[meteorite-list] Acapulcoites and NWA 725

From: bernd.pauli_at_paulinet.de <bernd.pauli_at_meteoritecentral.com>
Date: Thu Feb 16 14:02:15 2006
Message-ID: <DIIE.000000250000442B_at_paulinet.de>

> 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
Received on Thu 16 Feb 2006 02:02:12 PM PST