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: [email protected]
website: http://cosmochemists.igpp.ucla.edu/Rubin.html
----- Original Message -----
From: "Bernd V. Pauli" <[email protected]>
To: "Jim Wooddell" <[email protected]>; "Meteorite Central"
<[email protected]>
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: [email protected]
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