[meteorite-list] NWA 1109 Howardite or Eucrite?

From: fcressy <fcressy_at_meteoritecentral.com>
Date: Thu Apr 22 10:07:00 2004
Message-ID: <001001c271a7$3f535c20$c96f1c43_at_pavilion>

Hello Adam and all,

Your post from the other day brought to mind some questions I had about
polymict eucrite/howardite classifications. You wrote the following
regarding NWA 1109:


> It looks like NWA 1109 may turn out to be a very
> interesting and rare polymict eucrite with over 20 different basaltic
> textures (sizes) incorporated and a huge array of accessory minerals. I
do
> not believe there is enough orthopyroxene(diogenite component)
incorporated
> to qualify it as a Howardite. The requirement is 10%. David Weirs web
site
> addresses this ratio in an excellent manner.
>

I checked David Weir's website on Howardite classification (found under his
Kapoeta link) and it states:

"The basaltic achondrite group is a complicated one to classify due to the
diversity in the structural and mineralogical relationships among its
members. This group is composed of brecciated and unbrecciated, monomict and
polymict, eucrites, diogenites, and howardites, and has recently undergone a
redefinition.

The monomict subgroup containing eucrites, cumulate eucrites, and diogenites
is further subdivided into brecciated and unbrecciated members. The polymict
subgroup samples a compositional and textural continuum of regolith and
surface breccias consisting of eucrites, cumulate eucrites, diogenites, and
howardites. Those meteorites containing more than 90% of a single component
are given the prefix "polymict" attached to their present description (e.g.
polymict eucrites contain less than 10% non-eucritic material; polymict
diogenites contain more than 90% orthopyroxenite or hypersthene). Those
meteorites that contain less than 90% of any single component are defined as
howardites. While this 10% level is still an arbitrary dividing line based
simply on mineral proportions, it represents an amount of orthopyroxene that
can easily be detected by X-ray diffraction techniques. An additional tool
to distinguish between polymict eucrites and howardites involves pyroxenes
in the basaltic clasts; within howardites they are mostly unzoned whereas in
polymict eucrites they are usually zoned. "

Sorry about the long introduction to my question but I wanted all the
information easily out there. If I understand what is happening with the new
classification, is that it is based on the orthopyroxene (diogenite)
component. The rock is a howardite if it contains greater than 10%
orthopyroxene and a polymict eucrite if it has less than 10%. However, David
also states that a howardite is a meteorite "that contains less than 90% of
a single component.

So hypothetically, if NWA 1109 has an orthopyroxene (diogenite) component of
8% plus another 5% of chondritic material, is it a howardite or a polymict
eucrite? Do we forget about "the huge array of accessary minerals" and other
components except the orthopyroxene when the meteorite is classified because
we are now relying so much on laboratory work? I ask this because I have a
specimen of NWA1109 and it looks to contain at least 5% black clasts
(carbonaceous or shock blackened chondrite?). It obviously once was
"asteroidal soil" and no doubt before X-ray diffraction techniques came
about, it would have been classified as a howardite.

Any comments are appreciated because:

                          Inquiring minds want to know.

Regards,
Frank
Received on Sat 12 Oct 2002 12:24:14 AM PDT