[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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