[meteorite-list] Classification tidbits & consequences

[Zelimir Gabelica]

Hello Bernd, List,

Officially St. Mesmin (pronounce "senmemèn") is a LL6 pmb.

The mention "pmb" (polymict breccia) means (as far as I understood) that 
there are other lithologies, clasts...present but that LL6 is predominant 
(not clear though whether this implies that LL6 is the impacted body or the 
impactor).

If one classifies St Mesmin as LL3-7, H6, would mean (to me) that this 
meteorite is composed of (at least ?) 3 lithologies, namely LL3, LL7 and H6 
(see also below), possibly in equal proportion (otherwise a minor component 
is comprised in the term "pmb" that follows the type).

Hutchinson (cited by Bernd) indicates that there are H6 dark fragments 
throughout the meteorite (but not their %).
For example, a classification of the type: "LL3-7,H6", or (to simplify): of 
the type  "LL6, H6"
would be meaningful if both lithologies (LL6 and H6) are in equal 
proportion (here 50%), which is never the case, at least in one small 
portion of a meteorite. And probably this % is even less evident to compute 
throughout the whole meteorite.

For this reason, I beleive the current classification (LL6 pmb) should be 
maintained and the other lithologies should not be explicited further, 
perhaps just mentioned in the side-description that currently follows the 
type.
Here we indeed badly need Jeff's further comments.

We have other such examples, the one quite well known coming to my mind 
being Dimmitt.
This meteorite is composed of at least 4 (very!) different lithologies, 
namely H3, H4, LL5 and CM(!), if I remember well from the original abstract.
The official classification, namely  "H4 rbr", is here sufficiently 
explicit, the term "regolith breccia" implying the other 3 minor types H3, 
LL5 and CM along with the H4 lithology that, I presume, is predominant.

Back to St. Mesmin and Hutchinson, a classification as LL3-7, H6 would not 
mean that "St. Mesmin contains type 3 material (only), in addition to the 
dominant LL6 lithology" (Hutchinson, p 49) because I understand that LL3-7 
means that there are LL3 AND LL7 clasts present (along with H6) but NOT the 
whole series from LL3 to LL7, including LL6.
Actually, according to the official nomenclature, LL6 is not included in 
the LL3-7 mention.
One should then rather write "LL3/7", this meaning that all types from LL3 
to LL7 are present.
Please correct me if I misunderstood something.

In any case, I agree with Bernd that perhaps Jeff Grossman could bring some 
more light on this not so simple problem of nomenclature.

Best to all,

Zelimir


A 14:12 02/01/05 +0000, vous avez écrit :
>Classificational tidbits culled from:
>
>HUTCHISON R. (2004) Meteorites: A Petrologic, Chemical, and
>Isotopic Synthesis (Cambridge Planetary Science Series, pp. 506):
>
>Page 49: " ... St. Mesmin is LL3-7, H6 (Dodd 1974). Occasional
>complexity, we believe, is the acceptable price of accuracy."
>
>also on page 49: " ... St. Mesmin contains contains type 3 material
>in addition to the dominant LL6 lithology. Some slices additionally
>show a shocked dark fragment of H6 lithology that signals mixing
>between parent bodies."
>
>and on pages 51-52: "It is proposed that the maximum amount of
>information should be incorporated into the classificatory shorthand
>for breccias - St. Mesmin for example, would be classed as LL3-7, H6.
>
>Any comments ? Jeff (Grossman) ?
>
>Best wishes,
>
>Bernd
>
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Prof. Zelimir Gabelica
Université de Haute Alsace
ENSCMu, Lab. GSEC,
3, Rue A. Werner,
F-68093 Mulhouse Cedex, France
Tel: +33 (0)3 89 33 68 94
Fax: +33 (0)3 89 33 68 15