[meteorite-list] NWA 1109 Howardite or Eucrite?

From: fcressy <fcressy_at_meteoritecentral.com>
Date: Thu Apr 22 10:07:00 2004
Message-ID: <001201c2724e$4cb5e5c0$966b1c43_at_pavilion>

Hi David, Adam and all,

That's what I think too and the point I was making. It seemed to me in
Adam's past post and also in his ebay write-ups on NWA 1109 that a greater
than 10% orthopyroxene for NWA 1109 was needed for it to be called a
howardite. I wanted to point out that other material was present in the rock
and texturally it certainly looked like a howardite to me.
It seems to me that the new HED classification scheme is relying too much on
laboratory analyses and forgetting the textural features. As an analogy, you
might look at the famous black sand beaches of Hawaii. They're composed
primarily of small rounded fragments of basalt. Using the compositional
classification scheme, they could easily be classified as a "polymict
basalt" instead of a basaltic sand if they didn't have greater than 10%
other sand material ;-) The obvious textural fact that it is composed of
sand grains could be forgotten. In a howardite we have a meteorite that was
part of the regolith of an achondrite asteroid and to me, that is the most
important fact in the classification, not its composition. Maybe instead, a
classification scheme using something like diogenetic howardite, polymict
howardite, etc. might really give more useful information as to what the
rock actually is.
Just my 2 cents.
Regards,
Frank
----- Original Message -----
From: David Weir <dgweir_at_earthlink.net>
To: fcressy <fcressy_at_prodigy.net>
Sent: Saturday, October 12, 2002 5:24 AM
Subject: Re: [meteorite-list] NWA 1109 Howardite or Eucrite?


> Hello Frank and list,
>
> fcressy wrote:
> > The rock is a howardite if it contains greater than 10%
> > orthopyroxene and a polymict eucrite if it has less than 10%.
>
> Frank I don't believe this interpretation of the rule is correct. From
> what I gather, a polymict eucrite contains more than 90% eucrite
> components, but the remaining 10% may consist of anything else,
> including a small, less than 10%, component of orthopyroxene, together
> with a small, perhaps 5%, component of your dark material along with
> accessory minerals.
>
> However, given the specific composition you proposed ...
> "hypothetically, if NWA 1109 has an orthopyroxene (diogenite) component
> of 8% plus another 5% of chondritic material" ... then I would say it
> would be a howardite because it is inconsistent with the definition of a
> polymict eucrite or diogenite, which specifies that it must contain
> greater than 90% of a single component, and that would be impossible
> since 12% is accounted for.
>
> I would also imagine the case could arise where part of a meteorite is a
> howardite and part is a polymict eucrite. I would think that the
> additional method of distinguishing them apart, zoned versus unzoned
> pyroxenes in basaltic clasts, would be helpful in this case.
>
> David
Received on Sat 12 Oct 2002 08:20:02 PM PDT