[meteorite-list] Irons DON'T form Fusion Crust's - yes they DO

[Dr. Svend Buhl]

Eman and list,

when it comes to irons there is but one authority to refer to: Vaugn 
Buchwald.

There is not the slightest doubt that the melted exterior of freshly fallen 
iron meteorites is called a fusion crust.

In volume 1 page 49f. of his Handbook of Iron Meteorites Buchwald explains:

Cuts perpendicular to the surface of a freshly fallen iron meteorite 
disclose fusion crusts and heat-affected rim zones. While the fusion crusts 
on stone meteorites are usually the product of simple melting, the crusts on 
iron meteorites are complex, displaying mixed melts of fully and partially 
oxidized metal. The fusion crusts are the adhering remnants of ablated metal 
from the last part of the trajectory left on the surface when the velocity 
decreased below about 3km / sec, and ablation ceased. [...] The fusion 
crusts are, in principle, composed of an exterior fully oxidized, rapidly 
solidified nonmetallic melt, and an interior only slightly oxidized metallic 
melt. The oxide melts have solidified and decomposed to wuestite-magnetite 
aggregates, while the metallic melts, often forming numerous crossbedded 50u 
thin sheets, have solidified to dendritic -cellular aggregates, that below 
700°C have transformed diffusionless to fine grained martensitic alpha2 
products.

End of quotation. (pages 50 - 53 show fantastic photograhies of differnent 
types of fusion crusts on iron)

By the way. I do not agree that the Glossary of Geology of the American 
Geological Institute is a sound reference for nomenclature and terminology 
of meteorites.

Best regards

Svend Buhl

www.niger-meteorite-recon.de

----- Original Message ----- 
From: "Mr EMan" <mstreman53 at yahoo.com>
To: "Jason Utas" <meteoritekid at gmail.com>; "Meteorite-list" 
<meteorite-list at meteoritecentral.com>
Sent: Sunday, January 07, 2007 9:10 AM
Subject: [meteorite-list] Irons DON'T form Fusion Crust's... was Iron Falls& 
NJO


> 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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