[meteorite-list] Ablation Zone 5 Layers...AND Crust

From: Greg Stanley <stanleygregr_at_meteoritecentral.com>
Date: Mon, 23 Nov 2009 09:12:04 -0800
Message-ID: <SNT117-W35F733E1B80212EC6C7B59D29E0_at_phx.gbl>

Elton and List:

I just wanted to point out that in much of the literature (and on the web) it is stated that there is a fusion crust on irons, so I do not think this is the your argument.? It is how the term is should be used.? I think you make many valid points and I have done some reading on the subject - and thus, learning more about it.? It appears that the so called "crust" that forms on stony meteorites and irons may be very different, both in their content and how they form.? On a stony, the outer layer heats very rapidly and starts the process of ablation.? Then material is removed (causing the vapor trail) and a protective coating forms as the melt cools and forms a glass like Material.? As we know, the different mineral content produces different fusion crust: light brown on Aubrites, dark brown to black on chondrites and a very glossy black on Eucrites and Martian Meteorites due to the Calcium content.? Now on irons, the stone ablates (as we can see flow lines) but the so called crust may be different; not a glas
sy substance like on a stony but more of an iridescent coating.? This very well could be the result of minor amounts of silicate and perhaps even carbon related substances within the iron.? I think a paper discussing the similarities and differences of the "fusion crust" or thermal coatings on stony, stony-iron and iron meteorite would be fascinating; if it has not be done already.

Please let me point out that I enjoy these posts as it provides a great learning tool.?

Much Thanks,

Greg S. ? ?

----------------------------------------
> Date: Sun, 22 Nov 2009 22:12:29 -0800
> From: mstreman53 at yahoo.com
> Subject: Re: [meteorite-list] Ablation Zone 5 Layers...AND Crust
> To: altmann at meteorite-martin.de; meteorite-list at meteoritecentral.com; stanleygregr at hotmail.com
>
> - Greg Stanley wrote:
>> See Fig. 1 on this UCLA web page.
>> http://www.ess.ucla.edu/research/cosmochem/meteorite.asp
>>
>> Not sure if Dr. Wasson has written any papers regarding
>> fusion crusts on irons, but I would think he would endorse
>> anything on the web page.
>
> So Gregg, if your statement is true, Dr.Wasson and/or UCLA endorses the formation of "rusty" fusion crust actually formed during decent? ( Fig 1 or is it the regmaglyts that formed during passage and they are on top of the rusty fusion crust?)
>
> The issue is deeper than the semantic arguments, glossary obsolesence and lies in the differences of origin, composition, and presence of crust on different classes of meteorites. In the old school, a crust is generally removable from the underlying substrate and a dipping in molten metal alloy pushes the envelope of being a crust, however Buchwald illustrated that this was a combination of free metal and oxides. I conceed that irons--most all freshly fallen ones anyway , do have a fused surface that we can by convention call "crust" but the question remains as to what is the crust which is a few microns thick versus coatings and halos etc. There is danger of promoting ignorance by making assumptions that everything which is called crust is identical.
>
> I am old school and I have the (un)reasonable expectation that where a distinction does lie, that the collector can understand why things are the way they are and not abuse the descriptions when they try to peddle their specimens. Very few understood the distinction of silicate content when addressing irons as the presence of silicates,oxides, carbides,phosphides, etc largely govern the nature of a particular crust and gives insight into what went on during that miliseconds of exposure when the final surface was formed.
>
> Why it matters to me is 1) the abuse of the term in describing meteorite conditions 2) the science of understanding the differences in crust origin and composition and 3) how do we make the distinction amongst flight markings and do they differ from "fusion" crust?
>
> I am satisfied that 1)one of the iron specimens cited in this discussion apparently do have a silicate content that provided for a glassy fusion crust and its origin is intriguing scientifically. 2)That by definition alone (fused + coating) the irons do routinely have a "fused coating/crust" which differs significantly from our traditional concept of "crust" even though it is indistinguishable from the interior to the naked eye. (Do folks really "see" the crust or do they see if because it is supposed to be there?) At what point in weathering do we "see" that the welded crust/layer on an iron has rusted away given it is a few microns thick? 90% of the time, the seller will be claiming "crusted" long after the crust is gone, IMO.
>
> I've seen little to nothing so far that invalidates describing the ablation zone nor crust in layers--Nor did Buchwald, apparently.
>
> Finally, the term "big-head" someone used probably translates to "arrogant" Either way it was used in error as I was misunderstood. I did not disparage the pioneers in the field or meteoritical study. They gained their meteoritical knowledge largely autodidactly when they came over. I mentioned their former fields because when an individual crosses over into a different field of research, one can not just presume that they immediately acquire the entire knowledge base of those who were formerly educated in the same field.
>
> Elton
>
                                               
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Received on Mon 23 Nov 2009 12:12:04 PM PST