At least a couple times a year I receive specimens from someone who thinks they have found tektites at a new location. Often it is somewhere in Arizona or Utah. So this month we are reviewing the differences between tektites and other natural glasses.
Sometimes the finders have some knowledge of locations where non tektites glasses have been found in the past. They will say something like these were not found near Safford or some other location where the apache tears and similar obsidian individuals have been recovered. I guess the first thing to say is there are apache teardrop type stones all over the place. Some are definitely nicer and prettier colored then others. But, they are obsidian types. Most obsidian is gray when seen in thin sliver. It is black in any thicker amount. However, it may have a lavender hue to it and these are quite attractive. When seen thin it will nearly always have some cloudiness. By contrast tektites are essentially mineral free and totally amorphous. Everything in tektite glass was completely melted and cooled too quickly for mineral crystals to form.
Obsidian and tektite share the fact that they are both natural glasses. Obsidian is a igneous rock that can contain phenocrysts of minerals that were present in the magma. Obsidians can also be banded. But splashform tektites will always be very homogeneous in color. As glasses both are supercooled liquids and should not be classed with minerals since they do not have any orderly arrangement of atoms, in other words no crystal form.
The most dramatic test for determining tektite origin or obsidian glass is to drive the elevated levels of water and trapped volatiles from the obsidian with high temperature. Putting tektites and obsidians in an oxyacetylene flame will result in the obsidian frothing up into essentially pumice. While the gas and water free tektite will only melt slightly with the same heat applied to it. There is nothing real scientific about my test methods. However, it does make use of one of the most important aspects of tektite nature, that being their formation at such high temperatures that all water was disassociated into oxygen and hydrogen gas and never incorporated into the glass itself. The few bubbles found in tektites have gas at a rarified pressure further suggesting their formation at extreme altitude. Obsidians in contrast reflect by frothing up in the flame test the driving force of volcanic eruptions. They still contain the evidence of the gas and steam which blew them from under the earth.
So with that brief background allow me to present the results of Heating Test 2.0. I offered some pictures a few years ago. These are all new.
Specimen on the left is a piece of Central California obsidian collected by the author. The specimen second from the left is a slice of a suspect stone submitted already cut by the finder from a location in Arizona. Third from the left is a small slice cut by the author from a complete individual stone submitted by the finder from the same location in Arizona. And finally, on the right is a Thailand teardrop tektite that had a broken point which I was willing to sacrifice.
The left three specimens have all responded to the test as would be expected for obsidians. They were all gray and cloudy stones. The California obsidian was somewhat clearly and had a hue of lavender. It frothed the easiest since it was a broken flake with a razor thin edge. It therefore melted the easiest and fastest. But, it frothed well into the thick interior of the top surface. The two suspected tektites are clearly frothed up as obsidian would. The tektite as expected shows only minimal melting since the glass is remarkably high temperature stuff. As almost always seen there is a slight gunmetal sheen on the tektite which this author attributes to something in the composition in the tektite glass which deposits on the surface during the flame test. This might be wrong but is my first guess for why this iridescence is always seen after heating.
Whether they are called Americanites, Columbianites, lavander glasses, Apache tears, or Utah glass. If they are gray and cloudy they are very likely obsidian. If they foam up in a heat test they are obsidians. Some are very attractive and some are colorful enough to be collectable on the own. But, once again we have seen they are volcanic in origin; full of water and gases.
More often then not I get the same reaction from individuals after telling them their obsidians are not tektites; that I get when I tell people their rocks are not meteorites. They know that they are what they think they are, and they do not want to be confused by the facts or the knowledge they could gain. So I generally do not do any examinations anymore. I send individuals to others who have more time and patience for the work. But, with these obsidians it was a topic we have not discussed in a number of years and I hope the information is useful. And besides there is nothing more fun then firing up the torch and burning some rocks.
Till next month, Jim
