Philippinites, grooves, navals, tektite, sculpture, australasian, rizalite
PHILIPPINITE SCULPTURE: GROOVES AND NAVELS
Tektites are made of glass. When glass is heated (during re-entry) it will crack and spall. This is due to the thermodynamic properties of the tektite glass - see the bottom of this page. So the cracks, now etched into U-grooves, only occur on the anterior of specimens.
Navels appear to have formed prior to the straight U-grooves cutting them. It is fairly certain now that the U-grooves and navels have been enhanced by chemical etching along a pre-existing crack. Here I present my current thoughts on navel formation and orientation. My final conclusion is that the U-grooves and navels formed on the anterior of the tektite, linked to thermal stress-related shell loss. I am confident of this answer having explored many other avenues!
ABOVE: A selection of navels, all occurring on the grooved side of the specimen.
Two Philippinites, reportedly from Paracale, Camarines Norte, Bikol demonstrated very fine cracks and navels with shell material still attached to the navel. These specimens (and a few other finely grooved ones) look very similar to Javanites that I have seen figured/photographed. These very finely U-grooved/ cracked specimens, atypical of Camarines Norte, have given us some tremendous clues.
ABOVE: Very fine U-grooves and navels with shell material still attached. Specimen reportedly from Paracale, Camarines Norte, Bikol, Philippines. Interestingly this material looks very similar to Javanites.
ABOVE: A 70g tektite from Davao, Philippines (image from Des Leong of www.tektiteinc.com) and a larger specimen from Paracale, Philippines. These images nicely show the 'crescent' fractures around the navels.
A few key facts about navels:
- They are almost exclusively occurring on specimens under about 90g.
- They typify oriented biscuit-forms.
- They only ever occur on one side (the anterior where shell loss has taken place).
- The U-groove making up the circular navel can be of variable width.
- The straight U-groove/ gutter always cuts the navel when joining up with it, indicating the navel formed first.
- Before the straight U-groove joins to the navel, the navel is always cut on the edge closest to the rim.
- Navels, similar to those seen in Philippinites, occur on the anterior surface of Australite cores.
- Anda sculpture (formed by chemical etching) can overwrite U-groove gutters.
Thermal stresses on tektite glass during re-entry
When glass is heated it expands, when it is cooled it contracts. If parts of the glass body are not heated/cooled in an even manner then thermal stresses will result. Tektites are an interesting example in that the semi-molten interior will be cooling whilst the exterior surface will be rapidly heated during re-entry and probably rapidly cooled once the inherited cosmic velocity of the tektite is lost in the final freefall stages of re-entry. Steady and even cooling should not cause thermal stresses - it is the rapid temperature changes during re-entry that will create cracks in the glass. In Australites it is evident that following spalling, ablation can result in re-growth of a flange. This suggests that thermal stress related shell loss can occur from the early stages of re-entry.
As an example of thermal shock, when a drinking glass is warmed up quickly the thinner parts, like the sides, expand quickly. These are attached to the thicker base of the drinking glass, which is not heated to the same extent because glass is a poor conductor of heat. This difference in expansion may cause cracking. Thermal expansion in solid materials is an extremely strong force. I have found a post made by Bob Erck on the internet. He stated 'If you put a solid piece of glass in boiling water, it will expand about 0.02%. Maybe thousandths of an inch. That is not a lot. But if you tried to prevent it from expanding by putting a large clamp on it, you would need to squeeze it with a force of 2000 pounds per square inch to completely eliminate the expansion. The piece of glass does not expand far, but if you try to prevent it from expanding, it will push VERY hard. Clearly, if part of the tumbler is warm and part is cold, thousands of pounds of force can occur, which will break the glass'.
A good thermal shock explanation can be found at 'Glass on Web'. I have now also added a page on meandering or oscillating thermal cracks. This page includes a link to an excellent page on oscillating cracks - click here. Also I have done some experiments with thermal shock on solid and hollow glass spheres on my Bauble Destruction page!
Maybe the positioning of navels is telling us something. Perhaps on the nose of the tektite, where linear flow exists, little heat is transferred to the tektite body. As one moves towards the middle edge of the tektite a more turbulent air flow may exist, possibly resulting in sharper temperature changes as the hot gases come into contact with the tektite. This division between linear and turbulent flow is well demonstrated in a nose cone seen in Plate II of the Origin of Tektites by John O'Keefe (in Tektites, Ed. John O'Keefe, 1963). Australite buttons can also exhibit similar patterns.
Above: From Plate II of the Origin of Tektites by John O'Keefe (in Tektites, Ed John O'Keefe, 1963). a) Nose cone, view from above. Note central zone with ring-waves and outer zone with turbulent flow. b) Nose cone, seen from the side. Note grooves paralleling the direction of the air stream. (Courtesy of George C. Marshall Space Flight Center, Huntsville, Ala.)
In australites you see navel-like features on the leading edge of the flaked equatorial zone. Some nice images (although treat some of the conclusions with caution) are found in: Baker, G. 1940a. Some australite structures and their origin. Mineralogical Magazine. 25: 487-494. Full article available free at http://www.minersoc.org/pages/Archive-MM/Volume_25/25-168-487.pdf. A fine image of a indicator core, well worth seeing can be seen on page 40 in: Heinen, G. 1998. Tektites, Witnesses of Cosmic Catastrophes. 'Imprimerie' Linden, Luxemburg. Also published in German as ‘Tektite, Zeugen Kosmischer Katastrophen’. Another fine indicator core (although not demonstrating navel-like features as well as the previous specimens) is found on http://www.australites.com/page2.htm.
So, by means of summary, rapid changes of temperature during re-entry resulted in thermal shock. Shell loss was probably aided by the high forces/pressures on the anterior of the tektite, caused by the hypervelocity tektite entering denser atmosphere. Is the navel an etched Hertzian Cone or bulb of percussion? These are formed at the point of applied pressure (or impact). The shell was lost in the same way that Australite cores were formed.
The U-grooves and navels have been enhanced by chemical etching, probably by fresh water in the ground. Over time I have become confident that this is the correct solution. Still, unsolved problems exist. What is the precise mechanism of formation of the navels? Also, both V-grooves/Anda sculpture and U-grooves are probably the result of etching and enhancement of pre-existing features/weaknesses. I am puzzled that Anda Sculpture isn't a bit more common than it is, especially on posterior surfaces of biscuits. I'm sure and answer will come in time!
Please see my next and related page - Navel Protrusions