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 <[EMAIL PROTECTED]> 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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