[meteorite-list] Let's talk about meteorites

From: Arlene Schlazer <piebear_at_meteoritecentral.com>
Date: Thu, 3 Sep 2009 19:08:29 -0700
Message-ID: <31D3723F8ABA4EA5AEE3CB4813F2666C_at_PiePC>

Very interesting--thank you for that explanation--that explains all my
silicated irons....now
I have a question regarding the bandwith of the Widmanstatten pattern on
various irons--what exactly determines if it's a "fine", "medium" or
"course" octahedrite....is it the length of time it took to cool in space?
I have a fairly large collection of irons and I'd like to have a clearer
understanding of what determines the pattern.....thanks....Arlene Schlazer
----- Original Message -----
From: "Phil Whitmer" <prairiecactus at rtcol.com>
To: <meteorite-list at meteoritecentral.com>
Sent: Thursday, September 03, 2009 6:51 PM
Subject: [meteorite-list] Let's talk about meteorites


>
>
> Alan: Well done! Carl: thanks for persistently asking a simple question
> with a complex answer. I've often wondered about this myself. I never
> understood how some Campos (or Odessas) could be silicated and others not.
> I thought it had to do with Earth impact, (duh!) Thanks for saving me
> hours of reading, and I still wouldn't have drawn the proper conclusions.
>
> Phil Whitmer
>
>
>
> Sorry,, but I guess I was not clear. The only group of silicate-bearing
> irons widely agreed to have come from an asteroid core is the IVA group.
> This group has little silicate, mainly small grains of silica, which some
> think may have been vapor deposited in the core. There are no collisions
> involved in forming the IVA irons except the one or ones that shattered
> their parent differentiated asteroid and liberated them. The other
> silicated irons, i.e., the IAB, IIICD and IIE groups, may be from
> chondritic, not-differentiated asteroids, that never experienced global
> melting. These irons may have formed after an impact into the chondritic
> surface of these bodies involving local melting and separation of the
> metallic and silicate liquids because they were immiscible. The metal
> liquid sank to the crater floor, incorporated some rapidly melted silicate
> debris and cooled. This is a controversial model and not universally
> accepted. Mesosiderites are differentiated rocks consisting of roughly
> half
> metal and half silicate. The silicate is basically basalt and
> orthopyroxenite, i.e., eucrite and diogenite material. The metal is
> similar
> to that of the IIIAB iron meteorites (a differentiated iron group) and so
> is
> most likely from the core of a differentiated (i.e., globally melted)
> asteroid. My model from some years ago was that the iron core (plus
> overlying mantle) of the projectile impacted the basaltic/orthopyroxenitic
> surface of another (target) asteroid and formed the mesosiderites. The
> large gabrroic clasts in many of the mesosiderites seem to have formed by
> two or more episodes of impact melting, and grain settling. Their origin
> appears rather different from that of the silicated irons.
> Alan
>
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Received on Thu 03 Sep 2009 10:08:29 PM PDT