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

From: j.divelbiss_at_att.net <j.divelbiss_at_meteoritecentral.com>
Date: Thu Apr 22 10:29:56 2004
Message-ID: <20030917014619.C3A6053659_at_pairlist.net>

Jeff,

Thanx for keeping me and I'm sure many others straight on these issues. We as
a group are too loose about meteoritic nomenclature, and I think many(non-
scientists especially) out there who publish work need to pay attention to
these and other common practices in naming. Much of my confusion is self
generated, but I see discrepancies all the time that adds to it.

A little bit smarter now,
 
John





  
> 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
>
>
>
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Received on Tue 16 Sep 2003 09:46:15 PM PDT