[meteorite-list] Glass in meteorites

[jbaxter112 at pol.net]

Hi Jeff,

Congratulations on the cover story in Science on formation conditions of
chondrules. Your color photograph of a Semarkona cross section is
fabulous.

Cheers,
Jim Baxter

> 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
>
>
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