[meteorite-list] NWA 2871: Rocks From Space Picture of the Day - November 5, 2007

[Kashuba]

Mexico Doug, Bernd and List,

Bernd has been leading a few of us, off list, in a discussion of
Acapulcoites v Lodranites.  Doug's fine list of references, including David
Weir's wonderfully up to date pages, shows the likelihood that a more
comprehensive classification system will evolve.  Grain size is an artifact
of metamorphism and correlates well but tells the story only incompletely
and imperfectly.  

A while back I put up a page with a few pictures of winonaites and
acapulcoites in thin section.  Now these classifications are clearly
distinguished with olivine and oxygen isotope composition.  Still, there are
pictures of one former and two current acapulcoites there where grain sizes
can be compared.  If anyone wants to check out another guy's picture of NWA
2871 for grain sizes I just posted a large version of one of mine.  Just
click on the last picture on that page for a new page or use the second link
below.  Do remember, though, that most grain portions we see in thin section
do not represent the full size of the grain since the plane of the section
will cut through the thickest portion of only a small percentage of the
grains it encounters.

http://johnkashuba.com/Pages/Meteorite%20Pages/Topics/Acapulcoites_and_Winon
aites.html

http://johnkashuba.com/Pages/Meteorite%20Pages/Pictures/NWA2871Acapulcoite.h
tm

Best regards,

- John

John Kashuba
Ontario, California

-----Original Message-----
From: meteorite-list-bounces at meteoritecentral.com
[mailto:meteorite-list-bounces at meteoritecentral.com] On Behalf Of
bernd.pauli at paulinet.de
Sent: Monday, November 05, 2007 1:41 PM
To: meteorite-list at meteoritecentral.com
Subject: [meteorite-list] NWA 2871: Rocks From Space Picture of the Day -
November 5, 2007

Doug kindly wrote:

"Just a click away and an interested person without the materials and 
microscope can vicariously participate with Bernd's clear and colorful 
image of his nice thin section of Lodranite/Acapulcoite material."


Thank you, Doug. Much appreciated! Yes, that's one of my best pics
that I've taken so far in cross-polarized light and still without a gadget
to firmly attach the camera to the microscope, camera only hand-held!

Doug: "Bernd, could you kindly mention which of the grains were measured for
those 
of us who want to confirm our little fun with a tape measure on our computer
screens?"

I could but I won't ;-) Let me explain my rudeness: You have the field of
view
in the upper right corner, so it's easy to figure out the rest - an easy
exercise!

Doug quoting from C. Floss: "The simple bimodal classification of these
meteorites
based primarily on petrographic criteria, which has been used to date,
appears to be 
inadequate to describe this diverse group of samples, ..."

Hence I wrote and also quoted on Friday, Oct 26:
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But: It is not only grain size! ... and that's why this excerpt 
from an article in MAPS may be of interest in this context: 

TERRIBILINI D. et al. (2000) Evidence for common breakup events of the 
acapulcoites-lodranites and chondrites (MAPS 35-5, 2000, pp. 1043-1050). 

"GRA 95209 is considered a lodranite based on petrography (McCoy and 
Carlson, 1998), but Mittlefehldt and Lindstrom (1998) find that the
bulk-rock 
composition is more similar to acapulcoites and quite different from that of

lodranites. Thus, a clear-cut classification is difficult."

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Best wishes,

Bernd

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