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by Paul Harris

 

Schlieren
by James Tobin

I thought for a change that I would offer some photomicrographs of some tektites and some other glasses.

Splashform tektites are made of very high quality natural glass. Some of the estimates in recent years of the temperatures present in their creation are just staggering. Tens of thousands of degrees from which vaporous and melted rock condenses and cools to molten blobs that are water and volatile free. However, there is enough time in a liquid state for them to develop a very complex internal structure that in splashforms is invisible to the naked eye. It Maybe seen on the surface in the way that chemical corrosion exposes it. But internally one can not see it in a hand piece. But, when tektite glass is sliced very thin and polished on both sides the contorted fluid patterns can be seen. These patterns are called schlieren. They can be seen best by throwing the back lighting of a microscope off at an angle to hit the schlieren on the side of the lines.

Schlieren are contact lines where zones of different density or transparency meet. They can be highly complex or can be gently sweeping and wide. The pictures below will show the differences in schlieren between several types of tektites. I have also included a photograph of the schlieren in Libyan Desert Glass.  All the slides were prepared by me.


Moldavite

Moldavite tektite has a very complex schlieren. It is tightly folded and swirled. The glass is highly stressed and thin slices will in almost all cases crack after thin slicing and mounting. It May be that the very intricate spiny form of many moldavite pieces is the result in part of this complex schlieren. There is good evidence that corrosion of tektite glass by chemical action in the soil is selective to differences in composition, which are themselves displayed in the schlieren pattern. In other words the schlieren pattern shows areas of lower and higher silica, and perhaps differences in other elements like metal oxides.

 

Indochinite

The following photo is of a slice from a Philippinite. The photo clearly show a far more relaxed pattern of schlieren. The lines are spaced widely and are short and even in the most complex areas are less contorted than moldavite.

 

Muong Nong

I offer this photo of Muong Nong Layered Tektite to show what is meant by that name. Muong Nong is not a glass product that spent time in flight. The layers are undulating at most and the twisting that May show relates to the flow along the ground that the entire mass made. They are not contortions of the liquid in flight. The corrosion of Muong Nong type glass is characteristically along the layers. This is strongly suggestive of selective corrosion based on slight chemical difference between layers. Clearly in the case of Muong Nong a relationship of external appearance and the internal schlieren and layering exists. The corrosion of Muong Nong is also enhanced by the presence of a much high percentage of bubbles than found in splashforms. This is very evident in the accompanying photo. The bubbles are also nearly always perfectly spherical.



Libyan Desert Glass

Libyan Desert Glass is found on the western edge of Egypt as exposed masses resting on the desert floor in spaces between large dunes. It has been considered to be tektites at times in the past. Today the more popular view is that it is an impact created glass that represents the remains of a melt sheet. As in the case of Muong Nong which has a similar purposed origin, the lines of the schlieren are more or less straight or undulatory. There is none of the contorted structure that was seen above in Moldvite. This glass is very homogeneous compared to Muong Nong. This May be related to an origin from sand or sandstone instead of a more differentiated rock as in the case of the indochinite impact.