Tektite, plastic, ablated, anda, australite, philippinite, rizalite, bikolite, bicolite, indochinite, strewnfield, formation, sculpture, australasian, sale, bristol, impact, asteroid, meteor, meteorite, orientation, navals, grooves, bald, stretch, tektites, aubrey, whymark
SOLID SPALLED TEKTITES
ABOVE: A perfect 245g Philippinite belonging to Desmond Leong at www.tektiteinc.com. This specimen is interpreted to have lost it's shell from the anterior, exposing the nucleus. Now in the Aubrey Whymark Collection.
Tektites from the Philippines are characterised by their U-grooving, making them readily distinguishable from most other tektites. The most readily recognisable, although not the commonest of morphologies, are the large breadcrusts and the smaller oriented biscuits or lenses, which have a smooth side and a grooved side. The U-grooving has been attributed to various processes from chemical weathering, to forming on the back surface when the tektite hit the ground (Beyer) to occurring in the ablation/spalling process. The accepted theory for the origin of U-grooves, however, is that they are chemically etched and enlarged thermal cracks, formed primarily on the anterior side (which was exposed to re-entry heat) and that the smooth posterior is the original surface. This idea is favoured as Australites show grooving on their anterior side and have a smoother posterior. Australites have been extremely thoroughly studied and are well understood. Clear indicator forms remove any uncertainty as to Australite formation.
Please click here for my observations of surface sculpture in Philippinites. The smooth side of Philippinites is the posterior and the grooved side is the anterior in flight.
The suggested sequence of events leading to the characteristic shapes and sculpture of Philippinites is interpreted as:
1. The molten tektite ‘blob’ is ejected from the source crater. The rotation, temperature and composition determine its primary shape (sphere, teardrop or dumbbell). Spheres are most common.
2. The tektite rapidly cools and is probably fully solidified and brittle by the time it re-enters the atmosphere over the Philippine islands. Re-entry probably took place some five minutes or so after impact.
3. During the initial phases of re-entry the tektite may have a fixed orientation or may be rapidly rotating. As it hits the denser parts of the atmosphere it will usually establish a fixed orientation – blunt side first. Of course a perfect sphere cannot easily establish a fixed position all orientations will be as stable as one another. Only when a break occurs and thus an imperfect sphere is formed can a stable orientation be found.
4. On oriented tektites the loss of the stress shell, due to thermal stresses, occurs during atmospheric passage. When glass is heated or cooled rapidly it will expand/contract at a different rate to the interior. Spalling begins when the exterior surface, cooled in the five or so minutes between impact and arrival in the Philippines, is heated on re-entry, although notably most heat is carried away form the specimen rather than being passed into it. Spalling probably occurs during initial re-entry stages. The thermal shock, as the glass contracts or expands at different rates results in cracks. The resultant morphologies of oriented specimens are the biscuit-form for smaller sized original spheres (up to roughly 70mm original sphere diameter) and the hamburger-form for larger sized original spheres (typically 80-110mm original sphere diameter).
Un-oriented bodies, which must have originally been perfect spheres, suffer much less shell loss. Due to the body not having a fixed orientation, the re-entry heat is not concentrated on one surface, instead it is evenly distributed over the whole body. So the tektite is heated more evenly, to a lesser extent and less rapidly. All of these features favour less shell loss. As soon as shell is lost, however, the tektite is more likely to establish a fixed and stable flight orientation. Un-oriented bodies are represented by small grooved spheres and by breadcrust tektites (many of which do actually show some degree of, probably later phase, orientation). The whole of the body is grooved as the whole of the body was thermally shocked. In small spheres and breadcrusts below roughly 70mm original sphere diameter the shell is largely retained as the temperature differential between the interior and exterior does not reach a critical break point. In larger un-oriented bodies the grooved exterior shell is always lost and this results in a perfectly smooth sphere and shell fragments. In these bodies, where the original sphere was larger than roughly 70mm diameter, the temperature difference between the interior and frictionally heated exterior, always results in shell loss.
ABOVE: A series of photos showing progressive loss of the shell and exposure of the smooth nucleus. The grooved shell is probably lost from the anterior first. The sequence runs from the top left to the top right (these show the probable anterior surface) then onwards from the bottom left to bottom right (these show a side view with the probable anterior at the bottom).
In more oriented specimens, which have a stable flight, 'hamburgers' and the classic 'biscuit' forms are produced.
ABOVE TOP: A 239g 'Hamburger' Philippinite belonging to Desmond Leong of www.tektiteinc.com. Hamburgers may be oriented large breadcrusts. ABOVE BOTTOM: A Biscuit formed from oriented small to medium sized bodies
ABOVE: Un-oriented tektites large (over 1 kilo) to small.
ABOVE: Oriented Philippinites - Large to small. Side view in a circle; Anterior view in a line; Posterior view in a line; Side view in a line.
5. Finally the tektite strikes the ground. Previously I noted that a bald spot may be created. I now put less weight on this due to my belief that Philippinites were wholly solid when they landed, but these are interesting features so I have left this section in. The bald spot may indicate that some Philippinites remained slightly plastic when striking the ground, or they may have formed by thermal flaking shell loss. Maybe these are even collapsed bubbles? These flat areas have been observed in both large and medium sized bodies on the smooth side, which is the posterior surface. It is possible that on striking the ground further polygonal shell fragments were broken from specimens. Further cracks and even possible rare starburst rays formed at this point. These features are examined in greater detail on the Philippinite Navels & Bald Spots page.
ABOVE: A bald spot and fracture occurring on a medium sized Philippinite.
6. Once on the ground it is interpreted that in some geographic areas Anda Sculpture and V-grooving occurs, probably due to etching of residual stresses and strains on original tektite surfaces by acidic freshwater. Water transportation has also had a detrimental effect on many specimens. It is interpreted that U-grooves are formed from thin thermal cracks that have been enhanced by chemical etching. Chemical etching is encouraged by acidic ground waters. In areas where the ground water is more alkaline the U-grooves are often less pronounced.
ABOVE: An attempt by the author to demonstrate an idealised sequence of Philippinite formation, based on his observations. 1) Original primary spheres. These re-enter as solid bodies. Some will have an orientation, some will be unoriented perfect spheres. 2) During re-entry the glass is heated. Due to the temperature differential between the exterior and interior the glass spalls. On un-oriented tektite the re-entry heat is more evenly distributed across the entire exterior and this results in less spalling. 3) The final forms (from left to right) Large smooth nuclei / sphere; Breadcrust (perfect, imperfect forms occur when some degree of orientation is established during re-entry); small grooved sphere; 'Hamburger'- a large oriented form; large 'biscuit' / core and finally a small 'biscuit' / core. 4) Terrestrial transportation and acid etching by groundwater creates U-grooves from the paper-thin cracks produced during re-entry. Anda sculpture rarely occurs on primary surfaces (posterior).
FOR MORE UP-TO-DATE MATERIAL SEE MY PRESENTATION (19 FEB 2011)
IMAGE - currently under modifaication. I am working on a decent classification system for Philippinites.
ABOVE: An attempt by the author to classify the different Philippinite morphologies.