[Rockhounds] when volcanoes drop bombs: giant crystal payload

[Tim Fisher]

It's sunstone, it is from a well know collecting site on a cinder 
cone in WY just over the ID border.

At 05:26 AM 8/1/2007, you wrote:
>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
>

Tim Fisher
Ore-ROCK-On!
Email address at http://OreRockOn.com