Is It a Tektite???
"I found a wierd rock that looks sorta like tektite pictures I've seen on eBay. How can I tell if it really is a tektite?"
This is no simple question, especially since we're still debating just what tektites are. However, they do have certain things in common:
1) They are glass. Good vitreous, conchoidal-fracturing glass.
2)they are exceptionally pure glass, free of volatiles and crystallites. They have a very low water content.
2)they commonly contain lechatelierite threads (high temperature amorphous silica).
3)they sometimes contain microscopic Ni-Fe sphereoids.
4)They have a transmitted light color of dark greenish- beer-bottle brown, olive- to emerald-green, golden amber-brown, to straw yellow (if we count Libyan Desert Glass). This may be visible only along the thinnest edges with strong back-lighting in some cases. The transmitted light color is never gray. (Lilac-violet remains an issue, but is probably negative.
I have always found it frustrating that the key recognition criteria are often accessible only to a major academic laboratory setting, so I've been on a campaign to identify simple tests that one can do at home, or that are readily available commercially.
Thanks to numerous respondents on the meteortite list, here is a synthesis of relatively simple tests. Note that Tektites and Impactites are different. these tests do not necessarily apply to impactites, and they also may not discriminate between tektites and various artificial slags.
1) The torch test****: You can indirectly assess the volatile/water content of an unknown by intensely heating a sliver in an oxy-acetylene torch flame. Virtually all natural volcanic glasses will readily bubble and froth. Tektite glass will glow incandescently , blinding white, but will not bubble at all, and generally, they will not melt at all in the torch flame. Here are a couple of photos supplied by James Tobin (thanks!) illustrating these results:
------A melted Wannabe from Arizona-------------- A torched Indochinite
If you try this and see any bubbles at all, it's all over. Negative!!
****NOTE: there has been some confusion about this test. It is a way to discriminate between most natural glasses and tektite glass. I can't vouch for glassy industrial slag which may well have lost most of its volatiles artificially. If you try this on slag and it doesn't froth, I don't find that surprising, and it does not mean the slag is tektite glass.
Jim Tobin wrote about tektite determination (including this particular test) in Meteorite Times, January 2003. Here is a link to that excellent article: http://www.meteoritetimes.com/Back_Links/2003/January/Jims_Fragments.htm
2) The Magnet test: Only crystalline substances show significant magnetic properties, and tektites have essentially no crystals. Further, oxygen is one of the volatiles that partitions out of the liquid glass phase in the extreme conditions of tektite formation; Hence, available iron occurs in a highly reduced state, literally dissolved in the glass. In terrestrial obsidian, iron often occurs as crystalline magnetite, an oxide.
Using fairly sophisticated equipment, I have verified that tektites have magnetic susceptibilities that are often an order of magnitude or more lower than terrestrial glasses. This difference is sufficiently large that it can be detected with a strong magnet.
Hang a good Neodymium-Samarium magnet on a thread, then pass your suspect material close by. If there is any visible deflection of the magnet pendulum, it is not a tektite (By reuben reid). This is conclusive. If nothing happens, the test is inconclusive, but permissive. There is reason to expect that all tektites will be non-magnetic, but not all non-magnetic materials are tektites.
What if Fe-Ni sphereoids are present? I'm not absolutely certain, but suspect that their concentration is virtually always too low to detect magnetically.
3) Transmitted light color Test: A meteorite list respondent passed along word that the late, great tektite enthusiast, Darryl Futrell, used transmitted light color as a screening criterion. While many tektites appear opaque black in reflected light, all will transmit light along thin edges with strong backlighting. Check the color.
Australasian , Ivory Coast, Tibetan, and Bediasite tektites are a molasses-brown with greenish tinges. Moldavites are shades of olive to emerald green, and can be quite transparent. Georgiaites are golden amber with olive green tinges. Libyan Desert Glass is straw yellow.
Key Point: No known tektite is gray. Colombianites with their peculiar pale lilac/violet coloration are still an open case in my mind. I'll update this matter as I learn more.
4) A maybe: Nuke it! A test that was suggested that should work in theory (but didn't in my trials) is microwaving. The theory is that microwaves interact with water, and that "dry" tektites, like fine laboratory glass, should not react at all. "Wet" glasses, like obsidian, should heat up. Cook your unknown (together with a known for reference) for about 30 seconds or so in a microwave oven. Terrestrial volcanics should get hotter than the reference tektite, and may even melt and froth. (Note: Robert Verish advises caution here on the microwave time. My original text said 3 minutes---on his advice, I reduced that to 30 seconds, but still, if you try this monitor it closely). My personal trials were inconclusive at best, and I don't know why. This is a good idea that should work.
5)Spend some money on Petrography Test: If you've made it this far and still think you've got a tektite new to science, it's time to spend a hundred bucks. Find a petrographer (there are lots that are hungry---) and have them prepare a thin section. Petroscopy should find absolutely no phenocrysts or crystallites, and should find some lechatelierite threads. Ask your petrographer to look specifically for these things.
A test that isn't a test: Skin Ornamentation. This is the first thing we all see, and I still hate to admit that it's no good. Some Colombianite (check the photos on our Colombianite/Amerikanite page) and "Arizonaite" ornamentation looks fine. So does a bunch of terrestrially-etched bottle glass and other ancient glass of certain human origin. I recently appraised the Darryl Futrell collection, and he had a great chunk of terrestrially etched bottle glass (an unquestionable bottle bottom) with great incipient ornamentation. Also, Robert Haag has a celebrated yellow 'moldavite' that is "right" in every (visual) aspect except tektitic origin---- it is man-made ancient glass.
This is not to wholly discount skin ornamentation as a helpful recognition feature. Just bear in mind that great Tektite-like ornamentation is permissive but not diagnostic.
One visible clue worth remembering is that tektite glass is totally free of inclusions or phenocrysts. If you can see any tiny crystals or breccia fragments or pebbles in the glass, it is not a tektite. However, you commonly will find ochre-yellow laterite adhering to pits and grooves in real tektites. This is okay. It's what's inside that counts. The only exception to this rule is in the case of Libyan Desert Glass which may contain devitrification spherulites of white cristobalite.
A 'test' that isn't worth the time it takes me to write about it:
"I showed it to a geology /astronomy (your choice) Prof at xxx University (your choice) and he said it's a tektite" .
There isn't one geology/astronomy prof in a hundred that has received more than ten minutes training on tektites. I have two degrees and can't recall ever having heard the word in school. Pick up any basic geology or astronomy text and look up "tektite". Often there will be a sentence or two, maybe even a whole paragraph and a picture. There will not be anything regarding recognition criteria.
A WARNING: eBay is wonderful and can be a great source of specimens, but some sellers are the devil incarnate. Unbelievable, incredible junk gets sold on eBay as if it were real. Sometimes the sellers are well-intentioned and honest, but simply naive and misinformed. Others know that what they are selling is not real. Before you buy a "tektite" from Tanzania or Utah or Arizona or Bolivia or some other place where they have not previously been reported, please do your research . Ask the seller questions. If you ever wonder about something offered for sale, contact us! We'll give you our best opinion.
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