[meteorite-list] Re: Tektite pitting

From: Sterling K. Webb <kelly_at_meteoritecentral.com>
Date: Sat Mar 26 18:25:35 2005
Message-ID: <4245EEA6.BCAE1B4F_at_bhil.com>

Hi, Bob, Marc, List,

    The question of what has caused the shapes and surface features of tektites
has been a long term argument for most of a century.
    When it comes to surface features, the opinions have been many.
Australites (for an example) come in a whole variety of surface textures. The
grayish randomly rough surfaces are almost certainly the result of erosion,
either aeolian, fluvial, or possibly chemical weathering. The gray color is
because they are essentially ground glass.
    It has been shown that acid environments will attack a tektite and produce
such a surface . But the dark mirror-like smooth and glassy surfaces are
usually regarded as pristine and original. They certainly look it.
    Oddly, it possible to have both etched and unetched surfaces on the same
tektite. I have a big chunker that is lenticular, hexagonal, with glassy
facets around the edges (both sides) and a frothy dark super-pitted chaos in
the centers. Was it spinning wildly as it re-entered? But the point is, the
froamy pitted surface (on both sides) could not be the result of erosion or
devitrification if the glassy surfaces on the same tektite are pristine and
untouched.
    These dark and unscratched surfaces without a smooth mirror finish are the
puzzle; they show pitting, "worm track," and a bewildering array of fine detail
and specialized characteristics. These have most frequently been held to be
aerodynamic re-entry effects.
    Explaining all the observed features as aerodynamic in origin is challenge,
though. For example, there is a sub-set of philippinites that have deep
prominent grooves, not numerous, that wander around the tektite in a connected
fashion. (I think they are all rizalites from bikol province, but that's from
memory.) These grooves are U-shaped in cross section, that cross section being
being very uniform in dimension.
    And, here's the kicker, the walls of the grooves are pristine, glassy
mirror surfaces. This would suggest that the grooves were cut by very hot
plasma in re-entry, but how? They are a maddening aerodynamic puzzle. The
grooves often intersect each other cleanly, usually at near 90 degrees. But if
the damn thing is in re-entry plasma, why does it not surround the object
(compare to meteorites and their fusion crust.) and etch it everywhere or at
least on all sides but one. And how to account for the differing orientations?

    Tektites, BTW, almost never show the aerodynamic features we all know from
meteorites (nose cones, rollover lips, secondary and tertiary surfaces) except
for possible exception of "frothy" backsides, but since tektites are not blocky
like most meteorites, a backside is hard to identify.
    This is damn odd, considering that both are rocks re-entering at the same
hypersonic velocities through the same atmosphere! Possibly it's because many?
most? all? tektites are in the liquid or at least plastic phase for the
duration of the trip and do not resist aerodynamic forces like a nice cold rock
would. Still, I'm bothered...
    We know that at least some tektites are plastic when they land. There's
the taffy-pull-and-break tektites of Ninninger. And just ask Norm Lehrman
about "splatforms"! And while you're at it, ask how come "starburst"
tektites? At any rate, many of the surface features remain a puzzle as far as
exact mechanism is concerned.

Sterling Webb
-------------------------------------------
Bob King wrote:

> I have always been fascinated with tektites and enjoy the topic. Regarding
> the pitted surfaces of indochinites and other tektites, is this a result of
> devitrification (or some other sort of erosive action) or were these pits
> formed at the time of creation from escaping gases and water vapor?
> Thanks for your help with the question.
> Bob
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Received on Sat 26 Mar 2005 06:22:14 PM PST