SECONDARY CLASSIFICATION OF
MEDIAL TEKTITES

Written By Aubrey Whymark 2017
Medial tektites suffered variable degrees of plastic deformation during the ejection phase, resulting in some bodies being oriented and others un-oriented. They re-entered the atmosphere as solid and brittle bodies when small and hot bodies with a solidified exterior when large. During re-entry they were heated, but did not ablate. Once their inherited cosmic velocity was lost they were rapidly cooled and suffered a spallation stage. The largest bodies were insufficiently brittle and avoided spallation.

Most important classification points:
1)      Primary morphology.
2)      Plastic deformation history.
3)      Orientation/lack of orientation during atmospheric re-entry. Medial tektites, although mainly oriented, also have un-oriented or late-stage oriented forms and rotationally oriented forms.
4)      Size of the body. This is important as a small and large body, although effectively the same, appear significantly different due to the cooling history and therefore the susceptibility to brittle failure.
5)      Spallation history.

In extending the classification scheme to the medial tektites a binomial scheme of classification is again used. This utilizes the anterior/posterior view, approximating closely to the primary tektite morphology. This is compared with a side view, as viewed perpendicular to the flight path, taking into account the surface sculpture. The side view reflects the secondary modification, both plastic deformation during atmospheric ejection and spallation, during the later stage of atmospheric re-entry. The spallation and brittle fracturing patterns are in turn determined by the orientation (determined by the plastic deformation) and by the size of the re-entry body. Larger bodies retain a hot interior during re-entry and this influences the susceptibility to brittle failure.

Medial Tektites

e.g. Philippinites, Billitonites, Bediasites

Medial tektites typically suffered a degree of plastic deformation during ejection. During re-entry they were heated, but not sufficiently to be ablated. They then spalled. Uniquely, medial tektites also include un-oriented forms.  Why? Well, the distal forms were ejected at high velocity but low angle, so it took time to traverse the atmosphere. Proximal forms were ejected at low velocity but high angle, the low velocity meant they didn't truly escape the atmosphere. Medial tektites were ejected at just the right angle and speed such that they were the fastest forms to traverse the atmosphere. Some formed at the greatest altitudes and with no atmosphere present, the molten spheres were not flattened by interaction with the atmosphere.
 
ABOVE: The classification of medial tektites as determined by this work.

Oriented Bodies

All oriented bodies have undergone a degree of plastic deformation during the ejection phase. This will determine the final morphology. During re-entry the anterior surface was heated and then once the inherited cosmic velocity was lost the anterior surface was rapidly cooled causing spallation, the effect of which varied dependent on the size of the body. Specimens are considered globular once the height of the final morphology exceeds four fifths of the width of the body.

Bifurcated Core:

Bifurcated cores are derived from the largest plastically deformed re-entry bodies over 55 mm in width diameter. The final morphology must have a height under four fifths the width of the body. Specimens that are too globular will not form the classic ‘hamburger’ morphology as it is locally known. Bifurcated cores typically weigh around 200 to 280 grams.

The resultant morphology has a smooth posterior, representing the primary surface. The anterior and anterior margins have a poorly developed breadcrust texture, comprising polygonal cracks. The anterior surface usually has less cracks/u-grooves than the margin. The specimen is bifurcated by an equatorial crack, usually etched to a u-groove. Bifurcated cores are almost always devoid of navels. The reduction of cracks on the anterior surface of large bodies is attributed to these larger bodies re-entering with a hot interior, which inhibits formation of cracks. The exterior surface had, however, cooled prior to re-entry.

Shield Core:

Shield cores form from oriented re-entry bodies up to approximately 55 mm width diameter and occasionally even slightly larger. They are typically 20 to 28 mm in thickness. Shield cores are typically lenticular in shape. The final morphology must have a height under four fifths the width of the body. Shield cores typically weigh under 60 grams, but can weigh in excess of 100 grams.

The resultant morphology has a smooth posterior, representing the primary surface. The anterior surface typically comprises radial cracks, etched to form u-grooves. On larger specimens these develop into a polygonal pattern. Navels, representing etched cones which diverge towards the exterior surface, are present on the anterior surface. Crack formation indicates that the smaller bodies re-entered as cool and brittle glass.

Globular Bifurcated Core:

Globular bifurcated cores are derived from large, slightly plastically deformed, re-entry bodies over 55 mm in width diameter. The final morphology must have a height over four fifths the width of the body. These oriented globular specimens show cracking or u-grooving on one side only and often show a bifurcation, albeit into two very uneven portions. Globular bifurcated cores typically weigh around 200 to 450 grams.

The resultant morphology has a smooth posterior, representing the primary surface. The anterior surface has a poorly developed breadcrust texture, comprising polygonal cracks. Sometimes a single polygon may be formed and at other times, with increased plastic deformation, the morphology moves more towards a ‘hamburger’ shape. As with bifurcated cores, navels are not present. The reduction in cracks is again taken as evidence that these larger bodies retained a hot interior throughout re-entry.

Globular Core:

Globular cores form from oriented re-entry bodies up to approximately 55 mm width diameter, although appear to be typically under 40 mm width diameter. Globular cores are by definition globular in shape. The final morphology must have a height over four fifths the width of the body. Globular cores typically weigh under 60 grams.

The resultant morphology has a smooth posterior, representing the primary surface. The anterior surface typically comprises radial cracks, etched to form u-grooves. On larger specimens these develop into a polygonal pattern. Navels, representing etched cones which diverge towards the exterior surface, may be present on the anterior surface. Crack formation indicates that the smaller bodies re-entered as cool and brittle glass.

Un-oriented Bodies

Un-oriented bodies appear to have formed above atmospheric effects and thus suffered no plastic deformation during the ejection phase. The lack of orientation resulted in a tumbling motion during re-entry. The re-entry heat was distributed over the entire surface then, once the majority of the inherited cosmic velocity was lost, the anterior surface was rapidly cooled. This cooling resulted in crack formation in medium and smaller bodies, but usually not spallation. If spallation occurred then a late-stage orientation was established. In larger sized bodies, which retained a hot interior, crack formation did not occur as the body was probably insufficiently brittle. This resulted in large smooth spheres.

Fragmentation:

Smooth Spheres

The largest of the philippinites are usually only evident as fragments. This is because very large spheres, in excess of around 110 mm diameter, are thermodynamically unstable. The internal tensile stresses, developed during cooling, are greater than the strength of the glass (Centolanzi, 1969). The resultant morphologies are often conical/pyramidal shards, triangular in outline. One also finds L-shaped fragments, with two points comprising the original primary surface. A triangular fragment neatly fits between these points creating a quarter wedge. These bodies may fragment on the ground, shortly after impact, as they cool.
Some folk believe that smooth spheres are the result of loss of a breadcrust sphere. However, I have never seen an indicator form. I believe that smooth spheres were simply of sufficiently large size that they retained enough heat to resist brittle failure and cracking during re-entry.
 

Smooth Sphere:

Smooth spheres may be a wide variety of sizes, but typically the larger a specimen, the more likely it is to be smooth. Most smooth spheres appear to be over approximately 500 grams in final weight and over 70 mm in diameter. Smooth spheres often appear to be slightly off-spherical and this would appear to be due to minor planar spalling (similar to that found in proximal indochinites) as oppose plastic deformation. Smooth spheres probably retained a hot interior throughout re-entry, inhibiting crack formation.

Transitional Smooth Sphere:

These bodies are similar to smooth spheres, but may show sparse cracks or u-grooves. They typically weigh over 400 grams and are transitional between breadcrust spheres and smooth spheres.

Breadcrust Sphere:

Breadcrust spheres are spherical bodies with polygonal cracks over the entire surface. They are typically 50 to 70 mm in diameter, sometimes a little larger or a little smaller. These bodies are entirely un-oriented. Typically the smaller, more thermodynamically stable, end of the spectrum, below 60 mm are more likely to retain their shell and thus remain un-oriented throughout their atmospheric passage. The presence of polygonal cracks over the entire surface is indicative that bodies of this size, and below, cooled throughout and were brittle during re-entry.

Small Grooved Sphere:

Small grooved spheres are spherical un-oriented bodies typically derived from primary spheres under 50 mm diameter and represent a continuation of breadcrust spheres, but within this size range the polygonal cracking cannot develop to perfection. Instead meandering cracks, which later etch to form u-grooves are the norm. Crack formation indicates that the smaller bodies re-entered as cool and brittle glass.

Breadcrust Core:

Most breadcrust spheres are in fact oriented breadcrust spheres. The tektite originally tumbled and gained a polygonal cracking over the entire surface. We know the polygonal cracking is in response to re-entry heating then cooling and not simply cooling as it does not occur in indochinites that simply cooled. When most of the inherited cosmic velocity is lost the tektite is rapidly cooled. This may result in spallation. The moment a piece of glass spalls, the spherical tektite is able to gain a stable orientation. From this point onwards spalling will occur on the newly established anterior surface. This leaves a breadcrust sphere that has suffered shell loss to the front, resulting in perhaps a fifth or a third of the frontal surface being lost to spallation.

Rolled Grooved Tektites:

Elongate specimens, excluding teardrops and asymmetrical dumbbells, may gain a stable orientation with regards pitch, but may continue to roll about the long axis. This results in thermal cracking, later etching to become u-grooves, over the entire surface. There is no anterior or posterior surface.

Shell Fragments:

Shell fragments are often approximately polygonal in shape, although may be random or wedge shaped. They comprise one u-grooved, breadcrusted convex side and a smooth concave side. They are usually around 15-28 mm thick, although 20-23 mm is a common range. Shell fragments have been spalled from breadcrust tektites that gained a late-stage orientation.