[meteorite-list] New Iron

[Impactika at aol.com]

Forwarded for Robert Matson:
 
 
In a message dated 4/5/2007 3:23:32 P.M. Mountain Standard Time,  
ROBERT.D.MATSON at saic.com writes:
Hi Anne,

Based on  probabilities, one would assume an octahedrite of some type. The
principle  spatial frequency in the upper right corner of the cut surface 
looks
to be  around 2mm, though it isn’t clear to me whether these 2mm features  are
indicative of taenite/kamacite boundaries (which would put it in the  middle 
of
the coarse octahedrite (Og) range), or something else.  For  now I’ll assume
the ~2mm basic frequency is showing lamellae boundaries, so  the question
is which Og type?

Without knowing anything about  the quantity of trace metals, it’s really not
possible to decide among IAB,  IC, IIE, IIIAB, IIIE, IIIF or simply anomalous.
IC is very rare (and this  iron doesn’t look like any IC’s I’ve seen), so I’
ll play
the percentages and  exclude it.  Similarly, IIE is rare, and I don’t see
anything that looks  like silicate droplets, so I’ll drop it from contention.
IIIF is unlikely  because of the high amount of schreibersite inclusions.
So I’m down to IAB,  IIIAB, IIIE or anomalous.

It certainly ~looks~ anomalous, but that  seems like a bit of a cop-out;
braver to choose an actual group.  Going  back to IAB, inclusions of
troilite [e.g. schreibersite] and cohenite (and  graphite) are often abundant
in this group, so that’s a check mark in favor  of this common group.  IIIE’s
are abundant in carbide, which makes them  a bitch to cut.  Their bandwidth
is 1.2 - 1.6 mm, which seems a little  smaller than what I’m seeing in the
image.  Since no mention was made of  cutting difficulty, it probably makes
sense to exclude IIIE.

IAB  and IIIAB are the two most common iron meteorite groups, so it’s
pretty  “safe” to pick one or the other.  I slightly favor IAB over IIIAB  
since
IAB’s on average contain more trace elements, and the cut pattern  looks
closer to IAB to me than IIIAB.

Final answer:  IAB,  Og

Best wishes,
Rob

------------------------------------------------------------------
Very  interesting!!
Anyone keeping track of the bets?
 
Anne M. Black
www.IMPACTIKA.com
IMPACTIKA at aol.com
President,  I.M.C.A. Inc.
www.IMCA.cc
 



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