[meteorite-list] Iron Breccias - Gibeon - Arispe - Udei Station

From: Bernd Pauli HD <bernd.pauli_at_meteoritecentral.com>
Date: Thu Apr 22 09:53:59 2004
Message-ID: <3C5AD76F.D81FD9DA_at_lehrer1.rz.uni-karlsruhe.de>

Eric wrote:

> Here is another example of an iron breccia - arispe. Check out the paper
> on it especially the 5th paragraph and plate XI figure 1. It is a drawing
> of the three different etch pattern directions and how they intersect.

Excerpt from the fifth paragraph:

There remains to notice a point in the structure or construction
of the iron mass which is of the highest interest, and is, in some
respects, quite unique. The section across the meteorite shows
it to belong to the limited group of b r e c c i a t e d siderites,
and that its individual pieces, or soldered fragments, are by far
the largest which have ever been recorded (by Henry A. Ward
- read before the Academy Nov 10, 1902).


Hello All!

Should we call Arispe an "iron breccia"? Remember the article was
written in 1902! Let's once again have a look at some definitions
of a breccia:

WASSON J.T. (1974) Meteorites Classification and Properties
(Springer-Verlag, Berlin, Heidelberg, New York, Appendix I,
Glossary, p. 242):

A fragmental rock type including components (the larger pieces called
xenoliths or clasts) which were previously part of another rock. In
a monomict breccia all components originated in the same rock; in a
genomict breccia the components originated in distinct but genetically
closely related rocks; in polymict breccias the components originated
in two or more unrelated rocks.


NORTON O.R. (1998) Rocks From Space II, Glossary, p. 422:

A rock made up of cemented angular, broken pieces of rock.


McSWEEN H.Y. (1999) Meteorites and Their Parent Planets
(glossary, p. 288):

A rock composed of broken rock fragments (clasts) cemented together
by finer-grained material; a common product of impact processes.

Well, the jury is still out on this question ...

Rick wrote:

> I've seen Gibeons with three different etch patterns. I was told it was due to
> the mass cooling in three areas and the patterns "growing" together. You can
> see a slice with two patterns on my web page.
>
> My vote for an iron breccia would be Udei Station. It is very granular looking
> with at least three distinct and different types of metal visible as different
> shades of gray and silver.

Buchwald writes about Udei Station:

It appears that the brecciation is primary, as if granulated silicates
and metal were compressed, brecciated and sintered to a massive
composite during a mixing process on the parent body [BUCHWALD
V.F. (1975) Handbook of Iron Meteorites, Volume 3, p. 1251].


Although Buchwald himself uses the word "brecciated" here, he also
states:

A number of irons were previously termed "b r e c c i a t e d" e.g.,
Holland's Store, Barranca Blanca, and N'Goureyma (Krinov 1960a: 488;
Hey 1966; and elsewhere).

=> The term is unfortunate and should be abandoned.<=

The meteorites are not brecciated in the geological sense but are
polycrystalline aggregates of ferrite or austenite grains that have
grown to their present size by solid state diffusion. Holland's Store,
originally a normal hexahedrite, probably acquired its peculiar
structure by recrystallization of a shock-deformed single kamacite
crystal. It is here classified as a hexahedrite, with the qualifying
statement that it has been shocked and partially recrystallized.
Barranca Blanca and N'Goureyma apparently still display their primary
cosmic structure, formed by sintering and some grain growth. They are
classified as anomalous; their grain size and inclusions are noted, and
their possible relationships with other meteorites mentioned.
There are numerous irons which, upon sectioning, can be shown to be
composed of several taenite crystals. As noted above, most of these are
anomalous; however, others are normal members of their classes. Gibeon
(Of), Bodaibo (Of), Arispe (Og), Mertzon (Om) and Toluca (Og) are thus
classified according to their bandwidth, but the prefix polycrystalline
indicates that the samples examined were composed of several taenite
crystals. If this prefix is not given, the rneteorites in question are
believed to be - or at austenitic temperatures have been - single
taenite crystals larger than foot-sized.
The largest documented single crystal is that of Cape York; the
sectioning of the 20-ton Agpalilik fragment demonstrated that the
parent taenite crystal measured at least 2 x 1.5 x 1.5 m. It may
well have been larger if all of the Cape York fragments were uniformly
oriented. On the other hand, the fragmentation may have taken place by
preferential fracturing along inclusion-rich grain boundaries, so that
the present fragments represent the true grain size.
Campo del Cielo, which has been in collections since about 1800, was,
until 1967, only known as monocrystalline material. The cutting in Mainz
of a 2 ton mass revealed, however, a polycrystalline array of 10-30 cm
precursor taenite crystals with silicates in the grain boundaries.
Further sectioning of other masses, particularly from the chemical
group I and from the anomalous groups, will probably reveal other
polycrystalline iron meteorites [BUCHWALD V.F. (1975) Handbook
of Iron Meteorites, Volume 1, pp. 69-70].

In summary, we have a jury, a verdict, but still no answer to Mark's
original question whether there are iron meteorites that are definitely
composed of two d i f f e r e n t types.


Best wishes,

Bernd
Received on Fri 01 Feb 2002 12:59:11 PM PST