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

From: Dave Carothers <david.carothers_at_meteoritecentral.com>
Date: Sun, 07 Jan 2007 16:58:18 -0500
Message-ID: <05ee01c732a6$f182f580$6401a8c0_at_WINBOOKJ>

Good evening Elton and list.

Your point was that the definition of fusion crust needs to be changed
because, by definition, it doesn't "fit" iron meteorites. Elton, you wrote:

> 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".

If the definition as posted included the phrase (NOTE it is composed of
GLASS), I would concur that this make the definition exclusive to meteorites
composed of stone (including forms of silicate material).

The word "glassy" as it relates to the phrase "a thin glassy coating... " is
an adjective and qualifies the description of the "coating" as being
glass-like or something shiny, very smooth, and mirror-like. It does NOT
mean the fusion crust is composed of glass. The definition continues to
state that "fusion crusts may be knobby, striated, ribbed, net, porous,
warty, or scoriaceous(bubbly). This appears to contradict the declarative
statement that "a fusion crust is a thin glassy (i.e a shiny, very smooth,
and mirror-like) coating". As a result of this contradiction, I would agree
that the definition could be changed to eliminate the contradiction.

My $0.02.

Dave


----- 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 3:10 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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Received on Sun 07 Jan 2007 04:58:18 PM PST