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

From: Mr EMan <mstreman53_at_meteoritecentral.com>
Date: Sun, 7 Jan 2007 00:10:30 -0800 (PST)
Message-ID: <577192.58708.qm_at_web51005.mail.yahoo.com>

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!!!
Received on Sun 07 Jan 2007 03:10:30 AM PST