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Tektite/Impactite

Ann wrote:

> Could any of you give me a precise, clear-cut definition of
> "Tektite" and "Impactite", and the exact difference between the 2.

Rob's  e x c e l l e n t  response (excerpts):

> # Both impact glasses & tektites are
> products of a high energy impact event ...

Some (debatable) tektite/impactite definitions from selected articles:

(1) Tektites are natural glasses formed from terrestrial material
     that was melted and displaced by the impact of an extraterrestrial
     body.

(2) The bulk composition of tektites suggests that tektites are the
     end products of shock melting or vaporization of terrestrial
     soils, sediments or rocks, during impact cratering.

(3) Tektites are typically centimeter-sized, glassy blobs of
     formerly melted material that sometimes show evidence of
     aerodynamic shaping while molten.

(4) Tektites are to be regarded as droplets of molten rock thrown
     high into the air and outward over long distances from the impact
     site

(5) Tektites are solidified droplets of molten rock splashed into
     the air by the impact of giant meteorites.

(6) Tektites are a special type of glass produced by the hypercelocity
      collision of meteorites or comets with the earth.

(7) Tektites are [glasses] projected to the earth by explosive lunar
      volcanism.

(8) Tektites are fragments ejected from the Earth itself by cometary or
      meteoritic impact.

(9) Tektites are small, pebble-like, naturally-occurring objects made
      of glass that vary in color between pale green and black.

(10) T e k t i t e s   are natural silicate-rich glasses and, at first
glance, resemble obsidian. They are small pieces up to a few cm in size.
The color ranges from green or brown to black.  Tektites are generally
symmetric in shape, homogeneous in chemistry, and have very low water
contents. I m p a c t   g l a s s e s  are often irregular in shape,
less homogeneous, and have higher water contents. Tektites occur on
Earth in four geographically restricted areas, the so-called strewn
fields; distribution in strewn fields is part of the definition of
tektites. Numerous geochemical and isotopic studies have confirmed the
terrestrial impact origin (Taylor 1973; King, 1977; Glass, 1990,
Koeberl, 1986 and 1990). Normal  i m p a c t  g l a s s e s  are known
to occur at impact craters on Earth, and are considered to have
originated during a meteorite impact. Thus, while both  t e k t i t e s
and  i m p a c t  g l a s s e s  have been produced by impact melting of
terrestrial surface rocks, there are differences between tektites and
impact glasses; e.g., impact glasses are found directly at a crater,
while tektites occur in a strewn field away from the source crater. 

Source for No.10 :  MATSUDA J.I. et al. (1993) Origin of
tektites: Constraints from heavy noble gas concentrations
(Meteoritics 28-4, 1993, 586-599).


(11) J.MATSUDA et al. (1996) Noble gas study of a philippinite
with an unusually large bubble (Meteoritics 31-2, 1996, 273-277):

Tektites are natural glasses that are found in four distinct strewn
fields on the Earth (North American, Central European, Ivory Coast, and
Australasian). They are commonly up to a few centimeters in size and
have chemical and isotopic compositions that are very similar to those
of terrestrial upper continental crustal rocks, and they are unlike any
intrusive or volcanic rocks. They also have much lower H2O contents (ca.
0.001-0.05 wt%) than terrestrial volcanic or other natural glasses on
Earth. Research on tektites during the last decades has shown that they
have formed during hypervelocity impacts on Earth and that they show
some similarities to "normal" impact glasses (which occur directly at a
source crater).

> # Water content is ~10 times higher in impact
> glasses than that found in tektites.

Tektites are strongly depleted in water compared to their source rocks
(<100 ppm of water, compared to a few percent for volcanic glasses of
similar compositions).


> # The gas bubble pressure within tektites corresponds to an
> altitude of up to 70km, whereas the gas pressure of impact
> glass is at surface level.

J.Matsuda et al. (1995) A philippinite with an unusually large bubble:
Gas pressure and noble gas composition (abs. Meteoritics 30-5, 1995,
542):

"... noble gases dissolved in tektite glass indicate that the
glass solidified at atmospheric pressures equivalent to about 40 km
altitude."


Best regards,

Bernd

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