust, a zone called "A2" (Alteration 2) This area is characterized
by distinct heat alteration of the widmanstatten pattern where the
metal was brought to a temperature just below melting. Not many iron
meteorites show an A2 zone. One of the best that I once had in my
collection was a 1500 gram complete slice of Orange River, South
Africa. It had all three components of an iron meteorite fusion crust.
It had an extremely thin blue-black magnetite fusion crust, then below
that the "A1" un-oxidized iron fusion zone about a 1/4 mm thick, and
the "A2" zone in some places 2-3 mm thick where the metal was brought
just below fusion.
I traded that specimen years ago and have regretted it since. I have
never had another like it. But I have seen photos of other fresh iron
meteroite slices that show the three fold nature of iron meteorite
fusion crust.
Steve Schoner/ams
IMCA 4470
[meteorite-list] Fusion Crust on Irons
Gary K. Foote
Mon, 04 Dec 2006 10:27:19 -0800
Hi All,
Recently I have read a few posts to this list that definitively claim that
irons do not form a fusion crust. Yet, in Norton's "Rocks From Space",
[pg 167 in my softbound edition] it clearly states the following;
"Iron meteorites have the thinnest crust of all, usually only a small
fraction of a millimeter thick. A fresh crust is blue-black to black and
looks like freshly welded steel. This crust is fragile and easily
destroyed if the meteorite weathers for even a short time."
So, which is true? Crust or no crust for irons?
Gary Foote
http://www.meteorite-dealers.com
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