[meteorite-list] Classification of Chondrites

From: Bernd Pauli HD <bernd.pauli_at_meteoritecentral.com>
Date: Thu Apr 22 09:50:24 2004
Message-ID: <3CB080A6.DC5B807F_at_lehrer1.rz.uni-karlsruhe.de>

Fred Olsen wrote:

> What are the criteria for determining the amount of change to
> chondrules and determining their ranking from 2 through 7?

Hello Fred and List,

I think "metamorphism" is the magic word here. H.Y. McSween, Jr.
gives the following definition in his book "Meteorites and Their
Parent Planets":

Recrystallization, in the solid state, of a rock in
response to high temperature and sometimes pressure.

> Is it totally subjective?

No, it is absolutely objective (in most cases :-)

Do these rankings ever get changed?

Yes, reclassifications do occur every now and then.

Davy (a) was classified as an L6 in the 4th ed. of the
Catalogue and had to be reclassified by McCoy et al.
(1997) - it is an L4 now.

Dhofar 008 had originally been classified as a H3 chondrite
but was then reclassified by M. Ivanova as an L3.2/3.3.

Diprivier was originally an L6 but Tredoux et al. said it
should be an L5 because it "did not experience such intense
metamorphism" (Here is the word again!)

Holbrook used to be an L6 chondrite but A.E. Rubin reclassified
it as L/LL6 because of "its intermediate Co concentration in
kamacite". So there are other parameters too. This leads me to
Cole Creek which is a good "not so good" example :-)

> L5 Cole Creek

Slight correction: Cole Creek is an H5 chondrite. So far, so good
(or bad). We discussed this about 18 months ago and when I got my
Cole Creek thin section from Michael Blood, I got almost drowned
in a sea of chondrules. But it does have a fayalite content (another
important parameter) of 18.5 and this makes it clearly an H chondrite
although not necessarily an H5 chondrite :-(

This is not the end of the problem. There are several other H chondrites
with a Fa content of 18.5 but they are H3 (Laundry East) or H3-4 (Tulia
(a)), or H4 (Dhofar 013), or H6 (Dhofar 165), etc. So there are quite
a few further parameters (the Fs value, the Wo content, shock stage,
chondrule boundaries, occurrence of maskelynite or ringwoodite etc.)
which define an "objective" classification.


Best wishes,

Bernd
Received on Sun 07 Apr 2002 01:23:50 PM PDT