[meteorite-list] Acapulcoites and NWA 725

Fri Feb 17 01:00:20 EST 2006

Hello

NWA 1054 was very affected, and its 100% confirmed is
a acapulcoite - many oldest why we have found complete
chondrules into - and is paired to nWA 1052, another
acapulcoite. The problem is, NWA 1052/1054 have the
same matrix of NWA 1058, and this have the same
problems of classification why one say its a primitive
achondrite like winonaite, another say its a
acapulcoite etc...all 3 examples have complete
chondrules and not relitic. In only one we have found
Melliniite, other 2 not....mistery....

Matteo

--- "Kashuba, Ontario, California"
<mary.kashuba at verizon.net> ha scritto: 

> 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: <bernd.pauli at paulinet.de>
> To: <Meteorite-list at 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 at meteoritecentral.com
> >
>
http://six.pairlist.net/mailman/listinfo/meteorite-list
> > 
> 
> 
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