[meteorite-list] Fusion crust vs desert varnish?

From: E. L. Jones <jonee_at_meteoritecentral.com>
Date: Thu Apr 22 10:17:43 2004
Message-ID: <3FD75A69.2050407_at_epix.net>

Good Question and a long standing sleeper discussion on the list.

Most of all of the NWA's I have actually held in my hand don't show what
is true desert varnish. Many however show "wind polishing" and that
abrades the original fusion crust away to a degree. The cheaper
NWA's are so weathered that the surface isn't even smooth.

Due to the way desert varnish forms it is hard to compare NWA's with say
Southwestern US Meteorites which do tend to have varnish and caliche as
well. The crust does change appearance with exposure and to tell for
certain if the original crust remains is to look at the edge of a slice
for a .33-.50 mm rim of black directly on the inner surface. This
dosen't help ordinarily when you are trying to look at an auction photo
to consider your bid.

Photo quality varies so much it is really difficult to determine from
most photos. My guess, and only a guess without dragging out my
collection and looking at it, is the more brown they are the less fusion
crust remains. The shinny ones are more "wind polished" in my mind. In
close up, fusion crust isn't all that shinny and and has a something of
course sand paper looking texture. So, I think the more shinny and more
brown the photo the lower percentage of fusion crust remaining--but this
opinion has many exceptions. This is further complicated by sellers
calling anything that isn't a fractured surface as covered in "fusion crust"

Dean has seen more NWA's than any 10 of us here and perhaps he could
shed some light on the question.

Elton

Some Desert Varnish Links
--><http://walrus.wr.usgs.gov/infobank/programs/html/school/moviepage/20.01.20.html><--Movie

<http://fgms.home.att.net/desertv.htm> <--Latest Research

<http://www.desertusa.com/magdec97/varnish/dec_varnish.html>
<http://geology.about.com/library/bl/images/bldesertvarnish.htm>


jcowsar_at_pol.net wrote:

> When I look at some of photos of the hot desert NWA meteorite Ebay
>auctions, I can't tell if the nice looking blackish brown coating on the
>individual meteorites is fusion crust or just desert varnish with
>probably little or no fusion crust remaining. Advice please on helping
>this novice collector determine the difference. Shiny appearance =
>desert varnish?
>
>thanks,
>
>jd cowsar
>
Received on Wed 10 Dec 2003 12:39:53 PM PST