[meteorite-list] Ordinary chondrites - rarest to the most common classes

From: Jeff Grossman <jgrossman_at_meteoritecentral.com>
Date: Wed, 16 Dec 2009 05:33:01 -0500
Message-ID: <4B28B75D.2000406_at_usgs.gov>

I agree with Doug... the rarest and most valuable type of OC from a
scientific perspective is petrologic type 3.00-3.01, from any of the
chemical groups. Only one is known... Semarkona. If we take a more
expansive definition of "ordinary chondrite" than most of my rather
conservative colleagues are normally willing to accept, I would say that
the rarest group of OCs is the R chondrites (only ~100 are known and
many of those are paired). In addition, a number of unique ungrouped
meteorites are OC-like. But again, I don't know of any colleagues who
agree with me that R chondrites are in the OC class. [I would say that
the OC class has two clans, the H-L-LL clan and the R clan].

Jeff

Mexicodoug wrote:
> Hi Melanie and thanks for the enthusiasm you add to the list ...
>
> Here's a high to low sorting of the "ordinary chondrites", for over
> 32,000 meteorites:
>
> 22.0% L6 ("most common")
> 19.9% H5
> 12.9% L5
> 12.3% H4
> 11.5% H6
> 7.8% LL5
> 4.2% LL6
> 3.3% L4
> 2.2% H3
> 2.0% L3
> 0.8% LL4
> 0.8% LL3
> 0.1% L7
> 0.1% LL7
> 0.03% H7 ("least common")
>
> But this "common" and "rare" is a misleading label. That is a harder
> question if you look too closely at the deails and consider
> inhomogeneous and brecciated ordinary chondrites. That can all become
> somewhat unique if you ask the right person. Then there are the motley
> crew of ungrouped ordinary chondrites where it is hard to generalize.
> Some may be a weak classification while others might truly be weird
> ("rare").
>
> Just a few notes: the H7, L7, LL7 types are not widely used in the
> literature and border on impact melts, so I'd take them with a grain
> of salt unless someone goes postal on me in which case they are right
> in whatever they say. The way I listed these, the meteorites are
> counted by the lowest number and won't show up in the higher thermal
> (metamorphosed) levels. In other words, for example, an LL3.8-6 is
> counted with the LL3's.
>
> If you have a special meteorite, it can sometimes be a "rarer" type if
> you start to split hairs, like H3.8 instead of just grouping it within
> the H3's, but there is some degree of arbitrariness to this. The
> tendency is that more virgin Solar system stuff (closer and closer
> 3.00) is more special and like a holy grail ("rare" in a sense) to
> some who study that - since it is more representative of the original
> material before water and heat were added and did their thing. From
> hat we can try to get the proof we need to work out early formation
> processes and theorize on the related dynamics happening. By this
> logic, and considering it is a very studied meteorite, the precious
> meteorite SEMARKONA (LL3.00 or is it 3.01 :-)), a witnessed fall from
> India, is rather unique being the only one with that 3.00
> classification, which makes it super intact since formation and
> especially interesting to experts, and most notably Dr. Jeff Grossman
> who reviewed and updated its classification upon careful study.
>
> By another measure, the "common" ordinary chondrite, L5, Canadian
> witnessed fall, VILNA, is one of those very few special meteorites
> that was imaged during atmospheric entry and a precise orbit was
> determined. It was not too far from Buzzard Coulee, and what makes it
> even more special is that it was classified from a (although witnesses
> heard pieces whizzing around) 94 milligram fragment with fusion crust.
> The only other specimen found was a 48 milligram piece! This becomes a
> wild anecdote of a meteorite tale when one considers that the bolide
> passed directly over the only camera recording the sky for 500 miles
> (over 800 km) and headed for the newly constructed and world's only
> UFO landing site which had been built for the Canadian Centennial
> exposition in St. Paul, Alberta, where it showered sparks
> ("retro-rockets" to some folks). In case you wondered, I believe the
> Japanese classified on Antarctic meteorite with 10 milligrams, if you
> can believe that!
>
> So what actually makes a meteorite rare can turn into a matter of
> semantics and who you ask. Even the scale of 3 to 6 (or 7) is somewhat
> arbitrary and just looks for convenient thermally changed cairns along
> the path toward melting. So if we went the other way, if H, L, and LL
> correspond to only three parent bodies, the frequency of the types
> follows:
>
> H 45.0%
> L 40.6%
> LL 14.3%
>
> Hope this helps a little with that general question!
>
> Kind wishes,
> Doug
>
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>
> -----Original Message-----
> From: Melanie Matthews <miss_meteorite at yahoo.ca>
> To: meteorite-list at meteoritecentral.com
> Sent: Tue, Dec 15, 2009 7:01 am
> Subject: [meteorite-list] Ordinary chondrites - rarest to the most
> common classes
>
>
> G'mornin' listites,,
> What is the least common type of ordinary chondrite, as well as the
> most common?
>
>
>
> Thanks
> -----------
> Melanie
> IMCA: 2975
> eBay: metmel2775
> Known on SkyRock Cafe as SpaceCollector09
>
> Unclassified meteorites are like a box of chocolates... you never know
> what
> you're gonna get!
>
>
>
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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

Received on Wed 16 Dec 2009 05:33:01 AM PST