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

From: Chladnis Heirs <news_at_meteoritecentral.com>
Date: Wed, 16 Dec 2009 15:27:36 +0100
Message-ID: <004601ca7e5b$ea29eba0$07b22959_at_name86d88d87e2>

Indeed,

it's for the first time, that I read that R-chondrites are included in the
OC-group. If so, why exactly them and not the K-chondrites, the Carbonaceous
from grade 3-6, the ungrouped and the enstatite chondrites too?
 
>valuable type of OC from a
>scientific perspective is petrologic type 3.00-3.01

Where one has to say, that it's maybe too early to say that,
Because the classification with decimal places, (even with two!), is a
relatively new occurrence - most classifiers seems still to prefer to use a
simple "3" - so that in case, there are still a lot known type-3ers awaiting
to be revisited regarding the degree of their (un)equilibration.

But I agree - "Ordinary" is a somewhat misleading term,
- as the ordinary chondrites have told us most about the origin and
formation of the solar system, the planets and ourselves, more than any iron
or any lunar rock!

Keep that always in mind, if you are tempted, now in the end of the
desert-era and the decreed end of meteorite finding in so many countries,
with all their weird and fancy exotic types, to wrinkle your nose about the
"ugly" ordinary 25$-a-kilo-chunk from NWA-wonderland!
Rare as brilliants they are - and they were our beginnings!

Happy holidays to all!
Martin


-----Urspr?ngliche Nachricht-----
Von: meteorite-list-bounces at meteoritecentral.com
[mailto:meteorite-list-bounces at meteoritecentral.com] Im Auftrag von Jeff
Grossman
Gesendet: Mittwoch, 16. Dezember 2009 11:33
An: Meteorite-list
Betreff: Re: [meteorite-list] Ordinary chondrites - rarest to the most
common classes

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
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Received on Wed 16 Dec 2009 09:27:36 AM PST