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Re: More Tektite questions.

Larry Davison schrieb:

> How they were formed is still very much a mystery to me and many
> others.From the evidence they were formed by impacts of either comet
> or asteroid origin. And flew in space for not more that 900 years.
> Why the different colors of tektites, yellow, gray, black, brown,
> and green? I assume the type of impact object and geology of site
> would influence the final product.

Hello Larry, hello List Members!

Tektite color:

1) In transmitted light, Libyan desert glass is mostly yellowish or a
pale green because of its very high silica (SiO2) content - almost pure
silica => 98%.
2) Moldavites from Austria are usually light bottle green due to a
relatively high silica content => 78%.
3) One Austrian sample (Koeberl’s AB-d sample - AB = Altenburg in
Austria) is of very light pale green color and this agrees with its
higher SiO2 content => 85%
4) The Southern Bohemia and Moravia moldavites show beautiful, clear
green colors with silica averages => 80%
5) Martha’s Vineyard has 80.5% (I’ve never seen it but it should be a
clear green).
6) Bediasite 30775-2 is said to be a dark-brown to light-brown glass -
SiO2 => 68-85%
7) An average value of 25 bediasites => 71.89-81.31
Most bediasites have a lower average silica content than moldavites and
so their colors should not display that clear green of moldavites. And,
in fact, the 6 gram specimen I got from Mike Farmer rather displays a
brownish color when held to the light (this 6 gram specimen and some
other beautiful bediasites can still be admired - and, of course, be
purchased - on Mike’s homepage = > farmerm@concentric.net).
8) DSDP* site off the New Jersey coast => 72-80% (*Deep Sea Drilling
Project)
B.P. GLASS (1989) North American tektite debris and impact ejecta from
DSDP
Site 612 (Meteoritics 24, 1989, 209-218):
The tektite glass is transparent in thin section; most is dark
olive-green in color, but the larger grains appear to be black and
opaque. A small number of the glass fragments are transparent and
yellow-green in color (p. 210).
9) Muong Nong => 67-79%
10) Microtektites from the Ivory Coast strewn field => 63-69%
11) Australian tektites => 66.9-68.5% / 70-73%
‘An exception to the rule’ is R.F. Fudali’s K-1 sample => SiO2 = 77.17
R.F. FUDALI (1991) Australites from Northern Australia (Meteoritics 26,
1991, 153-155).

Thus a decreasing silica content seems to correlate with a gradual
change of color from yellow over green towards brown. If you look at a
relatively thin, broken piece from an apparently ‘black’ Australasian
tektite, it turns out to be a beautiful, brownish translucent color.
Silica content is, of course, not the only factor defining tektite
color. Iron seems to play an important role, too. As far as I remember,
presence of Fe2 or Fe3 is responsible for green or brown/black colors.
Perhaps, some other list member has more information on that. And, as
you correctly assume, the type of impact object and geology of site
should also influence the final product.

Best wishes, Bernd
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