[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> This is a multi-part message in MIME format. ------=_NextPart_000_0230_01C2A6D6.7E228980 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable 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) ------=_NextPart_000_0230_01C2A6D6.7E228980 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD> <META http-equiv=3DContent-Type content=3D"text/html; = charset=3Diso-8859-1"> <META content=3D"MSHTML 6.00.2600.0" name=3DGENERATOR> <STYLE></STYLE> </HEAD> <BODY bgColor=3D#ffffff> <DIV><FONT size=3D2>List,</FONT></DIV> <DIV align=3Djustify><FONT size=3D2></FONT> </DIV> <DIV align=3Djustify><FONT size=3D2><STRONG>New subject!</STRONG> = At the=20 TektiteSource.com, we've been bombarded with a stream of tektite = wannabes=20 (Texas, Arizona, Tanzania----). I've been struggling to = find a=20 reasonably simple tektite test that doesn't require an ion beam = microprobe, etc.=20 to do. I've thought about the following approach for a while, = but=20 just never got around to trying it till today. 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). Only crystalline materials can = assume=20 magnetic properties, so without crystals, no magnetic = susceptibility. =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. 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. = Since=20 iron is abundant in the earth's crust and magnetite crystallizes = at=20 relatively high temperatures, it is an early-forming mineral---that is, = if there=20 were any crystals starting to form, magnetite would likely be=20 there. Magnetite is, in fact, common in obsidian, sometimes = causing=20 the black coloration.</FONT></DIV> <DIV align=3Djustify><FONT size=3D2></FONT> </DIV> <DIV align=3Djustify><FONT size=3D2>From this line of thought, a fairly = simple test=20 is obvious: check for magnetic properties. To nail this down = in=20 more 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). 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. </FONT></DIV> <DIV align=3Djustify><FONT size=3D2></FONT> </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: 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. 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> </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. I tried a suspended=20 neodymium/samarium magnet and got no reaction on any of my actual=20 tektites. 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. </FONT></DIV> <DIV align=3Djustify><FONT size=3D2></FONT> </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. 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> </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. 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> </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> </DIV> <DIV align=3Djustify><FONT size=3D2>Norm</FONT></DIV> <DIV align=3Djustify><FONT = size=3D2>(TektiteSource.com)</DIV></FONT></BODY></HTML> ------=_NextPart_000_0230_01C2A6D6.7E228980-- Received on Wed 18 Dec 2002 11:46:08 PM PST |
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