[meteorite-list] Question for type collectors

[Jeff Grossman]

"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