[meteorite-list] Petrologic Groups

From: Rhett Bourland <rbourlan_at_meteoritecentral.com>
Date: Thu Apr 22 09:41:08 2004
Message-ID: <IOEBKAHMGFBDJMOFGDFNEEKDCEAA.rbourlan_at_evansville.net>

I've done my best to answer your questions. Scroll down to read them. Hope
this helps and that there aren't too many errors.
Rhett Bourland

Can or would anyone on the list take a moment and explain to me the 1-10
subgrouping system on the petrologic group 3's?

Actually, its just 1-6. 3 being unaltered. 2 is some alteration by aqueous
processes. 1 is totally alteration and obliteration of chondrules by
aqueous alteration. 4 through 6 is metamorphism due to heat and/or
pressure. 4 is only slight alteration or chondrules (more so of edges I
believe) up to total metamorphism of chodrules as well as olivine and
pyroxene uniformities as well in higher grades up to 6's. Some researchers
list a type 7 as well where chondrules are totally absent in a matrix only
rock.

Also, could someone please explain how a chondrite achieves a petrologic
classification with a range (ie H3-5 or L3-6)?

Brecciation. In the meteorites parent body in its original structure 6's
were in the center of the asteroid and were covered by progressively lower
metamorphic grades to type 3's on the surface. That's known as an onion
shell structure. It was later impacted and broken up but not enough to send
all the pieces flying. It reacreted but not in its original stucture.
Instead, all the metamorphic grades got mixed together in what's known as a
rubble pile stucture. This is how we have H3-6 and that type of meteorites.
An important distiction here is with meteorites that we see a H4/5
designation. That would indicate that it doesn't fit in nicely into 4 or 5
but somewhere between.

And question number 3... Is it typically, or even consistently possible,
with visual examination alone, to distinguish a Carbonaceous Chondrite from
an L or H class Chondrite? An LL from and L class? Or an Enstatite from a
more common H (or L) class?

I agree with the person who stated a good way to tell the difference between
carbonaceous and ordinary chondrites is its density. While sometimes
posible to determine an L from an H by looking to see how much free metal it
is not always the case which is where more detailed analysis by professional
researchers come in. I can't really answer this one very well though.

Thank you so much for taking time to help me with these questions.
-Robert Beauford : )



_______________________________________________
Meteorite-list mailing list
Meteorite-list_at_meteoritecentral.com
http://www.pairlist.net/mailman/listinfo/meteorite-list
Received on Fri 09 Feb 2001 06:43:56 PM PST