[meteorite-list] Question for type collectors

From: Jeff Grossman <jgrossman_at_meteoritecentral.com>
Date: Wed, 15 Apr 2009 06:47:24 -0400
Message-ID: <49E5BB3C.5050209_at_usgs.gov>

"Petrologic type" is really a term that only applies to chondrites. It
was popularized in the classic paper:

VAN SCHMUS W. R. and WOOD J. A. (1967) A chemical-petrologic
classification for the chondritic meteorites. Geochimica et Cosmochimica
Acta 31, 747-765

The term was meant to convey a sense of the degree of equilibration of
chondrites.

In the old days, there were only 6 defined petrologic types (1-6),
making it easy on a collector or museum wishing to establish a reference
collection. But now, the scale is interpreted more continuously, with
nearly 30 subdivisions appearing in the literature in one place or
another (1, 2.0-2.9, 3.01-3.05, 3.10, 3.15, 3.2-2.9, 4, 5, 6), as well
as transitional types like 1/2, 3.6/3.7 or 4/5, and breccia mixtures
like 4-6..

As for nonchondritic meteorites, petrologic type is undefined. For some
of these, there are groups, like the groups of irons you mention, which
are analogous to the groups of chondrites (H, L, LL, R, CV, etc.). Some
of these are themselves subdivided, as is the IAB complex. For other
achondrites, like mesosiderites, there aren't really groups defined, but
they have been subdivided into petrographic classes and metamorphic
grades, with designations like "B1" to show this. HEDs and ureilites
are really messy.

Textural terms, like the iron structural types you mention, or terms
like "polymict," "brecciated," etc., are not really classification terms
(in general). These are mostly descriptive terms. Use these to
subdivide a collection with caution, as they may not be applied
uniformly to all meteorites by all researchers.

Jeff



Galactic Stone & Ironworks wrote:
> Hi folks!
>
> I am not a type collector per-se, but I like to keep track of how many
> different petrologic types I have in my collection.
>
> I have a silly question about type collecting -
>
> Do type collectors consider each type of iron a seperate petrologic
> type? For example, are all octahedrites considered 1 type? Or is it
> different types for "coarsest", "coarse", "medium", "fine", etc?
>
> Right now I have 42 petrologic types - counting ALL irons as only one
> type. Should I go through my collection and correct that count to
> reflect the different types - IIAB, IAB, IVA, etc?
>
> Thanks in advance!
>
> MikeG
>
>
>

-- 
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 15 Apr 2009 06:47:24 AM PDT