SCULPTURE AND MORPHOLOGY

 

PRIMARY SHAPE FORMATION

Firstly the primary shape formation is studied. These primary shapes formed rapidly after impact whilst the glass was still molten and as it was being ejected from the lower atmosphere.

RE-ENTRY MODIFICATION

As the tektite re-entered the denser layers of the Earth's atmosphere frictional heating occured which modified the primary shape. In Indochinites, possibly the molten tektite was deformed. Indochinites do clearly have a posterior and anterior surface and this was likely produced by re-entry (otherwise formed at the point of impact). Philippinites suffered thermal cracks to the anterior, sometimes meandering on the surface. On larger specimens spalling occurred to the thermal stress shell. Australites suffered ablation leading to flange formation and thermal cracks leading to spalling and core production.

When studying re-entry modification, the most important thing to remember is that tektites are glass. Glass behaves very differently to rocky and/or metallic meteorites. During re-enty the glass varied from solid, with a slightly molten interior to 100% solid. Glass has specific properties, such as being brittle and having particular thermal expansion coefficients (related to the precise chemistry). These glass properties determine what happens to the glass tektite during re-entry heating. 

IMPACT MODIFICATION

When Indochinites struck the ground some suffered breakage and shape modification, due to the fact that they were still molten or molten on the interior with a thin brittle solidified outer surface. Philippinites possibly show some impact-related features. Australites show none (other than probable breakage along pre-defined lines of weakness).

CHEMICAL ETCHING

I used to believe that most sculpture seen in tektites was primary as it is not random. This assumption was incorrect: most sculpture is secondary, but follows pre-defined lines of weakness created at the time of formation of the tektite. This explains why tektite sculpture varies geographically. Tektites from different areas of a strewn field display different sculpture and the sculpture between the anterior and posterior surfaces is often markedly different. Some sculpture, such as the V-grooving and Anda sculpture, seen in isolated pockets in Australites, Philippinites, Indochinites, Bediasites and most notably Moldavites is certainly due to chemical etching. Anda sculpture preferentially develops on original (posterior) surfaces. Anda sculpture follows lines of weakness in the form of residual stresses and strains produced as the tektite cooled rapidly from the outside-in.

U-grooves (or gutters) and navels are found in Phillipinites, Billitonites (Malaysia) and on some Australite cores. They primarilly occur on the anterior surface. The arrangement is not random and follows thermal cracks produced during the re-entry stages. 

ABOVE: Philippinites with Anda sculpture on top two rows. Moldavites on the bottom row.

ABOVE TOP: Anterior; ABOVE BOTTOM: Posterior of Philippinite cores. Each row is from a different locality, showing different states of preservation. The ones at the top are the least etched. The tektites at the bottom are the most etched.

  

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