[meteorite-list] Irons DON'T form Fusion Crust's... was Iron Falls & NJ

From: Jason Utas <meteoritekid_at_meteoritecentral.com>
Date: Sun, 7 Jan 2007 02:43:04 -0800
Message-ID: <93aaac890701070243h1cc32dd7q91b972b285f41a9f_at_mail.gmail.com>

Hello "Eman," All,
Firstly, if one determines fusion crust to be made of siliceous material,
that, as far as I'm concerned, is up to them - irons most certainly form a
thin skin of magnetite (not loosely adhering, sorry, but about as hard to
peel off as that of a stone, if not more so) that most clearly displays flow
features, roll over lips, and froth as well as those seen on their stony
counterparts. If one chooses not to call this "fusion crust," I fail to see
the reason for calling any such feature a "fusion crust," and not a "cooled
siliceous crust."
The layer formed on your average iron while still at speed up there is most
certainly fused, and is definitely a crust. If you wish to call it
something else, that's up to you - I'll agree with Buchwald and Nininger on
this one and call it 'fusion crust.'

http://i50.photobucket.com/albums/f306/JUtas/DSCN1330.jpg

http://i50.photobucket.com/albums/f306/JUtas/dscn1570-damngood.jpg

http://i50.photobucket.com/albums/f306/JUtas/DSCN1315.jpg


Furthermore, if you'd like pictures of an evident widmanstatten pattern on a
torn meteorite face, look no further:

http://i50.photobucket.com/albums/f306/JUtas/DSCN1317.jpg

-A ~4 kg mas of Ziz (NWA 854), which fits perfectly onto the ~20kg mass (the
sides are only slightly fused over), both of which exhibit clear figures,
though they may be somewhat coarse (it's a coarse octahedrite...).
It's a clear octahedral pattern. Call it what you like, the photo's not
great (I was fortunate enough to take these a number of months ago), and
even I can still see it. Those are octahedral structures, just as you'd see
if you broke a piece of optical calcite (the crystal form is different, I
know, but the principal is the same). Don't worry, I don't lie to the list
- and if I make a comment such as that, I *can* back it up with pictures ;)

There are also a few Taza's (NWA 859) around the show some nice 3D torn
surfaces with such crystalline features, though I think they're due to the
kamacite needles within them being structural weaknesses - where they occur
in bands, the iron might have a bit of a weak spot. See:

http://www.arizonaskiesmeteorites.com/AZ_Skies_Links/Etched_Meteorites/Etched_Taza/512gEtched_Taza/index.html


This is a perfect example of such a layer of kamacite needles, though I'm
feeling a bit lazy now to actually snap some photos (it's a quarter past two
in the morning...I get a break here) of a nice torn surface.

Reheated zones, which you briefly, and incorrectly mention, as layers a mere
few microns thick, are typically a number of millimeters in thickness.
Tafrawet (NWA 860) in particular had a spectacular example of this, but just
grab your nearest fresh iron slice and have a look...again, it's
two...twenty in the morning now, and there's no way I'm taking new photos
for this argument that could so easily be resolved by any number of books on
the subject.

You can play with semantics as much as you like - it doesn't change the
nature of things...
Regards,
Jason




On 1/7/07, Mr EMan <mstreman53 at yahoo.com> wrote:
>
> Someone wrote:
> "...recovered recently after falling had been
> beautifully fusion crusted, ...(snip)
> Why anyone should doubt the existence of fusion crust
> on a freshly fallen iron is beyond me - have a look at
> Cabin Creek if you want proof that it still forms on
> smallish irons falling at terminal velocity.(sic)"
>
> No I am not wacky. I am a purist trying to save this
> hobby from choking on a long-accepted, urban myth.
> (Just kidding folks, I am only trying to save some of
> you.)
>
> OK, let me reverse it, when shown otherwise, "Why
> would anyone continue to claim that freshly fallen
> irons typically show a fusion crust?" We say this
> over and over but never stop to consider what the term
> actually means. We use "fusion crust" because
> fundamentally we don't, as a collective, bother to
> understand "fusion crusts" in the first place. Heck,
> half of you think the weathered chalky ocher surface
> of a W10 NWA is "fusion crusted" ,to read your Ebay
> ads.
>
> Here is the technical point explained ... a
> (meteoritical) fusion crust is a thin glassy coating
> (NOTE it is composed of GLASS). Owing to effects of
> atmosphere and composition, fusion crusts may be
> knobby, striated, ribbed, net, porous, warty, or
> scoriaceous(bubbly) (Glossary of Geology, American
> Geological Institute,2nd Ed) To be composed of glass
> it must have a "silicate" content which can be
> vitrified; that is turned amorphous/glassy by
> melting/fusing(the technical term is fused or fusing)
> ; and that is the operative word in the phrase "fusion
> crust".
>
> Everyday, normal, common Irons do not contain silicate
> in sufficient quantity to make glass and thus form a
> FUSION CRUST; A silicated iron might, a pallasite
> could, a mesosiderite should-- but not an
> Iron/Siderite. While a technical point, it is a valid
> and important distinction to note that the post flight
> surface of an iron is different from that of
> meteorites containing silicates.
>
> Irons do not have a fusion crust. They may have lines
> of molten flow that pool in regmaglypts and while this
> illustrates the state of fusing ( aka melting) it does
> not a fusion crust make.
>
> Irons will have an "ablation surface" which may be
> coated by:
> a RIND of loosely adhering magnetite, bunsenite, other
> oxides, phosphates, carbides, and sulphides,
>
> a FILM of carbon which is readily wiped off,
>
> a ZONE of "melted amorphous recrystallized metallic
> alloy, also called a zone of thermal
> alteration(microns thick)----but they DO NOT have a
> fusion crust unless they contain ample silicate.
>
> Eman
>
> PS: As to "widmanstatten pattern of some sort", Ok,
> from a photo? if you say so <wink> I agree that one
> might see boundary lawyers if segregated by
> schreibersite. Sorry but seeing crystal structure is a
> stretch and surely you misspoke-- as I too often do,
> but not about seeing fusion crust on irons!!!
>
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Received on Sun 07 Jan 2007 05:43:04 AM PST