[meteorite-list] Ablation Zone 5 Layers...Not

From: al mitt <almitt_at_meteoritecentral.com>
Date: Fri, 20 Nov 2009 11:59:38 -0500
Message-ID: <65E0BA72D5444D288B0FD505E0B70954_at_StarmanPC>

Hi Elton,

You've brought up some very good discussion on the definition of fusion
crusted specimens. I went to the authority, Buchwald's Iron Meteorites to
see what he called it. He has written a lot about it. He states: "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 a product of simple melting, the crusts on iron
meteorites are complex. 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 3 km/sec., and ablation ceased. The
fusion crusts are, in principle, composed of an exterior fully oxidized,
rapidly solidified nonmetallic melt."

He shows a number of samples that are iron meteorites with various fusion
crusts and identifies them that way. In some cases thick metallic fusion
crust to describe flows and so forth. While I think there is some agreement
with what Buchwald said and your trying to say, he still calls it fusion
crust. Not to say that it is a term that is accurately describing a
scientific effect on the outside of iron specimens.

I have always felt and called some of my fresh iron falls fusion crusted
because that is what Buchwald has defined them as in his books and feel it
is a fair term to use unless a better term is identified and used by the
scientific community that would label it different. I do know as you have
pointed out that the term is often exaggerated way beyond the term that
accurately defines it in Buchwald's Books and certainly abused by some
seller of meteorites. Perhaps with this discussion, the overuse of the term
on irons will be more carefully applied. All my best!

--AL Mitterling

Mitterling Meteorites

----- Original Message -----
From: "MEM" <mstreman53 at yahoo.com>
To: "Meteorite-list" <meteorite-list at meteoritecentral.com>; "Jason Utas"
<meteoritekid at gmail.com>
Sent: Friday, November 20, 2009 9:01 AM
Subject: Re: [meteorite-list] Ablation Zone 5 Layers...Not


Dear Jason If everything is a part of the fusion crust than every meteorite
is fusion crusted end of discussion. So are you really saying that every
meteorite regardless of how condition has fusion crust even if all the extra
trans-located material is missing?

I don't have the luxury of going point by point as you have but apparently
you are unfamiliar with the Oxford and Cambridge dictionary definitions as
well as all their Dictionaries of Geology. Most of the literature and
practically all of the referenced websites use the silicate glassy/glassy
term in defining fusion crust--and in context they are most always speaking
of stoney meteorites.

 We've been down this discussion before and while I respect Buchwald's
observations: he is an industrial metallurgist and had no training I can
see in mineralogy nor geology. He became a iron meteorite subject matter
expert in his own right( I don't know that he ever did any work on any other
type. I see no incentive on his part to reevaluate the surface changes as he
was focused on cataloging the interior features. Somehow I don't think
"crust" was an issue for him and in the absence of inquiry into the use of
the term there was never a need to rethink it. He has an email
address--write him and ask him what he meant.

Nininiger was a biology teacher and while another legitimate self-made
expert in the field it wasn't technically oriented until late in his career.
This shouldn't be taken as disrespectful and doesn't mean that everything he
assumed was gospel-- especially given the state of scientific tools in his
lifetime. He laid the foundation for meteoritical study but that doesn't
mean he knew all there was to know about meteorites. His book about
meteorite surface features was mainly a photo documentation with little
analysis and generally lacking in comparative studies of the crust.

Much of this argument that they are "the experts" and as such are
infallible, is misguided and out of context, as the tools available now are
vastly more quantitative than tools of their day. So is our body of
knowledge more complete than during their careers. (

I am calling the ablation surface below any "crust" feature because
well...it is. The ablation surface is the last level we can ascertain the
fusion has occurred. When the crust is worn away the ablation surface is
revealed. I am also not calling the oxide coating a "fusion crust"
because,... well...it isn't ( necessarily) fused material and represents
either condensation or contact metamorphism of the final flight air soaked,
modified surface. I am also not calling the surface of SA's which show
aero-thermo-dynamic interaction that form the troughs "crust" because
partial melting/softening/gas jet ablation does not meet the definition of
fusion/fusing. How you see it as fusion crust illustrates my point that we
call everything fusion crust when it is not even fused material. I think it
deserves a more objective review and understanding of the complexities and
not reduced to a universal simplicity. As to my point about extensively
rusted/shalely Canyon
 Diablos being said to have "fusion crust", sounds like we are in violent
agreement.

The point of addressing the loss of the coating over time was to suggest
further inquiry into what the actual chemical composition was and to
indicate I felt it was a class of mineral/compounds which were unstable in
an oxygen rich atmosphere.

Again --some irons have apparent classical fusion crust but, I have to
disagree that all irons have fusion crust--that is why ablation surface is
an important distinction and is better nomenclature that serves as a
starting point for discussing all meteorite surfaces and where crust begins
and ends. I believe when and where it is found it needs as much analytical
scrutiny as we spend on the interior so we know its source material and how
it came to be crust.

Rather than me reiterate what I've already addressed perhaps you would like
to read it more collaboratively as some of what you replied to skipped over
where there is agreement and also you've challenged the studies about how
deeply thermal alteration occurs in different meteorites.

I am not ready to roll over on the claim that a chemically bonded oxide
constitutes fusion crust unless you want to drop the word fusion. I
proposed some terms for use in defining a meteorite's surface more
descriptively. Other than disagree out of principle, you didn't give a
counter argument as to why the model I laid out was in error.

Finally I will reiterate the problems with trying to have a reasonable
succinct discussion when out of context examples are introduced as if they
were the rule rather than the exception they are.

Elton
Received on Fri 20 Nov 2009 11:59:38 AM PST