[meteorite-list] Stony-Iron Nuggets

From: Frank Prochaska <fprochas_at_meteoritecentral.com>
Date: Thu Apr 22 09:52:24 2004
Message-ID: <NDBBICFKNKHAAEEJLDALEEHCDCAA.fprochas_at_premier1.net>

Hello Mark and list,

Glorieta Mtn is not a "normal" pallasite in that the vast majority of the
samples by weight show no pallasitic texture at all. Most of the mass
appears to be a relatively ordinary iron. Only a fraction of the mass is
pallasitic. If you picture a meteoroid that is mostly iron with a pocket(s)
of pallastitic texture, that then ruptures/explodes in the atmosphere
creating a strewnfield, I think one would expect at least some of the small
pieces collected to be "iron nuggets." There is also great variety in the
metal phase / silicate phase ratios in mesosiderites. Some appear fairly
evenly proportioned while others appear to be mostly silicate with some iron
blebs mixed in or mostly iron with some silicate mixed in. Estherville has
large "nuggets" of iron within the matrix, and some sections look as if
there is a silicate "glue" holding these metal nuggets or blebs together.
When Estherville exploded over the resulting strewnfield, it's reasonable to
assume that these silicate areas were weaker than the iron, and in some
cases iron nuggets were released in the rupture event.

Imilac is different in that the texture of intact specimens (to my
knowledge) do not show large regions or "nuggets" of iron. Imilac is a
thorough mixture of iron and olivine, without large sections of metal alone.
I have several pieces of Imilac, and none show fusion crust, including a 154
gram complete slice. I am aware of the occasional iron with a hole or large
depression, but not like the Imilac skeltons. In examples like Sikhote
Alin, you can see samples that are angular from being torn from the main
mass. Imilac skeltons are more symmetrical, not like they were torn in a
particular direction from the main mass. The size, shape, and distribution
of the pockets also match the general features and distribution of the
olivine in complete Imilac specimens, so are consistent with the idea of
simply weathering out the olivine. They look kinda like Marjalahti samples
in which olivine crystals have been plucked for research standards. I
believe there are also transitional forms in which pieces show pockets with
most but not all of the olivine weathered away. The olivines at the edge of
my complete slice are complete but do tend to be indented a little from the
neighboring metal, where the exposed olivines have weather faster than the
exposed iron, which I think is quite common in pallasites in general. Other
irons from the Atacama region however, such as the North Chile group of
hexahedrites, do not show such a weathering texture. They are relatively
evenly sandblasted, and do not show deep, angular pockets like the Imilac
skeltons found in the same region. If the Imilac skeletons were due to
weathering iron alone, one would expect the same texture to be found at
least some examples of other irons from the area.

It appears that iron nuggets in the Glorieta Mtn and Esterville cases can be
explained most simply by their texture at formation and the resulting
effects during impact with the atmosphere, while Imilac skeletons can by
most simply explained by weathering/sand blasting.

Frank Prochaska

-----Original Message-----
From: meteorite-list-admin_at_meteoritecentral.com
[mailto:meteorite-list-admin_at_meteoritecentral.com]On Behalf Of MARK
Sent: Thursday, August 29, 2002 10:42 AM
To: farmerm_at_concentric.net; meteorite-list@meteoritecentral.com
Subject: Re: [meteorite-list] Stony-Iron Nuggets

Hello Mike (and list),

Yes, the Atacama is the driest place in the world. I think I remember
hearing places where it hadnt rained in over 500 years. Dean Bessey had
some artifacts made of wood that were remarkable well preserved after
thousands of years. A few grave markers he had (or has) were really neat.

I have seen the Imilacs with the holes you are referring to. I have also
seen large Imilacs cut in half that just look yellow and with small holes
throughout and without brillant olivine. Wind seems to be only part of the
weathering process there.

If there was however a rain of nuggets I do not see how a person would tell
the difference, from a weather nugget to a weathered skeleton. Do you know
of any Imilacs with any fusion crust? Please do not take my questioning in
a negative way, I am just being curious. I have always referred to Imilac
skeletons as pieces were the olivine has weathered away, as it says in the
auctions I run.

Thanks again, Mark

>From: "Michael Farmer" <farmerm_at_concentric.net>
>Subject: Re: [meteorite-list] Stony-Iron Nuggets
>Date: Thu, 29 Aug 2002 10:09:53 -0700
>The imilac pieces had olivine, many of them have holes where the olivine
>been weathered out.
>the olivine is softer and the blowing sand of Chile eats the olivine away.
>It is not water weathering, The Atacama is the driest place on earth.
>Mike Farmer
>----- Original Message -----
>From: "MARK BOSTICK" <thebigcollector_at_msn.com>
>To: <meteorite-list_at_meteoritecentral.com>
>Sent: Thursday, August 29, 2002 9:58 AM
>Subject: [meteorite-list] Stony-Iron Nuggets
> >
> > Hello,
> >
> > I recently recieved an e-mail on the Admire Pallasite crystals I have on
> > eBay. Basically asking why the Admire metal likes to rust away leaving
> > mostly just stained olivine crystals. In the e-mail the meteorite
> > collector, referred to the Imilac "skeletons" as an opposite. The metal
> > were the olivine has weathered away. Which brings me to the
> >
> > The Esterville (MES) and the Glorietta Mountain (PAL) stony irons rained
> > lot of metal nuggets. I traded for a few Glorietta Mountain iron nuggets
> > few weeks ago and they appear quite stable (and do look a lot like
> > Taza...with less rust). Anyway it got me thinking that most of the
> > "skeletons" might never had any olivine on them. That they might just
> > the "nuggets" that have weathered. While I am sure that is not always
> > case I was wondering if any one thinks this is possible.
> >
> > Thanks, Mark
> >
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Received on Thu 29 Aug 2002 02:42:08 PM PDT

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