[meteorite-list] Dr. Jeffrey N. Grossman Chondrule paper in Science

[Jeff Grossman]

People who study the origin of chondrules try to select chondrites in 
which the effects of secondary shock and heating are 
minimal.  Semarkona, in our paper, is a good choice because it is 
petrologic type 3.01 and only lightly shocked.  Except for a small 
amount of glass in the outer parts of some chondrules that was 
affected by squeous alteration, the silicate phases are pristine and 
in the same condition as when the chondrule originally formed (not 
necessarily by condensation, however).

Shock and metamorphism can certainly affect the structure of 
chondrules in a variety of ways, although I'm not sure what you're 
seeing that could be described as growth from chemical reactions 
within the chondrule.

Jeff

At 12:34 AM 6/26/2008, STARSANDSCOPES at aol.com wrote:
>Hi list,  I just read Jeff's paper on  chondrule and chondrite formation.
>I'm still trying to get my brain around  some of the points.
>
>Well done!!!  A whole new way of looking at  everything!
>
>I had a question related to the paper but just a little off  topic.  It is
>directed to Jeff but any one with knowledge in this area  should jump in.
>
>The many structures found in chondrules, are they formed  during the
>condensation process at formation or later during subsequent 
>heating  and shock events?
>
>Perhaps both?  I often see structures within  chondrules that give the
>appearance (at least) of having grown out of chemical  reactions within the
>chondrule.  Structures that I can not imagine shock  forces having caused.
>
>Tom Phillips
>
>In a message dated 6/25/2008  7:27:00 A.M. Mountain Daylight Time,
>jbaxter112 at pol.net writes:
>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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>
>
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Dr. Jeffrey N. Grossman       phone: (703) 648-6184
US Geological Survey          fax:   (703) 648-6383
954 National Center
Reston, VA 20192, USA