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" <prairiecac...@rtcol.com>
To: <meteorite-list@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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