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Re: More Tektite questions.
Dear list, dear Bernd,
in addition to your recently made statement on tektite-colours it can
pointed out:
-a) a tektite-colour scale has been introduced by Bouska and Povondra
1964 which is distinguishing 6 mayor colours in the visible region of
the spectrum:
pale green; light green; bottle green; olive green; poisonous green and
brown
(in German and Czech poisonous is related to green [Giftgrün]; in
English I believe it is related to yellow).
-b) Stair 1955 pointed out that the colour of tektites can be the result
of combined adsorption by Fe- Ni- Cr- and Mn-oxides.
-c) Thorpe and Senftle 1964 stated that the colour of tektites is mostly
given by the Fe(II) content and that the brown colour is the result of
he presence of Fe(III) and dispersed colloidal particles of metallic
iron. [Fe(0)].
-d) On a study based on moldavites Bouska et al. 1982 pointed out that
the colour is affected both, by the total iron content and by the
Fe(II)/Fe(III) -ratio. The Fe(III) content is relatively higher in brown
moldavites.
-e) the colour of „poisoned green“ may be related to Cr2O3 (Glass 1984)
or to Ni-oxides (Bouska et al. 1982).
Your observation on the correlation between the silica contend and the
gradual change of colour is good explainable with the in c) and d)
sketched statements.
Detailed information can be read in Bouska et al 1993.
Best wishes
Wolfgang
Bouska, V.L et al. 1993: Natural glasses. Academia Prag, 354p. (Also
Ellis-Horwood Chicester but factor 10 higher priced). Detailed (but not
a complete) references in this volume.
Bernd Pauli wrote:
>
> 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
References: