[meteorite-list] a question on fusion crust

From: Mr EMan <mstreman53_at_meteoritecentral.com>
Date: Wed, 21 Mar 2007 23:48:04 -0700 (PDT)
Message-ID: <219955.88263.qm_at_web51004.mail.re2.yahoo.com>

The "soot" coating you are talking about is mostly
freshly created magnetite (micro /nano crystals) from
the oxidation of iron whilst passing through the
incandescent phase. It adheres by magnetism and can
be wiped off with rough handling. There is probably a
carbon component however graphite turns to carbon
dioxide upon burning.

There are several other terms for the zone associated
with "ablation" heating, one of which is in the
literature: "zone of thermal alteration". When there
is an oxidation coat of native elements such as found
on sodium or phosphorus it is sometimes called a rind.
This is what I favor to describe the coatings on
irons--the one before terrestrial oxidation also known
as rust.

In my view there can be no crust when the chemical and
physical characteristics of the surface do not differ
from the donor material. That lets the door open for
irons having crust and at least most agree that it is
unlike the fusion crust of stoney meteorites.

While at the Smithsonian inspecting the collection up
close and personally, I was advised to use caution in
examining a fresh iron as the coating would rub off
easily. They didn't call it fusion crust but the issue
never came up. The point being, not everyone is in
agreement nor is there universal usage.

Critical analysis on the assumptions about fusion
crust on irons would likely lead to a more descriptive
table of composition and relate that to the metallurgy
and chemical composition of the meteorite itself. In
fact I believe research on crustal petrology would be
reveling for all types of meteorites. I yet wonder why
some lunarites have a brown bubbly crust. The crust of
a iron meteorite on Mars will be different from one
here and I'd like to know what to expect and why.

Intuitively, I know there would be rare minerals such
as nickel oxides,nitrates, phosphates perhaps even a
sulphide and yes even O3 and O4 silicates. However
current wisdom is that crust is crust ego no research
needs to be contemplated-NOT.

Some of the oft quoted god status experts who write of
fusion crust on irons monitor this list and have
remained curiously silent on the topic. I hope this
topic is thought provoking for all parties.

I suppose that the use of the term "fusion crust"
universally with respect to irons is acceptable if one
doesn't have a need for technical accuracy. One thing
yet to be refuted here on the list is that the "crust"
on irons is not composed of "glass".

Elton

--- Michael Murray <mmurray at montrose.net> wrote:

> Hi List,
> Are you ready for a dumb question! Here goes... Is
> there a more definitive description of the
blackened outer layer on the surface of iron
meteorites than simply referring to it as "fusion
crust"? From what I can gather looking at different
pictures of stonys, they seem to have a glassy or
melted layer of the material of the stone. I can see
that being a 'crust'. On at least some irons, there
is a coating but it doesn't appear glassy, just
blackened. I'm guessing that that coating is
partially resulting from burning of the gases in the
atmosphere? If it is a 'crust', it is not much of
one. It looks
like a very thin coating of soot that is adhered to
the stone, although more durable than soot. This is
probably one of those dumb questions that the
Collection of Wisdom would answer.
 
 The stone in this picture has the blackened layer
 I'm referring to:
<http://i127.photobucket.com/albums/p124/mmurray_02/IMG_0319.jpg>
 Michael Murray
Received on Thu 22 Mar 2007 02:48:04 AM PDT