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Tektite / Impactite
Impactika@aol.com schrieb:
> ... moldavites and indochinites are tektites but the other types of
> impact glasses (Darwin, Libyan, bombs) are impactites. Did I get
> it right? Anne
Hello Anne, Lou, and List!
Here's an excerpt from a post that Lou sent to Bill* on Wed, 18 Aug
1999:
* <billcamp@portjeff.net>
Retired NASA astronomer and tektite authority John O'Keefe proposes
that LDG - as well as Aouelloul Glass, Darwin Glass and glass from the
Zhamanshin Crater - are actually remnants of the impacting body which
formed the craters (though curiously no crater exists as a source for
LDG). Thus, what we are looking at may be a very rare glassy meteorite.
This is an intriguing theory, but I caution you that it is strongly
challenged by many researchers.
As for LDG (Libyan Desert Glass), Wasson et al. proposed in 1998:
"Libyan Desert glass (LDG) consisting of samples ranging in size up to
22 kg is found in a region with dimensions of about 50 km E-W and 130 km
north-south in the Western Desert of Egypt. The process that formed
these high (980 mg/g) SiO2 glassy objects is poorly understood. Although
most past researchers have attributed LDG to formation during a
cratering event, there remains serious doubt that impact cratering can
create such clast-free materials. We suggest that an aerial burst,
similar to Tunguska but about 104 x larger, may have been responsible."
Another expert, Virgil Barnes, wrote:
V.E. Barnes (1990) Tektite Research 1936-1990 (Meteoritics 25-3, 1990,
149-159 - Libyan Desert Glass Expedition, p. 154, excerpts):
"On the return trip we visited the Oasis astrobleme in Libya, which is
about 150 km west of the Libyan Desert glass area (Underwood and Fisk,
1980). The Oasis astrobleme is about 11.5 km in diameter... Dr.
Underwood and his colleagues estimate that about 1.4 x 10^9 grams of
Libyan Desert glass is present in an area of about 6500 km^2 (Murali et
al., 1989). In the dark streaks occasionally encountered in LDG, they
found an extraterrestrial component comparable to chondrites. This
finding strongly supports the impact origin for LDG, first mentioned as
a possibility by Spencer and Clayton (1934) in their announcement of the
discovery of LDG, and it substantiates Cohen's (1961) surmise, based
chiefly on his gallium and germanium studies, that LDG "was produced by
impact of a body low in nickel-iron such as a comet or stony meteorite"
on the desert sand."
As for Darwin (impact) glass:
Th. Meisel and Ch.Koeberl (1988) Geochemical Studies of Impact Glass
from the Darwin Crater, Tasmania (abs. Meteoritics, 1988, vol. 23, pp.
289-290):
"Darwin Glass has been known since the beginning of the century and was
later identified as impact glass. The main occurrence of the glass is in
western Tasmania, SSE of Queenstown. Less than 20 years ago, a crater
was found to be associated with the glass and was termed Darwin Crater
(Ford, 1972). The crater has a diameter of about 1 km, but is poorly
discernible on aerial photographs."
Impactites
O.R. Norton (1997) ROCKS FROM SPACE II, Glossary, p. 424:
"[Impactites are] small, roughly spherical glassy masses, melted as a
result of heat generated by an impacting meteorite."
O.R. Norton (1997) ROCKS FROM SPACE II, p. 98:
"Nininger also discovered fused silica bombs, called impactites, on the
west flank of the crater. Impactites were first found around the Henbury
meteorite craters in central Australia in 1932. They are small, black
masses made entirely of glass, containing minute iron fragments. The
iron fragments tested positive for nickel, which is always found in
combination with iron in meteorites. The fused silica bombs (they were
called bombs for their similarity to small volcanic bombs typically
found around volcanic vents) were produced by the melting of silicate
rocks during the impact of a large meteorite. These bombs were ejected
from the crater along with other rock debris that formed the raised rim.
Nininger found iron particles in the silica bombs that also tested
positive for nickel. The search for impactite bombs is now standard
procedure in crater investigations."
Best regards,
Bernd
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