From: Jason Utas <[email protected]>
Subject: Re: [meteorite-list] Ablation Zone 5 Layers...Not
To: "Meteorite-list" <[email protected]>
Date: Friday, November 20, 2009, 6:58 AM
Hello Elton, All,
I'm going to go through this bit by bit to do it
justice...
> Yes on a freshly recovered iron, there appears to be a
"film" of what we believe is "magnetite-like oxide/nitride
micro-crystals, probably including some sulfide and
phosphide minerals" which form through interaction with hot
atmospheric plasma. Even though some of it is magnetic,
some of it is easily dislodged with a wipe of the finger.
I assume this coating is relatable to the iridescent film
which often coats stony meteorites - the film that often disappears
within days of a fall.
>I surmise that this rapidly goes to hematite or
limonite but I've not thought through the chemistry and I
suspect a valence discrepancy that makes this type meteoric
"magnetite" unstable. The mineral assemblage in the
coating/film is a result of passage through the atmosphere
and not per se the resulting changes that occur with the
passage of time on the surface.
I shall point you toward this photograph of the external
surface of a Sikhote-Alin. This iron was found ~50+ years after
falling and still retained its exterior surface. The features you see
are not made of melted Fe, but of an outer coating of iron oxide which
formed during atmospheric descent.
http://www.flickr.com/photos/cameteoritefinder/2335664239/sizes/l/
> I think I can proffer an argument for what is and is
not a scientifically underpinned definition of "crust" but
I'll work on that later. For the time being the use of
"crust" by present definition involves glass and last time I
checked there is no such thing as "iron glass".
Where did you get this definition? Why is it more
valid than the one
accepted by Buchwald, Nininger, Krinov, and the folks at
the USNM?
Why does fusion crust *have* to have glass in it?
Honestly, this
whole thing seems like a semantics battle on your part.
>We expect to find something analogous to "crust" so we
call what we see "crust"-- I understand that. But when we
stray too far everything including dust mites, rust and,
fungus gets called fusion crust.
Right.
Here:
http://www.aerolite.org/catalogue/sikhote-alin-aaa-33-2.htm
What you're looking at is the original external surface of
the iron, made of that FeO layer that you keep insisting isn't fusion
crust.It's perfectly analogous to the crust of stony meteorites,
except, of course, in that it contains no glass.
> As there is also a surface bluing occasionally
observed (much like after welding) this may be a directly
formed oxide/nitride layer of chemically altered meteorite
while emerging from the incandescent phase of flight but
since I am unsure of the composition I'll leave it out of
the below discussion.
I've seen that on stony meteorites as well. But since
you're leaving it out, there's no real need to address it.
> Chances are it is also quickly lost to weathering on
the surface--even in the museum drawer.
Maybe. I saw some pretty Oum Dreyga's with such a
film still present as of this winter - in a drawer in Alain Carion's shop
in Paris. As such, I have the feeling that such layers may not simply
sublimate with time, but they do seem to disappear rather rapidly
when meteorites are left in the field.
>The bottom line here is: we have to accept the
probability of an ever-evolving surface on our meteorites.
Some happen quickly and may be gone in a flash and some oh
so slowly. This should not deter us however from discussing
the basis for each step that comes and goes.
This also has nothing to do with our argument, for the most
part. I don't think there's anyone here who denies such a fact, so
stating it is somewhat superfluous.
> I believe to discuss meteorite surface features e.g
crust, non crust, flow lines, ripples, regmaglypts, pits,
bubbles, and all the variations, we should come to a working
definition in general principle of what to call them so we
know we are discussing the same thing.
Sounds good to me.
> Part of that is acceptance that there is an
"ablation/ablated zone" generally 2-6mm from the physical
surface where the meteoroid last interacted with the heat of
reentry. This zone my eventually be proved a new type of
"rind", geologically speaking.
Unfortunately, it's hard to gauge whether or not such a
feature truly
exists on stony meteorites because, due to their decreased
conductivity, this heating does not occur over distances
quite so large.
See page two.
http://adsabs.harvard.edu/full/1982Metic..17...27R
So there's kind of an "ablationary rind," but it really
only exists to the extent that you just noted in iron meteorites...
Of the layers physically present, I see two
branches/variations which we may reintegrate but for ease of
discussion the first is mostly the non-silicate bearing iron
branch of "layers":
So we *are* talking about irons' fusion crusts...ok...
> The ablated/ablation zone includes amongst it layers:
> 1)Lost Layer/ Null layer: The material which is
missing, includes ALL the material which is no longer there
which we may conclude was lost from its pre-entry form due
to atmospheric interaction. It may be marginal but may be
needed to discuss surface depth in relation to cosmic ray
tracks 14C concentrations, etc.
O...k....the stuff that's no longer there. A wordy
description, but
sure. Call it what you will.
> 2) Oxide Film or Coating: There is a layer of
non-physically/non-chemically, bonded oxide film which is
not persistent, subject to rapid erosion/weathering,
abrasion etc. This represents a condensation coating which
is applied after ablation stops. (See bluing discussion
also)
Such a coating forms on irons and stones alike, though -
iridescent films have been reported on many freshly fallen meteorites,
regardless of type.
> 2a) This is where fusion crust might be found if there
were normally crystalline molecules that melted and quickly
quenched leaving an anamorphic solid. But what we know as
true fusion crust is more complicated than that and is
largely governed by the composition of the meteoroid.
You breeze over it so nicely, without addressing the
issue. Hum. Well, again, I don't know why you insist on the glassy
nature of a fusion crust: I really can't fight your definition of it,
because it simply doesn't make any sense. There's no reason to
draw the line there, and I can't think of a single reason why fusion
crust should *have to* contain glass. Knowing meteorites, I would
define the fusion crust as the layer of meteoric material transformed
into melt during a meteorite's ablative stages of flight, which later
solidifies into a solid coating of material on the surface of the
stone, iron, what have you. I see no reason to insist on glass - I
agree that making a distinction between the properties of stones'
versus irons' fusion crusts and their structures might well be a
worthwhile endeavor, but insisting on calling the crust that forms on
irons 'not a fusion crust' seems a pointless task.
> 3)Flow/ Thermodynamic Features:
> 3a)There may be a layer of flow streams/esker-like
inverted stream channels where molten material, which
escaped evaporation and,, was displaced from one spot to
another where it may have been redeposited. Regardless it
is an artifact of reentry and we may also include it in the
subset of features we refer to as "flight markings" This is
a gray area also because this is more akin to a surface
feature than a true layer but I throw it out on the table
for discussion. There will be occasional features which
represent movement of material over top of a previously
ablated surface and time and consensus will determine if it
merits a layer designation.
I disagree; such structures are merely features of the
aforementioned fusion crust layer, and should be deemed synonymous with
said layer.
They are, after all, composed of the same materials, and
one is not
below or above the other layer; you're talking about the
same stuff
here.
These features are made of the fusion crust noted above, so
calling
them a distinct layer seems pointless.
You're not even arguing the difference between icing and
frosting.
You're arguing a difference between frosting and thick
frosting. It
just doesn't make sense.
> 3b)There is also the occasional surface feature (semi
flow) (which may or may not be a layer) of plastically
deformed "ripple-marks" which give a satiny, wavy, micro
"ridge and valley" pattern not unlike the depth and texture
of fingerprints (NOTE this is not the same as "thumb-print"
regmyglipts) Not all irons have this very thin layer. These
ripple marks appear to form via fluid dynamics. I surmise
(but have yet to prove) these are ripple marks of a
extremely short-lived state where semi-molten metal is
plastic enough to deform along lines where superheated gas
eroding gas passing in both laminar and turbulent flow over
the continuously evolving surface of the meteoroid. It
leaves, a row and furrow/valley and ridge/ripple-like
marking, submilimeter in depth. This results in that "less
than glossy","satin-like" sheen seen on some
meteorites--This is legitimate flight marking and therefore
may actually be a surface feature and not a true layer but,
a
> variation on the ablation surface. I am leaning
toward this being a surface feature vs an independent
layer.
See the specimen in my flickr stream pictured above.
This "layer," as
the one before it, is synonymous with fusion crust.
> 4)Ablation surface: It is included to distinguish from
the newly fallen surface any weathered/flaking/rusting
surface all too frequently mistakenly called "fusion crust"
on Canyon Diablos, Natans etc. Crust if present sits atop
the ablation surface as it represents incorporated
atmospheric gasses and possible re-deposition of Physically
and chemically altered material from another location on the
meteoroid, etc. Surface features can be in the ablation
surface or above it depending on their origins. The ablation
surface is a distinct demarkation between what was removed
and what remained even if subsequently it bubbled into
fusion crust or represents a redeposition of condensate from
this ablative/ heating/ shearing process--which also needs a
generic but descriptive name!
False. Completely and utterly incorrect.
You're talking about the surface of the iron meteorite
itself, below
the fusion crust.
How on earth can you put this "layer" between the fusion
crust and the
reheated rim when many Canyon Diablos and Nantans have seen
so much
weathering so as to lose any trace of their original
reheated rims!?
The only irons I have *ever* seen to possess such a surface
are desert
irons, where the crust has been stripped from the fresh
metal,
allowing for a thin coating of desert varnish on the
exposed iron (any
substantial oxidation would destroy this "layer"), and on
antarctic
irons, where a similar process often occurs.
Canyon Diablo and Nantan are examples of irons where the
crust has
been removed - along with countless mm or cm of
material. This
"layer," as you define it, does *not* exist on such irons.
Oh - Sikhotes sometimes exhibit such a surface as
weathering has
removed patches of fusion crust while leaving the surface
of the iron
relatively unaltered. It's a good thing they're so
fresh or this
wouldn't be true...
> 5)TAZ: Thermally Altered Zone: in this zone is the
material which was not displaced nor reformed, per se-- but
was thermally altered to a major or minor degree. Some
volatile gases my have been out gassed but a major effect
would be resetting magnetic orientation within the zone.
There are means to analyze how deeply and to what range of
elevated temperatures this zone was subjected to.
Well, the major effect generally noted is the
recrystallization of the
meteoric material, but sure - this is a legitimate
"layer."
> 6) All the remaining material largely unaffected by
the change in address from solar orbit to our collections. A
place holder for the time being but all that which is not a
part of the ablation zone.
> I'll leave it there for tonight and for a straw man
suggest there are 5 layers(on irons at least) in the
"Ablation Zone". These layers are thick or thin; regions of
original material which were in someway altered /affected by
the dynamics of passage through the atmosphere.
Right, well...you have my point of view. It's based
on the fact that
the fusion crusts on iron meteorites and on stony
meteorites form
through the same general processes into analogous
structures and
function in the same way on both types of meteorites.
Your insisting
on glass being a component of fusion crust seems as likely
to be
present due to a misinterpretation of some archaic article,
as best I
can tell, so I really don't know what to say. You
keep stating the a
fusion crust must contain glass 'because it is defined that
way,' but
when I stand back and ask the obvious question - 'why is it
defined as
such, and does that make sense,' all I get in response is
a
reiteration of your conviction that fusion crust is defined
as such,
and the definition cannot be changed.
Science is change, Elton.
The trouble with this point, though, is that you've taken
up the
conservative mantle of "no change" when I cannot find a
single
reference anywhere that states that fusion crust *must*
contain glass.
All of the literature from NIninger to Buchwald, to
Krinov, to
modern-day descriptions of meteorites entering the USNM
from
Antarctica - they all state that irons have fusion crusts.
In other words, you're saying the definition shouldn't be
changed from
one in which glass is an indisputable component of fusion
crust when
that's not stated anywhere in the first place.
You can't advocate *maintaining* a definition when it's
*never* been
accepted as fact, because that's not how definitions
work. It needs
to be accepted before you can try to "keep it
accepted." Otherwise
you're just advocating a new theory based on the historical
merit of
the theory - which, if it has never been accepted in the
first place,
is simply circular reasoning.
You're the one advocating a backwards sort of change,
Elton. We're
going along with accepted meteoritics. And unless you
have a reason
to say that glass is an inherent component of what we are
to deem
fusion crust, I suggest that you come up with a better
reason than
"it's defined that way," because it's getting old.
Regards,
Jason
> --- On Thu, 11/19/09, Martin Altmann <[email protected]>
wrote:
>
>> From: Martin Altmann <[email protected]>
>> Subject: Re: [meteorite-list] Fusion Crust on
Irons--Not
>> To: [email protected]
>> Date: Thursday, November 19, 2009, 7:21 AM
>> Unlike in politics and public opinion
>> (and sometimes in science),
>> in meteoritics it sometimes can be more difficult
to adhere to
>> theories/legends,if one gets samples in ones very
hands, which exhibit the opposite of that, the theory
postulates.
>>
>> If you ever had an early picked Sikhote at hand,
>> or if you had taken from Andi Gren's Boguslavka
slices
>> (a fall, who simply hadn't enough time in field,
to develop
>> a magnetite, wuestite, limonite or whatever -ite
weathering crust),
>> you would be very surprised.
>>
>> Cause they don't display that ominous blue-ish
flimsy
>> luster, which is often reported as fusion crust,
but a thick and fat layer of a discernibly different matter
than the material beneath, of a dark color and rough to
silky surface.
>>
>> I never believed in iron fusion crusts neither,
but when I
>> got in these freshly picked up observed falls, I
was disabused.
>>
>> Main problem in that question is, as it was
correctly
>> mentioned here, that we simply have so few
pristine samples of fresh iron falls and that most irons we
get in our collections arrive with weathered or artificially
cleaned surfaces.
>>
>> Now you may argue about the word "crust" as a
>> (pseudo-)scientific term...well for me scientific
terms are best, when they keep most of their meaning they
have in their common use in the language. And there crust -
meant for me a layer on the outside of an object.
>
>> .....and we have the problem, that there exist
these
>> freshly fallen lumps with that strange crust.
Shall we hide them in the deepest corner of our drawers,
cause they don't fit in the axiom, that fusion crusts are
fusion crusts only, when silicates are melting?
>>
>> Sometimes, if the results don't fit into a theory,
one has
>> to think about modifying the theory,
>>
>> Else there wouldn't be no meteorites in our sense
at all,
>> Nada, Niente, Nix, Nimic, cause we all would know
that they are products of our Aristotelian atmosphere,
solidfied accumulations of terrestrial vapours and probably
created by lightning strokes,wouldn't we?
>>
>> Best!
>> Martin
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