Re-2: [meteorite-list] CH or CB Chondrite?

From: Jeff Grossman <jgrossman_at_meteoritecentral.com>
Date: Thu Apr 22 10:29:56 2004
Message-ID: <5.1.0.14.2.20030916205328.01b889e0_at_gsvaresm02.er.usgs.gov>

At 11:43 PM 9/16/2003 +0000, j.divelbiss_at_att.net wrote:
>Jeff,
>
>Thank you for the explanation and clarification that clans do not necessarily
>tie two groups together...though they are thought to be related due to
>similarities. So it goes something like this... (with a few questions at the
>bottom).
>
>Class: Carbonaceous
>
> Groups: CB3a, CB3b, CH, CR2 (CR clan)
> CV3, CK3-5 (CV-CK clan)
> CM2, CO3 (CM-CO clan)
> CI

I'm not sure what the consensus on C chondrite clans would be, or even if
there is one. This is a reasonable starting point, although the numbers
(pet types) don't belong here (see below), and there should only be one CB
group (maybe).

>Note 2: I've seen CH called CH2...is that not the case as of yet???

petrologic type 2 means that the meteorite is heavily aqueously altered,
with abundant hydrated minerals. CH chondrites are not, although they
contain hydrated clasts of matrix-like material. They have to be called
type 3.

>Note 3: I've seen Classes called Groups in one textbook.

I don't doubt it. What I have been saying is what I consider to be common
usage, but people use all kinds of terms for these things, sometimes out of
carelessness, and sometimes out of genuine differences of opinion.

>Note 4: Are subgroups appropriately called a class or should they be a type
>(ie: Class OC, Group H, class or type H3.8 ???) Some texts call H3.8 a
>class...which makes it even more confusing! Should it be a type?
There are only a few "subgroups" in the literature. The "CB" chondrite
group has the subgroups you mention, although not everybody accepts
these. There are two well-accepted subgroups of CV (oxidized and reduced),
and the former is divided by some people into two subsubgroups. One
researcher has proposed dividing the E chondrites into some subgroups
too. Subgroups are not classes or types, they are just subgroups, i.e.
subdivisions of accepted groups. In general, they are refinements to the
classification scheme, and in many cases may represent different processing
on the parent asteroid from similar starting material.

"H" is a chemical group name. Chemistry is what chondrite groups are all
about. The 3.8 refers to the petrologic type, which for values in the 3-6
range is synonymous with metamorphic grade (1 and 2 mean something totally
unrelated!). This number has nothing to do with meteorite classification;
it just tells you something very important about what happened to this
particular chunk of rock on the parent asteroid. This bit of information
is traditionally considered important enough that it gets reported
alongside the group name. From time to time, researchers have tried to
promote other tidbits of info to go next to the group name, like shock
stage, weathering grade, a bunch of greek letters meaning various things,
all kinds of subscripts and superscripts, and who knows what else. Most of
this has not stuck except for the petrologic type.

Anyway, your example would come out as class OC, group H, petrologic type 3.8.

No yes?

jeff


>Thanks again Jeff,
>
>John D.
>
> > At 10:30 PM 9/15/2003, j.divelbiss_at_att.net wrote:
> > >2. If all these variations on CB's and CH's are actually part of the
> CR clan,
> > >then why don't we start with calling them CR's, and then give them a
> suffix
> > >of some kind to differentiate them CRL(low metal), CRB-1 and CRB-2,
> CRH(high
> > >metal), etc.. Designations of CBa & b, CH, CR, and who knows what
> next...all
> > >under one umbrella(CR clan) does not make sense to this simpleton.
> Kinda like
> > >what we are heading for with olivine diogenites. I know tradition, known
> > >name, etc. Change before it is too late!!!
> >
> > When we say "clan" we mean a number of meteorite groups that are related,
> > not closely enough to be a single group, but by sharing enough properties
> > that it seems likely they formed in a similar way or in a similar time or
> > place (it's all very vague). It's a very loose term meant to convey a
> > relationship. Bencubbin and Renazzo look nothing like each other, but
> > share certain chemical and isotopic properties than lead researchers to
>
> > place them in the same clan. Nobody would or should ever put these in
> > the same group.
> >
> > "Groups," on the other hand, refer to groups of meteorites that are alike
> > in most of their primary properties (chemistry, texture, isotope
> > systematics), and which probably formed together in one parent body. If
> > you show an expert two members of the same group with identical secondary
> > histories (metamorphic, shock, aqueous alteration, and terrestrial
> > weathering), in most cases he/she would have a very hard time telling them
> > apart. (Of course, a 4-year-old could tell Bencubbin apart from QUE
> 94411,
> > which is part of the reason I object to putting them in the same "CB"
> > chondrite group.)
> >
> > Above both of these are "classes," which for chondrites include ordinary,
> > carbonaceous, and enstatite. I would put R chondrites in the ordinary
> > class, but others prefer to call it its own class. K chondrites also may
> > or may not belong in their own class. Classes are related by broad
>
> > chemical properties, especially oxidation state of Fe and ratios between
> > various major elements. Classes may comprise more than one clan and many
> > groups. Classes convey even broader relationships than clans, and
> possibly
> > indicate the general region or heliocentric distance at which the
> > chondrites formed.
> >
> > Your proposal to use terms like CRH, CRL, etc., would essentially elevate
> > the CR clan to "class" status. I don't think this is appropriate. It
> > could also be considered an attempt to formalize "clan" nomenclature, but
> > clans are so loose and poorly defined that this is not practical (AND, it
> > would require renaming many other chondrites, like the clans CV-CK, CM-CO,
> > and H-L-LL).
> >
> > jeff
> >
> > Dr. Jeffrey N. Grossman phone: (703) 648-6184
> > US Geological Survey fax: (703) 648-6383
> > 954 National Center
> > Reston, VA 20192, USA
> >

Dr. Jeffrey N. Grossman phone: (703) 648-6184
US Geological Survey fax: (703) 648-6383
954 National Center
Reston, VA 20192, USA
Received on Tue 16 Sep 2003 09:34:43 PM PDT