Re: [meteorite-list] Tektite identification criterion

[N Lehrman]

Mark and list,   Properties like those you've listed do show helpful variations, but the range between individual tektite types tends to overlap with terrestrial materials to the point that none of this allows one to discriminate between tektites and other materials.   Despite the thousands of papers debating the myriad mysteries of tektite lore, I don't know of any that directly address the fundamental question "how do you tell if something is or is not a tektite?".  I'd love to hear from any of you out there with ideas or suggested references.   I'm going to pull a synthesis of this subject together with time, and there is a reasonable stash of widely scattered data that bear on the subject.  The really big challenge though is coming up with criteria that can be used outside of a major university laboratory setting.  For example, one of the hallmark characteristics of tektite glass is its exceedingly low water content.  However, you'd be hard pressed to find any commercial laboratory that could provide an accurate determination of this property at the levels of resolution we require.  Ditto a good ion microprobe analysis.  This is all great stuff in the academic laboratory settings where most technical publications originate, but what are we supposed to do out here on the front lines?   Of course, there are great folks in academia who will collaborate on worthy issues, but such matters cannot extend to passing judgment on suspect materials that arrive in the mail every other week.  You meteorite freaks know the routine well---and have developed a pretty good bag of tricks to screen the winners from the losers.  With tektites, we've barely emerged from debating the very definition of the word.    Cheers,   Norm (TektiteSource.com) ----- Original Message ----- From: [EMAIL PROTECTED] To: N Lehrman Cc: [EMAIL PROTECTED] Sent: Wednesday, December 18, 2002 8:46 PM Subject: Re: [meteorite-list] Tektite identification criterion Hi Norm and list   Norm, are there any other tests that may prove useful like refractive indice, specific gravity or thermal conductivity?  I mention these because of tektites having mineral contents not normally associated with terrestrial rock or glass, and I don't know if tachylytes would be of concern or not. Mark flexing new old knowledge without a lot of experience ----- Original Message ----- From: N Lehrman To: [EMAIL PROTECTED] 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)