I think it is fairly clear that the glass in chondrites, which forms
in chondrules because of their rapid cooling from a partially molten
state, is stable on the time-scale of the age of the solar
system. In the most primitive chondrites, the ones unaffected by
reheating or alteration on asteroids, the glass is preserved in
pristine condition to this day. In metamorphosed chondrites, glass
may survive in protected areas of type 3.9-4 material, but the
reheating caused most of the glass to crystallize into feldspar early
in solar system history. In aqueously altered chondrites, like CMs,
the glass was mostly replaced by phyllosilicates and other phases due
to the chemical action of water on the asteroid. Water is apparently
a key ingredient in devitrifying silicate glasses, especially
important in earth rocks.
The image on Tom's website is almost certainly one of dendrites
(probably olivine) in what was once glass. These dendrites were the
result of rapid crystallization during cooling of a chondrule
melt. Because this is a metamorphosed chondrite, the glass is now
most likely replaced by fine-grained feldspathic material.
Jeff
At 12:24 AM 6/25/2008, [EMAIL PROTECTED] wrote:
Hi, Several years ago I ran onto an unusual chondrule in JaH 055
that looks like glass but it is forming in crystals. I have had
various explanations presented to me and all involved "Glass" This
might be "On topic"? If any one is up to taking a look and sharing
their observations, I would greatly appreciate it. Just go to my
Meteorite Times Micrograph
Gallery
http://www.meteorite.com/meteorite-gallery/meteorites-alpha_frame.htm
and select alphabetical sorting, JaH 055, and then
crystal structure. These shots were produced using incident
(reflected light). Thanks, Tom Phillips In a message dated
6/24/2008 10:02:55 P.M. Mountain Daylight Time, [EMAIL PROTECTED]
writes: Have any studies been done on "decay" of glasses in
meteorites into crystaline configurations? Is there a mesurable
rate, or does it not happen? This story brought that to mind-- if
impact-generated glasses in meteorites HAVE NOT "decayed" into
crystaline material in 4 billion years, it's fairly good evidence
that it won't happen "in billions of years", as the story speculates.
Dr. Jeffrey N. Grossman phone: (703) 648-6184
US Geological Survey fax: (703) 648-6383
954 National Center
Reston, VA 20192, USA
______________________________________________
http://www.meteoritecentral.com
Meteorite-list mailing list
[email protected]
http://six.pairlist.net/mailman/listinfo/meteorite-list
