[meteorite-list] Glass in meteorites

Wed Jun 25 00:03:26 EDT 2008

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.

http://www.foxnews.com/story/0,2933,370864,00.html

Scientists have made a breakthrough discovery in the bizarre properties of
glass, which behaves at times like both a solid and a liquid.

The finding could lead to aircraft that look like Wonder Woman's plane. Such
planes could have wings of glass or something called metallic glass, rather than
being totally invisible.

The breakthrough involved solving the decades-old problem of just what glass is.

It has been known that that despite its solid appearance, glass and gels are
actually in a "jammed" state of matter — somewhere between liquid and solid —
that moves very slowly.

Like cars in a traffic jam, atoms in a glass are in something like suspended
animation, unable to reach their destination because the route is blocked by
their neighbors.

So even though glass is a hard substance, it never quite becomes a proper solid,
according to chemists and materials scientists.

Work so far has concentrated on trying to understand the traffic jam, but now
Paddy Royall from the University of Bristol in England, with colleagues in
Canberra, Australia and Tokyo, has shown that glass fails to be a solid due to
the special atomic structures that form in a glass when it cools.

Icosahedron jams

Some materials crystallize as they cool, arranging their atoms into a highly
regular pattern called a lattice, Royall said, but although glass "wants" to be
a crystal, as it cools the atoms become jammed in a nearly random arrangement,
preventing it from forming a regular lattice.

In the 1950s, Sir Charles Frank in the Physics Department at Bristol suggested
that the arrangement of the "jam" should form what is known as an icosahedron,
but at the time he was unable to prove it.

An icosahedron is like a 3-D pentagon, and just as you cannot tile a floor with
pentagons, you cannot fill 3-D space with icosahedrons, Royall explained. That
is, you can't make a lattice out of pentagons.

When it comes to glass, Frank thought, there is a competition between crystal
formation and pentagons that prevents the construction of a crystal.

If you cool a liquid down and it makes a lot of pentagons and the pentagons
survive, the crystal cannot form.

It turns out that Frank was right, Royall said, and his team proved this
experimentally.

You can't watch what happens to atoms as they cool because they are too small,
so Royall and his colleagues used special particles called colloids that mimic
atoms, but are large enough to be visible using state-of-the-art microscopy.

The team cooled some down and watched what happened.

What they found was that the gel these particles formed also "wants" to be a
crystal, but it fails to become one due to the formation of icosahedra-like
structures — exactly as Frank had predicted.

"It is the formation of these structures that underlie jammed materials and
explains why a glass is a glass and not a liquid — or a solid," Royall said.

The findings are detailed in the June 22 issue of the journal Nature Materials.
The research was supported in part by a grant from Britain's Ministry of
Education, Culture, Sports, Science and Technology as well as the Royal Society.

Preventing jetliner disasters

Knowing the structure formed by atoms as a glass cools represents a major
breakthrough in the understanding of meta-stable materials and will allow
further development of new strong yet light materials called metallic glasses,
Royall said, which is already used to make some golf clubs.

This stuff is generally shiny black in color, not transparent, due to having a
lot of free electrons (think of mercury in an old thermometer).

Metals normally crystallize when they cool, but stress builds up along the
boundaries between crystals, which can lead to metal failure.

For example, the world's first jetliner, the British built De Havilland Comet,
fell out of the sky due to metal failure.

When metals are be made to cool with the same internal structure as a glass and
without crystal grain boundaries, they are less likely to fail, Royall said.

Metallic glasses could be suitable for a whole range of products beyond golf
clubs that need to be flexible such as aircraft wings and engine parts, he said.

Glass is not what it seems

Royall is part of a group of scientists who think that if you wait long enough,
perhaps billions of years, all glass will eventually crystallize into a true
solid.

In other words, glass is not in an equilibrium state, he believes, although it
appears that way to us during our limited lifetimes.

"This is not universally accepted," Royall told LiveScience. "Our work will go
some way to making that point more accepted. I think there is a growing weight
of evidence that certainly many glasses 'want' to be a crystal."

Still, glass "looks like a liquid and this is one of the great riddles that we
have gone some way to solving," Royall said. "It has always been thought that
glass has same structure as a liquid, and that's why it looks like it. It does
not have same structure as liquid."