[meteorite-list]: Lunar origin of tektites

From: Marc Fries <m.fries_at_meteoritecentral.com>
Date: Sun Mar 27 11:23:34 2005
Message-ID: <1157.69.140.192.34.1111940606.squirrel_at_webmail.ciw.edu>

Howdy

   For the glass cooling bit, this scientist also happens to make
glassware. I needed a hobby. :) The catastrophic breakdown you're
talking about here isn't from rapid crystallization, but rather from
uneven cooling. The
finishing steps of making glassware subject the glass to rather fast
cooling rates (for a glass), and the change in volume that comes with the
change in temperature is enough to shatter your work. We get
around that by placing the work in an annealing oven and slowly reduce it
to room temperature, allowing the work to slowly change size without
breaking. For lumpy shapes like tektites (or Prince Rupert's drops),

http://glassworker.bravepages.com/2003symp/competitions.html

this effect is much less important because the shape is massive relative
to its surface area and roughly spherically symmetric.

   Here's my thoughts on tektite formation. Silicates were melted and
blasted out of the source crater into a vacuum on a suborbital
trajectory. They remained molten or at least cooled very slowly, since
they only had radiative cooling available to cool off. When they
dropped back through the atmosphere, they simultaneously cooled on contact
with the atmosphere (conductive cooling was re-established), were heated
somewhat by friction, and it was shaped into ...whatever it ended up
looking like. Note the competing thermal effects; which one dominates
depends on the temperature of the tektite at that point, I suppose. That
would explain Australian buttons - they had cooled to the point that
frictional heating dominated upon atmospheric interface, and so they
partially re-melted. This is in keeping with their far distance from the
source crater.

    The pic posted earlier of the tektite with the dimples was impressive;
I'd agree that those might be from volatile exsolution. In general,
however, if you jam a piece of glass in the dirt for tens of thousands of
years to millions, it will turn to powder no matter how little
water there is in it. I think the grooves in the edges of moldavites are
a good example - radial cracks from cooling were more susceptible to
devitrification. Makes 'em pretty.
   Now - the whole issue of volatile exsolution raises some problems. If
tektites formed by condensation from a very hot, dense, short-lived
silicate vapor, then there should be no volatiles left to come out of
the tektites. SO - if there are bubbles, then the tektites probably
formed as a liquid slush splashed out of the crater (or nearby?). BUT
most tektites don't have exolution bubbles, so did most tektites form
by condensation? Could there be two types - condensation and
liquid-splash? It seems to me that such a thing is likely, and
represents tektite formation at varying distance from the source crater
center.

   One thing is clear, not the least of which from earlier posts -
tektites don't come from the moon.

   If I can offer a suggestion about your lenticular tektite, could it
have a gradient of chemical composition from its' center? That may create
the features you described.

   There are plenty of questions left in tektites. Good stuff.

Cheers,
MDF

> Hi,
>
> The major composition is not quartz. Quartz is a mineral, i.e.,
> crystal.
> There are small quartz inclusions, partially melted, in tektites.
Tektites are
> a glass. "Glass" is a rock that has been completely melted (or almost
completely) then cooled so rapidly that no crystals have time to form.
No
> crystals -- it's not a rock any more, Baby Blue.
> There is a big argument about whether "glass" is a solid or a liquid
> of
> super high viscosity. Since there are no crystals in a glass, it is
often
> impossible to determine what rocks were melted to produce the glass.
The
> bulk
> composition (and common sense knowledge) shows that quartz must a major
component, up to 70% silicon, probably in the form of sand -- that's how
humans
> make glass from scratch.
> Glass-making, however, is a craft technology in which scientists are
> never
> involved and about which scientists, frankly, know almost nothing.
Arguments
> about tektites that put forward inconvient facts about the
characteristics
> of
> glass that must be considered in studying tektites are usually ignored.
> O'Keefe, an astronomer, spent years learning about glasses and
> everything he
> learned was ignored. Shouldn't have been, but there you go. For
example,
> cooling a molten mass down to create glass must be done veeeeery slowly,
for
> many, many hours, or crystalization will suddenly commence at some point
inside
> the mass, spread catastrophically, and tons of melt will turn back into
rock,
> mineral, in moments. From the glass-maker's viewpoint, a disaster.
> So, how did tektites get formed in a 50,000 degree plasma, get blown
> into
> space to cool rapidly for 10-25 minutes in a vaccuum, then get reheated
again
> almost to the melting point by re-entry, and soft land, all in 30
minutes
> or
> less, and stay glass?
> Beats the hell out of me.
>

-- 
Marc Fries
Postdoctoral Research Associate
Carnegie Institution of Washington
Geophysical Laboratory
5251 Broad Branch Rd. NW
Washington, DC 20015
PH:  202 478 7970
FAX: 202 478 8901
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Received on Sun 27 Mar 2005 11:23:26 AM PST