[meteorite-list] NWA 3099 (L/LL3) and why not 3.X ???

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From: j.divelbiss_at_att.net <j.divelbiss_at_meteoritecentral.com>
Date: Thu Apr 22 10:32:47 2004
Message-ID: <031020040053.12805.55eb_at_att.net>

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Jeff, Tom, Adam and others:

Thanx for the explanations for my question about the petrologic extensions to the grade 3
subtype. I've always wondered how it happens...I guess I now know how it can be done...just not
the reasons to do it or not to do it for any particular meteorite name/number. The method Jeff
is proposing that Adam alluded to will hopefully make it easier for the institutions to do it
on a more regular basis.

As for Tom's question about whether I thought the scientists were being "lazy"...no, I wasn't
asking that question, but I do think the reason some of these classifications of
particular meteorites are not subtyped with a 3.X may have more to do with ability, time and
the cost to do it. So, no Tom...no one has attacked me over my ignorance on this particular
subject. And as you all can tell, I am ignorant about a lot of this stuff. That is why I ask
the questions.

But as you have learned Tom, one has to be careful how they phrase a particular
question/subject...so as to not ruffle the feathers of other listees with off-based, misleading
or "cute" statements. Book burning and Norton in the same subject line was cause for
retribution in this case...in my opinion. Accept it move on would be my tact.

We all need to treat subject lines and post content with more care.

John

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Metamorphic subtypes (the tenths place in the petrologic type) are
routinely determined in a variety of ways, some more accurate and reliable
than others. For a given meteorite, you have to look into what was done
before you can evaluate the usefulness of the assigned number.

The most reliable method, and the one you should have the most confidence
in, is the thermoluminescence (TL) sensitivity measurement. Only a handful
of labs around the world do this, notably Derek Sears' lab at the Univ. of
Arkansas where the method was pioneered. Basically, the more metamorphic
feldspar in a chondrite, the higher the induced TL. This method is good to
+/- 0.1, although severe weathering or shock can cause problems.

Another commonly used method is to look at the distribution of olivine
compositions with an electron microprobe. The more scattered the
compositions the lower the subtype. The definition of a type 3 chondrite
is based on this method: any chondrite where the percent mean deviation
(PMD) of fayalite content of olivine is over 5% is type 3 and all others
are type 4 or higher. As you go down in petrologic type from 3.9 to about
3.4, the PMD rises from 5% to over 33%. But below 3.4 all meteorites have
highly scattered olivine compositions, so you can't tell a 3.0 from a 3.4
on this basis. Bottom line: subclasses done this way may be good to about
+/- 0.1-0.2 in the 3.4-3.9 range, but below that all one can say is <3.4.

Other claims are sometimes made based on the appearance of the thin
section. If a meteorite has a lot of isotropic glass, lots of black
matrix, and also shows evidence for certain kinds of alteration reactions
in its metal and sulfides, one can say with a fair degree of confidence
that it is type 3.2 or less. Other estimates based on appearance are hard
to evaluate, like when you read "visually estimated to be type 3.6."

In summary, you need to find out what was done before evaluating any
claim. The Nomenclature Committee tries to filter out claims based on
improper methods, so really bogus ones should not appear in the
Meteoritical Bulletin. We do, however, readily accept meteorites that are
simply called "type 3," and do not force people to assign a subtype.

Hope this helps.

Jeff
At 07:43 AM 3/9/2004, j.divelbiss_at_att.net wrote:
>Jeff, Bernd and others:
>
>Great stuff guys, especially the pictures of these great unequilibrated
>stones. Will we ever
>understand how these anomalies ever happened...probably not. But it is
>sure is worth trying.
>
>Questions: I've often wondered why some of these stones are not evaluated
>further to determine
>the level of feldspar change/metamorphism that grades this level of change
>from the original
>material. Instead of simply stating it is an LL3...the grading goes
>further to say it LL3.2,
>LL3.5, etc. If I remember right it is a measurement of the feldspar glass
>illuminescence.
>
>1. How/who decides when to this evaluation?
>
>2. Is it really just a matter of available equipment in many cases...the
>facility may not
>have the ability to do the test...so meteorites out of those institutions
>are never beyond LL3.
>
>3. Is it a cost issue for doing the test?? Seems to me that LL3's are
>cheaper to buy than say LL3.5 or lower...yet I've looked at plenty of
>awesome LL3's that rival some the others with the
>extra desingation or test.
>
>Maybe of the dealers that have stones evaluated can tell us why this test
>is done or not.
>I'd like to see it done more often with beauties like NWA 3099, and NWA
>1933. Why not is my question.
>
>
>John
>
>
>
>
>
>
>G'day all,
>
>For those of you who purchased NWA 3099 or are just interested, I spoke
>with Bernd over the past couple of weeks regarding this particularly
>remarkable meteorite. Bernd had some interesting things to say it which
>are posted at the end of the page here:
>
><http://www.meteoritesaustralia.com/features/nwa3099.html>http://www.meteoritesaustralia.com/features/nwa3099.html
>
>Cheers,
>
>Jeff Kuyken
>I.M.C.A. #3085
><http://www.meteorites.com.au>www.meteorites.com.au

Dr. Jeffrey N. Grossman
Chair, Meteorite Nomenclature Committee (Meteoritical Society)
US Geological Survey
954 National Center
Reston, VA 20192, USA
Phone: (703) 648-6184 fax: (703) 648-6383

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<html>
Metamorphic subtypes (the tenths place in the petrologic type) are
routinely determined in a variety of ways, some more accurate and
reliable than others.  For a given meteorite, you have to look into
what was done before you can evaluate the usefulness of the assigned
number. 
The most reliable method, and the one you should have the most confidence
in, is the thermoluminescence (TL) sensitivity measurement.  Only a
handful of labs around the world do this, notably Derek Sears' lab at the
Univ. of Arkansas where the method was pioneered.  Basically, the
more metamorphic feldspar in a chondrite, the higher the induced
TL.  This method is good to +/- 0.1, although severe weathering or
shock can cause problems.  
Another commonly used method is to look at the distribution of olivine
compositions with an electron microprobe.  The more scattered the
compositions the lower the subtype.  The definition of a type 3
chondrite is based on this method: any chondrite where the percent mean
deviation (PMD) of fayalite content of olivine is over 5% is type 3 and
all others are type 4 or higher.  As you go down in petrologic type
from 3.9 to about 3.4, the PMD rises from 5% to over 33%.  But below
3.4 all meteorites have highly scattered olivine compositions, so you
can't tell a 3.0 from a 3.4 on this basis.  Bottom line: subclasses
done this way may be good to about +/- 0.1-0.2 in the 3.4-3.9 range, but
below that all one can say is <3.4. 
Other claims are sometimes made based on the appearance of the thin
section.  If a meteorite has a lot of isotropic glass, lots of black
matrix, and also shows evidence for certain kinds of alteration reactions
in its metal and sulfides, one can say with a fair degree of confidence
that it is type 3.2 or less.  Other estimates based on appearance
are hard to evaluate, like when you read "visually estimated to be
type 3.6." 
In summary, you need to find out what was done before evaluating any
claim.  The Nomenclature Committee tries to filter out claims based
on improper methods, so really bogus ones should not appear in the
Meteoritical Bulletin.  We do, however, readily accept meteorites
that are simply called "type 3," and do not force people to
assign a subtype. 
Hope this helps. 
Jeff 
At 07:43 AM 3/9/2004, j.divelbiss_at_att.net wrote: 
<blockquote type=cite class=cite cite>Jeff, Bernd and others: 
Great stuff guys, especially the pictures of these great unequilibrated
stones. Will we ever 
understand how these anomalies ever happened...probably not. But it is
sure is worth trying. 
Questions:  I've often wondered why some of these stones are not
evaluated further to determine 
the level of feldspar change/metamorphism that grades this level of
change from the original 
material. Instead of simply stating it is an LL3...the grading goes
further to say it LL3.2, 
LL3.5, etc. If I remember right it is a measurement of the feldspar glass
illuminescence. 
1.  How/who decides when to this evaluation? 
2.  Is it really just a matter of available equipment in many
cases...the facility may not 
have the ability to do the test...so meteorites out of those institutions
are never beyond LL3. 
3.  Is it a cost issue for doing the test??  Seems to me that
LL3's are cheaper to buy than say LL3.5 or lower...yet I've looked at
plenty of awesome LL3's that rival some the others with the 
extra desingation or test. 
Maybe of the dealers that have stones evaluated can tell us why this test
is done or not. 
I'd like to see it done more often with beauties like NWA 3099, and NWA
1933. Why not is my question. 
 
John 
  
 
 
G'day all, 
  
For those of you who purchased NWA 3099 or are just
interested, I spoke with Bernd over the past couple of weeks regarding
this particularly remarkable meteorite. Bernd had some interesting things
to say it which are posted at the end of the page here: 
  
<a href="http://www.meteoritesaustralia.com/features/nwa3099.html">http://www.meteoritesaustralia.com/features/nwa3099.html</a> 
  
Cheers, 
  
Jeff Kuyken 
I.M.C.A. #3085 
<a href="http://www.meteorites.com.au">www.meteorites.com.au</a></blockquote>
<x-sigsep></x-sigsep>
Dr. Jeffrey N. Grossman 
Chair, Meteorite Nomenclature Committee (Meteoritical Society) 
US Geological
Survey          
954 National Center 
Reston, VA 20192, USA 
Phone: (703) 648-6184   fax:   (703)
648-6383 
</html>

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Received on Tue 09 Mar 2004 07:53:08 PM PST

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