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
-----Ursprüngliche Nachricht-----
Von: meteorite-list-boun...@meteoritecentral.com
[mailto:meteorite-list-boun...@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
-----Original Message-----
From: Melanie Matthews <miss_meteor...@yahoo.ca>
To: meteorite-list@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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