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 [EMAIL PROTECTED]
To: Jason Utas [EMAIL PROTECTED]; Meteorite-list
meteorite-list@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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