[meteorite-list] Re-2: Types of twinning in chondrites?

[Alan Rubin]

Okay.  As I said and Andy said, we can find polysynthetic twins in low-Ca 
clinopyroxene in type-2 and type-3 chondrites (those rocks that have not 
been metamorphosed).  We can actually find a few such twins surviving in 
type-4 OC.  We can see such twinning in plagioclase (also sometimes called 
albite twinning) in type-6 chondrites (OC, EH, EL, CK).  One final note: 
since the low-Ca clinopyroxene with twins forms from quenched protopyroxene, 
we can also see this in shocked chondrites (even if type 5 or 6), when they 
have been subjected to high temperature excursions and rapid cooling.  The 
low-Ca pyroxene is heated into the protopyroxene range and then rapidly 
cooled to form twinned low-Ca clinopyroxene that very much resembles the 
pyroxene phenocrysts in chondrules in unequilibrated chondrites.
Alan


Alan Rubin
Institute of Geophysics and Planetary Physics
University of California
3845 Slichter Hall
603 Charles Young Dr. E
Los Angeles, CA  90095-1567
phone: 310-825-3202
e-mail: aerubin at ucla.edu
website: http://cosmochemists.igpp.ucla.edu/Rubin.html


----- Original Message ----- 
From: "Bernd V. Pauli" <bernd.pauli at paulinet.de>
To: "Jim Wooddell" <jim.wooddell at suddenlink.net>; "Meteorite Central" 
<meteorite-list at meteoritecentral.com>
Sent: Saturday, February 22, 2014 1:39 PM
Subject: [meteorite-list] Re-2: Types of twinning in chondrites?


> My pleasure, Jim!
>
>> Would it be safe to say if I see polysynthetic
>> twinning, odds are it's low-Ca cpx, ...
>
> No, you might also be looking at multiple twinning of
> plagioclase but in that case you may be pretty sure it
> is *not* an unmetamorphosed chondrite!
>
> Bernd
>
> To: jim.wooddell at suddenlink.net
>    meteorite-list at meteoritecentral.com
>
>
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