[meteorite-list] urelite origins

From: Marc D. Fries <m.fries_at_meteoritecentral.com>
Date: Tue Sep 7 09:46:06 2004
Message-ID: <1134.10.17.14.1.1094564760.squirrel_at_webmail.ciw.edu>

Howdy Frederic

   The simplest answer is that there is a distinct Raman spectrum for
diamond, and another for graphite, and they are distinguishable from
each other even when the two are present together. It's more
complicated than that, actually, but that's the heart of the matter.
There are two major complications - the Raman spectrum of graphite is
about 50x more intense than that of diamond, so you need to have a lot
of diamond present to see it. Secondly, for very small diamonds (100s
of nm or less) the Raman spectrum peak becomes broad instead of a nice
sharp line. As an added difficulty, I've seen some Raman spectra of
urelites that look an awful lot like Raman spectra of diamond-based
polishing compound (oops?).
   Overall, however, it's clear that urelites contain very small diamonds,
and the Raman spectra I've seen all seem to show "relict" strain that
is consistent with formation by violent compression.

   About the lack of chondrules - that's not surprising if the original
material was a carbonaceous chondrite. The heat and pressure needed to
turn graphite to diamond should obliterate any chondrules, or at least
I'd expect it to - perhaps someone has calculated this...

Cheers,
MDF

> Hello,
>
> I am not a scientist and therefore not able to analyse Raman data futher
> than seing which type of mineral was hit by the laser beam. But I have
> also
> heard that some scientits suggest that some diamonds in Ureilites could be
> presolar, and therefore not necessarily formed by shock... Also, I wonder
> if
> Ureilites were formed from carbonaceous meteorites, as we would still find
> some chondrules, even very rare, which is not the case... Ureilites are
> definitely achondrites.
> Well I do not know really, and I also wonder. I know from Raman data
> obtained on one Ureilite that we found, that diamonds are located most of
> the time in areas of the meteorite where mostly graphite can be found,
> which
> is quite understandable. But I do not know how the reason why some of this
> graphite turned to diamond can be found within Raman data. Can you explain
> to me with words as simple as possible, as once again I'm not a scientist!
> Thank you very much.
>
> Frederic Beroud
> www.meteoriteshow.com
> IMCA #2491
>
> ----- Original Message -----
> From: "Marc D. Fries" <m.fries_at_gl.ciw.edu>
> To: <meteorite-list_at_meteoritecentral.com>
> Sent: Monday, September 06, 2004 7:51 PM
> Subject: [meteorite-list] urelite origins
>
>
>> Greetings
>>
>> I've got a meteorite question. Having seen enough Raman data to be
>> convinced that the diamonds in urelites are formed by shock
>> compression, I'm wondering - does anyone have an opinion on what sort
>> of meteorite they started out as? They had to have been carbonaceous
>> meteorites of some sort to begin with, but the articles I've seen don't
>> seem to offer a clear picture of what they were like before they were
>> shocked. CM, perhaps?
>>
>> Cheers,
>> MDF
>>
>>
>> ---
>> Marc D. Fries, Ph.D.
>> Postdoctoral Research Associate
>> Carnegie Institution of Washington
>> Geophysical Laboratory
>> 5251 Broad Branch Rd. NW
>> Washington, DC 20015
>> PH: 202 478 7970
>> FAX: 202 478 8901
>> ______________________________________________
>> Meteorite-list mailing list
>> Meteorite-list_at_meteoritecentral.com
>> http://six.pairlist.net/mailman/listinfo/meteorite-list
>>
>>
>
>

---
Marc D. Fries, Ph.D.
Postdoctoral Research Associate
Carnegie Institution of Washington
Geophysical Laboratory
5251 Broad Branch Rd. NW
Washington, DC 20015
PH:  202 478 7970
FAX: 202 478 8901

Received on Tue 07 Sep 2004 09:46:00 AM PDT