[Rockhounds] when volcanoes drop bombs: giant crystal payload

[Pmodreski at aol.com]

Cool stories, Alex and Don, and that's a great photo of the glassy feldspar  
crystal in the volcanic bomb from Spencer ID.
 
I should try to guess what it is; I'm torn between guessing orthoclase or  
sanidine; I'll go with orthoclase.  Let me know what it turns out to  be.
 
And I hate to sound like the skeptic, but I cannot believe that any  feldspar 
crystal like this (or the one you describe, Alex) actually "grows"  inside 
the cooling volcanic bomb, no matter what Alex's volcanologist  friend thinks he 
might have seen on Vanuatu.  I think Don will agree--that  just ain't 
possible.  Those crystals are "megacrysts", also could be called  "xenocrysts".  They 
grew somewhere else when the magma was still deep down,  10s of kilometers 
deep, and then they were carried up by the magma and encased  by a glob of it 
when it was erupted.  Any crystal growth while embedded in  that rapidly cooling 
lava bomb would have been very very minor, on a microscopic  level along the 
very surface edges of the megacryst.
 
Pete Modreski



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