[meteorite-list] Am I missing something here?

[Jeff Grossman]

Dear Martin and list,

I can't comment on the CO3 oxymoron, but I am the inventor of the 
type 3.05 classification, so I can comment on that.

As we have studied type 3 chondrites over the last 30 years, 
especially ordinary and CO chondrites, we have been gradually 
refining the 1967 Van Schmus and Wood classification scheme.  In 
1980, we realized that type 3 chondrites alone showed as great a 
range of metamorphic effects as type 4-6 did, so Sears and coworkers 
including me, proposed subdividing type 3 into types 3.0-3.9.  In 
2005, Grossman and Brearley (2005) described a similar wide range of 
metamorphic effects just between types 3.0 and 3.2 and subdivided 
this into 3.00-3.15 by steps of 0.05.  Since then, we have even begun 
to recognize different levels of metamorphic heating between types 
3.00 and 3.05, and so we find Acfer 094 at type 3.00, Semarkona at 
type 3.01, ALHA77307 at type 3.03 (e.g., Bonal et al. 2007; Kimura, 
Grossman and Weisberg, 2008, MAPS in press).

These differences are quite real and important.  In type 3.05 
ordinary chondrites, the olivine in chondrules has begun to decompose 
from its high-temperature state, the matrix chemistry is quite 
different, especially for sulfur, and the metal has greatly changed 
in structure and composition (all compared to the much, much rarer 
type 3.00-3.01 chondrites).  Although these numbers do not tell you 
the peak metamorphic temperature, they are very useful in describing 
the various transitions that occur during metamorphic heating.

The fact that there may be many possible combinations of chemical 
group and petrologic type is a good thing, although there certainly 
aren't 1500.  Basically, current usage is 3.00-3.04 (5 categories), 
3.05-3.15 (3 categories), 3.2-3.9 (8 catagories), 4-7 (4 categories) 
= 20 categories.  I suppose if you double this for classifiers who 
can't make up their minds, you get ~40 categories, times 3 chondrite 
groups plus 2 transitional groups = ~200 total combinations.

Jeff

At 11:24 PM 6/15/2008, Dark Matter wrote:
>Hi All,
>
>Twice in the past few days, I found myself staring at the screen in
>confused disbelief. The two statements in sales ads:
>
>
>"a yet unclassified CO3"
>
>and
>
>"absolutely rare type L3.05 !"
>
>just seem to me to border on absurdity. How can a unclassified
>specimen be identified by its classification?
>
>And if we carry petrological grade to the hundredths, then
>theoretically we could have over 1500 ordinary chondrite designations
>not to mention all the transitional possibilities. I fail to see how
>that level of hypothetical opinionated hair splitting could do any
>good...except for ebay sales ads that is.
>
>Just an evening though when I should be working on something else.
>
>Cheers,
>
>Martin
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Dr. Jeffrey N. Grossman       phone: (703) 648-6184
US Geological Survey          fax:   (703) 648-6383
954 National Center
Reston, VA 20192, USA