[meteorite-list] NWA 3099 (L/LL3) and why not 3.X ???
From: Jeff Grossman <jgrossman_at_meteoritecentral.com>
Date: Thu Apr 22 10:32:47 2004 Message-ID: <5.1.0.14.2.20040309111103.02dbaed8_at_gsvaresm02.er.usgs.gov> --=====================_435640640==_.ALT Content-Type: text/plain; charset="us-ascii"; format=flowed 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 --=====================_435640640==_.ALT Content-Type: text/html; charset="us-ascii" <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.<br><br> 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. <br><br> 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.<br><br> 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."<br><br> 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.<br><br> Hope this helps.<br><br> Jeff<br> At 07:43 AM 3/9/2004, j.divelbiss_at_att.net wrote:<br> <blockquote type=cite class=cite cite>Jeff, Bernd and others:<br><br> Great stuff guys, especially the pictures of these great unequilibrated stones. Will we ever <br> understand how these anomalies ever happened...probably not. But it is sure is worth trying.<br><br> Questions: I've often wondered why some of these stones are not evaluated further to determine <br> the level of feldspar change/metamorphism that grades this level of change from the original <br> material. Instead of simply stating it is an LL3...the grading goes further to say it LL3.2, <br> LL3.5, etc. If I remember right it is a measurement of the feldspar glass illuminescence. <br><br> 1. How/who decides when to this evaluation?<br><br> 2. Is it really just a matter of available equipment in many cases...the facility may not <br> have the ability to do the test...so meteorites out of those institutions are never beyond LL3.<br><br> 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<br> extra desingation or test.<br><br> Maybe of the dealers that have stones evaluated can tell us why this test is done or not. <br> I'd like to see it done more often with beauties like NWA 3099, and NWA 1933. Why not is my question.<br><br> <br> John<br><br> <br><br> <br><br> <br> <font size=2>G'day all,</font><br> <br> <font size=2>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:</font><br> <br> <font size=2><a href="http://www.meteoritesaustralia.com/features/nwa3099.html">http://www.meteoritesaustralia.com/features/nwa3099.html</a></font><br> <br> <font size=2>Cheers,</font><br> <br> <font size=2>Jeff Kuyken<br> I.M.C.A. #3085<br> <a href="http://www.meteorites.com.au">www.meteorites.com.au</a></font></blockquote> <x-sigsep><p></x-sigsep> Dr. Jeffrey N. Grossman<br> Chair, Meteorite Nomenclature Committee (Meteoritical Society)<br> US Geological Survey <br> 954 National Center<br> Reston, VA 20192, USA<br> Phone: (703) 648-6184 fax: (703) 648-6383<br><br> </html> --=====================_435640640==_.ALT-- Received on Tue 09 Mar 2004 11:52:06 AM PST |
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