[meteorite-list] golden iron color and Widmanstätten" patterns

From: Jason Utas <meteoritekid_at_meteoritecentral.com>
Date: Thu, 1 Apr 2010 15:32:10 -0700
Message-ID: <g2l93aaac891004011532j18883bb3y2ff8d713f2b3a5b_at_mail.gmail.com>

Hello All,

It's a pretty coating of desert varnish on a (probably naturally)
sandblasted iron.

A winonaite would look like a stony meteorite. If it were a
"metal-rich winonaite," you might as well call it a silicated iron -
but there are no silicates visible anywhere on its (supposedly wind-)
polished surface, so it's not a winonaite, that much is obvious.
The fact that you can see a widmanstatten pattern on the sandblasted
surface means that we're not looking at a lump of troilite, and the
suggestion that the coloration could be due to iron sulfide
is...strange. Troilite occurs as inclusions within iron meteorites,
and it's clear that the sandblasted surface exhibits no such
inclusions. Don't get me wrong - I see why you might say that, Mike,
since troilite is a bronze-gold color as well, but...there's none
there.

So -

The coloration is due to a thin layer of desert-varnish-related iron
oxide that forms on the surface of irons exposed to abrasive desert
winds (and not your typical temperate or tropical oxidation).

See here for other examples:


http://picasaweb.google.com/MeteoriteKid/Irons#5444617684310649778


http://www.flickr.com/photos/cameteoritefinder/2336572790/


But note that both of these irons are fresh; where fusion crust has
spalled off, discolored, wind-polished iron is exposed below the
surface.

Many NWA 5549 fragments actually display similar surfaces:


http://www.encyclopedia-of-meteorites.com/test/49248_8446_2637.jpg


Aziz's iron looks significantly older, and, as you can see from the
patches of iron oxide covering lower, protected areas on the polished
surfaces, it was at one point completely covered in a rind of oxide.

Again, the complete and utter lack of visible silicate inclusions
rules out the remotest possibility that it's some sort of a stony iron
meteorite (with the very vague possibility of its being analogous to
something like a Brenham or Seymchan pallasite, but those are
indistinguishable from other iron meteorites, so...it would still be
an "ordinary" iron).

Conclusion:

The once-complete rind of oxide tells us that this is an old iron that
was at some point buried, or at least was fluvially moved or exposed,
at which point the upper half of its surface was exposed to arid,
abrasive conditions.

It's what appears to be a medium octahedrite from the definition of
the widmanstatten patter on its surface (large, straight,
clearly-defined kamacite boundaries rule out coarse or coarsest (see
Ziz, a coarse octahedrite, for comparison - the grain boundaries are
visible, but all you see are amorphous-shaped outlines and a general
orientation due to schreibersite crystals and weathering on a few
fragments), though this iron could conceivably be a fine octahedrite).

Regards,
Jason

On Thu, Apr 1, 2010 at 2:21 AM, habibi abdelaziz <azizhabibi at yahoo.com> wrote:
> hey all
>
> got some? time now and i? missed meteorite, sorry if i miss any emails or didn't answer;;
>
> ok many collectors ask me why? this iron has this golden color,
> what is the process , that make it looking like that.
> and why the?Widmanst?tten" patterns is showing on the surface,
> http://www.flickr.com/photos/azizhabibi/page3/
> can someone answer please ,as myself i do not know why its golden color,
> i know i have sold two small cut iron that was looking like gold, i may have photo in my archives.
>
> thanks
> aziz habibi
>
> ?habibi aziz
> box 70 erfoud 52200 morroco
> phone. 21235576145
> fax.21235576170</font>
>
>
>
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Received on Thu 01 Apr 2010 06:32:10 PM PDT