Hello Sean, Johnno,

The similarities between certain Bediasites and Philippinites from Mindanao (Davao) is uncanny.  This fact is, in my opinion, the equivilent of a 'Rosetta stone' when it comes to deciding when features such as navels and anvils are formed on the stones.

In Brazos county all Bediasites are found in soils and lag gravel deposits overlying the Jackson formation.  A similar lag gravel occurs in the formations just younger and just older but they contain no tektites.

Almost all Bediasites show moderate to strong evidence of abrasion by transport.  A few rare specimens have strong surface ornamentation and the rest show varying degrees of abrasion that has removed some/most of the ornamentation.  Features such as navels are well inset and resist abrasive obliteration.

Now let us look at three important observations in the above statements:
1- Bediasites have skin ornamentation exactly like Davao specimens.
2- Bediasites exhibit abrasion that obscures this ornamentation to varying degrees.
3- Bediasites are only found in specific gravel lenses on one formation and not in similar gravels in adjacent formations.

Put this all together and what starts to become clear is that early in their formation the Davao surface was present on Bediasites.  They were subsequently transported and left in the Jacksonian sediments, arriving there within the first million years as dates in the formation can attest.  Abrasive injury to the skin oranmentation occurred and was not altered significantly by more than 34 million years of burial.  And modern transport as explanation of the abrasion can be ruled out as the tektites show no inclination to appear on the surface of younger sediments.  They are still sitting where they surfaced on top of Jacksonian material.

So etching can be ruled out unless it occurred in the first million years and then took a 34 million year 'time out'.  That would make no sense to me.  Ablative forces in conjunction with thermal shock are the most likely explanations for navels/anvils.

Adding credence to this observation are the very rare examples of Bediasites with pristine surfaces little altered since they fell.  I have an oval core from Davao with wonderful spalling on the anterior and a beautiful anvil perched near the edge.  The spalls are still shiny and almost completely unetched.  I have been trying (in vain) to secure a Bediasite that could pass for its identical twin only slightly more etched on the spalled surface- it has the same anvil!  The only good anvil I have ever seen on a Bediasite.  Thirty five million years difference in age for these 'twins'.  Once again the only way to resolve the exsistence of these specimens is to favor original formation over subsequent etching as the process behind these features.

All readers please feel free to comment on this theory.

,
Brian

Hi Phil,

Thermal shock and the resultant spalling of the anterior is the first step in anvil formation.  For reasons not clear to me it appears that donut shaped bubbles exist just below the surface- they may be remnants of imploded bubbles trying to shrink their volume as the tektite returned to higher atmospheric pressures at the Earth's surface.  These ring bubbles act as 'moats' and thwart the leading edge of the thermal spall fracture as it advances across the surface of the tektite.  The fracture can separate all the material around the moat but it cannot influence the pedestal standing in the middle of the ring structure.  So now there is a thin 'onion peel' spall that has separated from the body of the tektite everywhere except at the attachment in the center of the ring.  The spall is now reduced by mechanical breakage and if the pedestal is severed only a 'moon mark' remains.  If the pedestal is not broken off a residual bit of skin remains stuck to the protrusion.  Subsequent etching does not produce but rather exaggerates the resultant anvil.

Does this make any sense?

,
Brian

btw the origin of this bubble idea comes from the Meteorite Magazine Article by Edamgaard Jenson http://www.edamgaard.dk/Copy%20of%20VietnamTektites%20edj.htm on Vietnam tektites. I believe that these interpretations are basically wrong. The pictures in this article are great though. For a start it is very apparent that the exterior of the tektite was brittle and solid (with the shape fully formed) before the Indochinites struck the ground. The interior may still have been molten. We know the shape had fully formed and was brittle before the tektite hit the ground because of the bald spots which are spalled areas. These occur only on the anterior surface affected by re-entry, so this surface was solid by the later stages of re-entry. If they are not due to re-entry then why are they not all over the surface - Indochinites with bald spots are poorly formed cores. The indochinite shapes themselves formed early on during the re-entry phase when they were still molten and this commonly created concavo-convex shapes - in exactly the same way raindrops form (look up raindrop shapes on Google). The starburst rays are cracks and splits radiating from the spalled areas - these cracks form when the exterior is brittle and the interior is molten. They may be subsequently etched to produce radial U-grooves. These radial U-grooves are not found on Philippinites (and Bediasites) because these were fully solidified. Navels are, however, found in these and not on Indochinites (except exceedingly rarely).

Bubbles are interesting. Indeed they must expand as the tektites are ejected into high atmospheric levels. Bubbles are mostly lost, but in proximal Indochinites many more bubbles are retained and this I think accounts for many Indochinite 'shell' framents that originally had large internal bubble cavities. I also suspect this may account for many asymmetrical dumbbells (not proven). In the Philippines I think the bubbles were frozen in the solidified mass before re-entry. I could go with limited bubble shrinkage in Indochinites during re-entry as these forms clearly distorted due to atmospheric contact, but I think the tektites were pretty much solid on the exterior before hitting the denser lower atmosphere.