----- Original Message -----
Sent: Wednesday, December 18, 2002 8:46
PM
Subject: [meteorite-list] Tektite
identification criterion
List,
New subject! At the
TektiteSource.com, we've been bombarded with a stream of tektite wannabes
(Texas, Arizona, Tanzania----). I've been struggling to find a
reasonably simple tektite test that doesn't require an ion beam microprobe,
etc. to do. I've thought about the following approach for a while,
but just never got around to trying it till today. The basic concept is
that true tektites typically contain absolutely no primary crystallites
(except for Muong nongs which occasionally do have remnant mineral grains from
incompletely melted target material). Only crystalline materials
can assume magnetic properties, so without crystals, no magnetic
susceptibility. Further, tektite glass is highly reduced (i.e., low
volatile Oxygen), so even if it were to have microcrystals, they would not be
magnetite. The black or green color of tektite glass is from elemental
iron literally dissolved in the glass, in which form it has no magnetic
properties.
On the other hand, nearly all volcanic glasses contain
crystallites or phenocrysts. Since iron is abundant in the earth's crust
and magnetite crystallizes at relatively high temperatures, it is an
early-forming mineral---that is, if there were any crystals starting to form,
magnetite would likely be there. Magnetite is, in fact,
common in obsidian, sometimes causing the black coloration.
From this line of thought, a fairly simple
test is obvious: check for magnetic properties. To nail this down
in more quantified terms, I used a digital magnetic susceptibility meter
available to me through work (mag susc. basically relates to the volume
content of magnetic minerals like magnetite, titanomagnetite, ilmenite,
pyrrhotite and native iron). In effect, the magnetic properties of the
specimen provide an indirect way of assessing the presence of crystallites AND
the redox state of the material, both of which are good solid criteria for
tektites vs. terrestrial volcanics.
I'll post details after they're a bit more
refined, but the basic pattern matches the theory: true tektites have
extremely low magnetic susceptibilities; obsidians, apache tears, and
amerikanites all yield values 2 to 10 times higher, with no field of
overlap. Impactites (which very commonly contain Ni-Fe inclusions, are
commonly 1 to 2 orders of magnitude higher than the tektites.
Most people don't have access to a magnetic
susceptibility meter, but these differences are sufficiently large to detect
with a strong magnet. I tried a suspended
neodymium/samarium magnet and got no reaction on any of my actual
tektites. I did get subtle deflections with the tektite wannabes I was
checking, and it goes without saying that some of the impactites jumped out
and clung to the magnet.
The sweet and simple conclusion to all this is
that when faced with a suspect tektite, test for subtle magnetic
properties. They may not always be detected by the simple magnet
test---but if the material deflects a strong magnet, it's not a
tektite.
This may seem like esoteric trivia to some of
you, but what you've just read is to my knowledge the first suggested simple
field test to discriminate between tektites, terrestrial volcanics, and
impactites. Of course, there are exceptions to most every rule, but so
far it's looking to me like this will put you on the right side of the
argument about 99% of the time.
Merry Winter Solstice and a Happy New Orbit to
All!
Norm
(TektiteSource.com)
