[meteorite-list] Re: Protecting irons again

From: geoking_at_notkin.net <geoking_at_meteoritecentral.com>
Date: Thu Apr 22 10:14:08 2004
Message-ID: <v04220809bad3022dc78b_at_[66.2.44.107]>

Lars posted [in regard to Sikhote-Alins]:

>But I thought that the blueish color is the original fusioncrust?


Dear Lars and List:

Your question is a good one Lars, and something that we've touched on
from time to time, but probably never covered in detail.

You can find -- for sale on the web, and elsewhere -- any number of
Sikhote-Alin individuals which are described as having "fusion
crust," or "gun metal blue fusion crust," etc. I'm sorry to say that
many or possibly even most of these Sikhote-Alins do not actually
retain all (or in some cases, any) of their fusion crust, and that
they have been cleaned at some point between the strewn field and the
sale page. They are authentic meteorites, of course, and I don't want
to step on anyone's toes here, so please don't think I'm inferring
that any dealers are misrepresenting their material. These irons have
been well prepared -- overseas in most cases -- and not everyone
realizes that they have been worked on.

Consider this: The fall occurred in 1947 in a meteorite-hostile
environment (lots of moisture, falling leaves, long winters, etc.). I
believe there is no chance that iron meteorites could have survived
in that type of environment for 56 years with intact fusion crust.
The specimens being recovered today (and there are at least two List
members I know of that have actively hunted at S-A) show moderate
rusting, and usually some pitting, and are more often than not
cleaned prior to sale. I am told that some of the harsher cleaning
methods have included, unfortunately, sandblasting and acid dipping.
Cleaning will likely strip away any remaining fusion crust, along
with the rust. Some cleaned specimens are then treated with an
oxidizing agent which simulates a fusion crust. Sikhote-Alins that
have been heavily cleaned often lose surface details such as flow
lines, but fortunately the majority of Sikhote-Alins are cleaned
gently (with a wire brush, or similar) and look great. I suppose most
collectors would argue that they are more attractive than the
as-found specimens.

Russian meteoriticists visited the Sikhote-Alin strewnfield several
times following the fall and numerous specimens with intact fusion
crust were no doubt recovered during those expeditions. When in
Moscow, I spoke at length with Valentin Tvetskov -- a noted Russian
astronomer who is an expert on the Sikhote-Alin strewn field -- and
he told me that the first recovered pieces were found lying atop
small piles of snow and had a blueish fusion crust. It's reasonable
to assume that some of these specimens have made their way onto the
collectors' market, and I believe I have seen a few of them over the
years. However, with the vast number of Sikhotes being offered for
sale today, it's also reasonable to assume that majority were not
collected during the early Russian expeditions. Irons which were not
picked up within a few years of the fall are likely to have started
rusting, and most will therefore have been cleaned prior to sale.

Among my own collection of Sikhote-Alins, there are only three which
appear to retain most or all of their original fusion crust; two of
these came directly from contacts in Russia, and one from an old
collection here in the States. They have a matte blueish tinge, and
do look somewhat different from cleaned specimens which generally
seem to have a blackish/grey hue.

There is absolutely nothing wrong with cleaned Sikhote-Alin specimens
(remember that any "silvery" Gibeons, Canyon Diablos, or Odessas that
you may see have also been cleaned), and I have many cleaned
specimens in my collection. It does, however, take a practiced eye to
tell which retain their original fusion crust, and in my opinion
there are very such specimens about.

Of course none of this should detract from your enjoyment of
Sikhote-Alin -- one of the best meteorites available.

Regards,

Geoff N.
Received on Mon 28 Apr 2003 01:23:47 PM PDT