Bernd,
I don't mind at all. I think I'm with you wishing I could lean over the
'scope and think "Ah, yes, heat and time makes crystals grow and chondrules
disappear. Metal migrates and other stuff does too." And thereby progress
in an orderly way from chondrites through these primitive achondrites.
Unfortunately, it looks like there is more to it than that. In
Discrimination of Acapulcoites and Lodranites from Winonaites Rumble et al.
state:
Combined petrological and oxygen isotopic analyses of five Northwest African
primitive achondrites . . . have clarified the relationships among
acapulcoites, lodranites and winonaites. It would not be possible to
properly classify some specimens in these groups using their mineral
compositions alone . . . .
http://www.lpi.usra.edu/meetings/metsoc2005/pdf/5138.pdf
To say nothing of mere appearances.
I don't know what to think about Marvin's NWA 1054 Winonaite. Matteo points
out that NWA 1054 is an acapulcoite. Further, I don't see the abundant 120°
triple junctions mentioned in the descriptions of NWA 1054. To me it looks
a lot like NWA 725 (Tissemoumine).
The differences I see under the microscope aren't very subtle. Nor are they
orderly. Please see these examples. The fields of view are all the same,
three and a half millimeters from left to right.
http://www.johnkashuba.com/Ach%20Acapulcoites_and_Winonaites.html
Regards,
- John
John Kashuba
Ontario, California
----- Original Message -----
From: <[EMAIL PROTECTED]>
To: <Meteorite-list@meteoritecentral.com>
Sent: Thursday, February 16, 2006 11:02 AM
Subject: [meteorite-list] Acapulcoites and NWA 725
I'm must be missing something. What could one conclude by comparing thin
sections of NWA 725 and a known winonaite? I understand that
distinguishing
among acapulcoites, lodranites and winonaites is not a textural exercise
nor
can they be resolved by just their mineral composition.
Hello John and List,
I thought other list members might also be interested in this thread,
so I hope you don't mind me sending this mail to the List as well.
No, you are not missing anything. Hand samples of acapulcoites, lodranites
and
winonaites look pretty much the same tan color (at least to me). Only the
micro-
scope will reveal their subtle textural differences and only a thin
section in
polarized light will show mineralogical differences or differences in
grain size.
Unortunately I still don't have a thin section of an acapulcoite but the
one pictured
in Marvin Killgore's "Color Atlas of Meteorites in Thin Section" on pages
208-211
(Acapulco) surely looks different than the winonaite on pages 232-235 (NWA
1054).
Something readily noticed is grain-size: The grains in winonaites are
usually smaller
than in acapulcoites. I know, unfortunately their grain sizes overlap as a
comparative
overview on p. 252 of Hutchison (Meteorites: A Petrologic, Chemical, and
Isotopic Syn-
thesis ) shows. To make matters even worse: almost all cited properties
overlap :-(
Maybe the amount of troilite in NWA 725 could help here. According to
Hutchison,
acapulcoites have 3-6 vol% FeS whereas winoaites have 1-19 %.
The mineral composition of NWA 725 may also be useful in determining
whether it
is a winonaite or an acapulcoite. Olivine Fo and orthopyroxene En have
(slightly)
higher values in winonaites, and the same is valid for plagioclase An.
Unfortunately both acapulcoites and winonaites can have relict chondrules,
so this
doesn't help either. But what may help is the fact that winonaites are
more depleted
in 16-O and so plot closer to the terrestrial fractionation line than
acapulcoites.
Well, you are right ... a thin section will not disclose such information
so I can
only repeat what I already mentioned above: grain size and visual
appearance of thin
sections of winonaites and acapulcoites - they just look different to me
and Hutchison
also states:
"The winonaites are texturally similar to acapulcoites, but the winonaites
contain
coarser grains and abundant crosscutting metal-sulfide veins."
So, maybe, the grain size of NWA 725 visible under the microscope does
hold a clue ...
Cheers,
Bernd
______________________________________________
Meteorite-list mailing list
Meteorite-list@meteoritecentral.com
http://six.pairlist.net/mailman/listinfo/meteorite-list
______________________________________________
Meteorite-list mailing list
Meteorite-list@meteoritecentral.com
http://six.pairlist.net/mailman/listinfo/meteorite-list
