[meteorite-list] Tektite identification criterion

From: N Lehrman <nlehrman_at_meteoritecentral.com>
Date: Thu Apr 22 09:53:35 2004
Message-ID: <023301c2a719$8c908e20$19e3fea9_at_homeportal.2wire.net>

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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. =20

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.=20

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)

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<DIV><FONT size=3D2>List,</FONT></DIV>
<DIV align=3Djustify><FONT size=3D2></FONT>&nbsp;</DIV>
<DIV align=3Djustify><FONT size=3D2><STRONG>New subject!</STRONG>&nbsp; =
At the=20
TektiteSource.com, we've been bombarded with a stream of tektite =
wannabes=20
(Texas, Arizona, Tanzania----).&nbsp; I've been struggling&nbsp;to =
find&nbsp;a=20
reasonably simple tektite test that doesn't require an ion beam =
microprobe, etc.=20
to do.&nbsp; I've thought about the following&nbsp;approach for a while, =
but=20
just never got around to trying it till today.&nbsp; The basic concept =
is that=20
true tektites typically contain absolutely no primary crystallites =
(except for=20
Muong nongs which occasionally do have remnant mineral grains from =
incompletely=20
melted target material).&nbsp; Only&nbsp; crystalline materials can =
assume=20
magnetic properties, so without crystals, no magnetic =
susceptibility.&nbsp;=20
Further, tektite glass is highly reduced (i.e., low volatile Oxygen), so =
even if=20
it were to have microcrystals, they would not be magnetite.&nbsp; The =
black or=20
green color of tektite glass is from elemental iron literally dissolved =
in the=20
glass, in which form it has no magnetic properties.<BR><BR>On the other =
hand,=20
nearly all volcanic glasses contain crystallites or phenocrysts.&nbsp; =
Since=20
iron is abundant in the earth's crust and magnetite&nbsp; crystallizes =
at=20
relatively high temperatures, it is an early-forming mineral---that is, =
if there=20
were any crystals starting to form, &nbsp;magnetite would likely be=20
there.&nbsp;&nbsp;Magnetite is, in fact, common in obsidian, sometimes =
causing=20
the black coloration.</FONT></DIV>
<DIV align=3Djustify><FONT size=3D2></FONT>&nbsp;</DIV>
<DIV align=3Djustify><FONT size=3D2>From this line of thought, a fairly =
simple test=20
is obvious: check for magnetic properties.&nbsp; To nail&nbsp;this down =
in=20
more&nbsp;quantified terms, I used a digital magnetic susceptibility =
meter=20
available to me through work (mag susc. basically relates to the volume =
content=20
of magnetic minerals like magnetite, titanomagnetite, ilmenite, =
pyrrhotite and=20
native iron).&nbsp; In effect, the magnetic properties of the specimen =
provide=20
an indirect way of assessing the presence of crystallites AND the redox =
state of=20
the material, both of which are good solid criteria for tektites vs. =
terrestrial=20
volcanics.&nbsp; </FONT></DIV>
<DIV align=3Djustify><FONT size=3D2></FONT>&nbsp;</DIV>
<DIV align=3Djustify><FONT size=3D2>I'll post details after they're a =
bit more=20
refined, but the basic pattern matches the theory:&nbsp; true tektites =
have=20
extremely low magnetic susceptibilities; obsidians, apache tears, and=20
amerikanites all yield values 2 to 10 times higher, with no field of=20
overlap.&nbsp; Impactites (which very commonly contain Ni-Fe inclusions, =
are=20
commonly 1 to 2 orders of magnitude higher than the =
tektites.</FONT></DIV>
<DIV align=3Djustify><FONT size=3D2></FONT>&nbsp;</DIV>
<DIV align=3Djustify><FONT size=3D2>Most people don't have access to a =
magnetic=20
susceptibility meter, but these differences are sufficiently large to =
detect=20
with a strong magnet.&nbsp; I tried&nbsp;a suspended=20
neodymium/samarium&nbsp;magnet and got no reaction on any of my actual=20
tektites.&nbsp; I did get subtle deflections with the tektite wannabes I =
was=20
checking, and it goes without saying that some of the impactites jumped =
out and=20
clung to the magnet.&nbsp;</FONT></DIV>
<DIV align=3Djustify><FONT size=3D2></FONT>&nbsp;</DIV>
<DIV align=3Djustify><FONT size=3D2>The sweet and simple conclusion to =
all this is=20
that when faced with a suspect tektite, test for subtle magnetic=20
properties.&nbsp; They may not always be detected by the simple magnet=20
test---but if the material deflects a strong magnet, it's not a=20
tektite.</FONT></DIV>
<DIV align=3Djustify><FONT size=3D2></FONT>&nbsp;</DIV>
<DIV align=3Djustify><FONT size=3D2>This may seem like esoteric trivia =
to some of=20
you, but what you've just read is to my knowledge the first suggested =
simple=20
field test to discriminate between tektites, terrestrial volcanics, and=20
impactites.&nbsp; Of course, there are exceptions to most every rule, =
but so far=20
it's looking to me like this will put you on the right side of the =
argument=20
about 99% of the time.</FONT></DIV>
<DIV align=3Djustify><FONT size=3D2></FONT>&nbsp;</DIV>
<DIV align=3Djustify><FONT size=3D2>Merry Winter Solstice and a Happy =
New Orbit to=20
All!</FONT></DIV>
<DIV align=3Djustify><FONT size=3D2></FONT>&nbsp;</DIV>
<DIV align=3Djustify><FONT size=3D2>Norm</FONT></DIV>
<DIV align=3Djustify><FONT =
size=3D2>(TektiteSource.com)</DIV></FONT></BODY></HTML>

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Received on Wed 18 Dec 2002 11:46:08 PM PST


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