Tektite Specific Gravity Observations

by Norm Lehrman, C.G.S.

2007

Recently I tried some density determinations on a variety of tektites in my collection, and got some interesting results so I decided to pass the observations along by means of this webpage.


Specific Gravity is a measure of density, the mass of a given volume of a substance. In the case of specific gravity, we are comparing substances to an equivalent volume of water. A specific gravity of 1 means that the material has the same mass as an equivalent volume of water (1 gram per cubic centimeter). A specific gravity of 3 implies three times the weight of an equivalent volume of water (=3 g/cc).

Whenever I suspect a big bubble in a tektite ("this one feels light----"), I measure the specific gravity. It's then easy to calculate the size of the bubble.


Two things largely determine the density of a tektite: 1) the presence of bubbles, and 2) the chemical composition of the glass. The big compositional players are the silica content (the higher the silica, the lower the S.G.), the iron content, and the magnesium content (for both of the latter, the higher the content, the heavier the tektite). Since there are compositional differences between the various tektite varieties, specific gravity measurements provide an easy, non-destructive clue about composition.


Within each tektite variety, there is a considerable range in composition (and consequently, density), so results can suggest affinities but are seldom completely diagnostic. There is substantial overlap between tektite varieties. However, I have found S.G. measurements helpful in some cases, and there are some thought-provoking patterns in the data.


Several years ago, I purchased 4 tektites on Ebay that the seller claimed were Bediasites. They were spectacular pieces, strongly resembling Rizalites in ornamentation. Subsequently, I purchased a huge collection of Bediasites directly from the finder in Texas. None were like the Ebay "Bediasites", so I was increasingly sure that the Ebay items weren't what they had been claimed to be. From my own measurements, I noticed that there is quite a contrast between typical Bediasite and Rizalite densities (Average S.G. = 2.386 and 2.450 respectively). The four supected imposters gave specific gravities of 2.452, 2.441, 2.450, and 2.437, all squarely in the middle of the Rizalite range and well out of the ordinary for Bediasites. Taken together with the visual characteristics, I am now certain that these are NOT Bediasites.


Notice in the following chart that my measurements for Javanites, Australites, Malaysianites, Billitonites, and Borneo tektites all give relatively high specific gravities. This is an interesting grouping, as all of these represent relatively far-traveled members of the Australasian tektites. (My findings are not supported by what I can find in the published literature, also shown on the chart below. I'll try to track down some additional references and will note what I learn on this page). Nonetheless, you can bet that when someone tries to sell me a Billitonite that looks like a Thailandite, I'll be quick to check the S.G. The results won't be absolute, but I'll feel a lot more confident if the numbers are high! It is interesting also that the controversial Tibetanites that I tested fell in with the "far-traveled" grouping. It's a subtle and probably insignificant difference, but surely an interesting coincidence if that's all it is---


In line with the preceding paragraph, note that the far-traveled members of the Chesapeake Bay event (Bediasites & Cuba tektite) are heavier than their brethren in Georgia that stayed closer to home. The Martha's Vineyard tektite looks just like a Georgiaite.


The chart below also presents something I've not seen in any reference: specific measurements for Anda tektites and Anda-style Thai tektites. Those in my collection came out a little different from their non-Anda geographic associates, hinting that Anda-style ornamentation may be associated with compositional factors.


Finally, notice that the Colombianites and Arizonaites nestle comfortably in the range spanned by known tektites. Unfortunately, the natural volcanic glasses have a wide range in compositions (and densities), negating the hope that specific gravity might help us to discriminate real tektites from wannabes.

Determining Specific Gravity: This was the discovery that had Archimedes running down the street nude shouting "Eureka! Eureka!"

What Archimedes had discovered was that the volume of an irregular solid could be measured by immersion in water. Since the immersed object displaces exactly its own volume, and 1 cubic centimeter of water weighs 1 gram, the volume of an object in cubic centimeters will be the difference in gram weight for the solid when it's mass is determined dry, then determined while suspended in water. The measured mass in water will be reduced by a number of grams exactly equivalent to its volume in cubic centimeters due to the bouying effect of the displaced water. Hence, volume in cubic centimeters = weight (in grams) dry less weight (in grams) suspended in water.

Specific Gravity is expressed as grams per cubic centimeter, so if we divide the dry weight (in grams) by the volume (in cubic centimeters), we get the specific gravity.

Specific Gravity (g/cc) = weight dry (in grams) / volume (in cc)

Recalling the volume equation above, we get:

Specific Gravity = weight dry / weight dry - weight in water

Here are some example set-ups:

Bend up a simple wire so that you can get the weight suspended under the center of your scale. Place a board on your counter with a nice big iron meteorite clamping it down to create a projecting platform for your scale. Use thread or very fine brass wire to suspend the specimen. Be sure to zero-out your scale with the wire and harness sling in place. Then get a dry weight on the stone. Follow up with an immersed weight. Don't let the specimen touch the sides of the vessel, and be sure there are no bubbles adhering to the surface of the stone. The specimen must be completely immersed.

You can also arrange a tripod or other overhanging system right on your scale if it can handle the weight. Just zero it out with all the rigging in place, then do your measurements. It's helpful to have some sort of rotating arm to support your water vessel. A thin board on a pile of books (again weighted with a nice big iron!) makes a crude but easily adjusted support that you can swing in and out of position.

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