[meteorite-list] Let's talk about meteorites

[Arlene Schlazer]

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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