[meteorite-list] Petrologic Groups - Summary of Answers

From: Robert Beauford <wendirob_at_meteoritecentral.com>
Date: Thu Apr 22 09:41:08 2004
Message-ID: <003501c092fc$a9a439e0$804897cc_at_wendirob>

My most sincere thanks again to everyone who supplied information. Below is
a summary of the answers.
-Robert Beauford : )

> 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?

Answer: The H, L, or LL designator is acheived by looking at the iron
content of
the meteorite. H = high iron, L = low iron, LL = low, low iron content.
The number after denotes the metamorphic history of the rock on a scale of
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.
For the least altered of the petrographic grades, type 3's, a further
distinction is made as follows:
Petrographic grades 3.0 through 3.9 are obtained by measuring changes of
thermoluminescence (TL) of chondrules. TL measures the luminesence of the
sample after it is irradiated.
During mild metamorphism tiny grains of feldspar start to grow in a glassy
chondrule. The higher the metamorphism, the more feldspar is present and
feldspar causes a marked increase in thermoluminence. This allows one to
subdivide the petrographic grade 3 meteorites.

> 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?

Carbonaceous can be distinguished from L, H, or LL in most cases.
The lower density of a carbonaceous chondrite would be a
big tipoff, even if they weren't so visually different. Telling H from L
is hardest.
LL should be easily distinguishable from H, and not that
much harder to tell from an L.

All of the above distinctions can be made visually with the aid of a
petrographic
microscope.









> Thank you so much for taking time to help me with these questions.
> -Robert Beauford : )
>
>
>
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Received on Fri 09 Feb 2001 07:59:03 PM PST