[meteorite-list] Glass in meteorites

[Jeff Grossman]

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, STARSANDSCOPES at aol.com 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, cynapse at charter.net 
>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