SECONDARY CLASSIFICATION OF
PROXIMAL TEKTITES

Written By Aubrey Whymark 2017
Proximal tektites were plastically deformed during the ejection to re-entry phase, having never truly left the atmosphere. Minor spallation and brittle failure may occur during the latter stages of flight.

Proximal:

Most important classification points:
1)      Primary morphology.
2)      Plastic deformation history.

Less important classification points:
3)      Size of the body is not important for classification, but does determine the final morphology of the tektite. Size vs. final morphology varies with melt viscosity/distance from impact.
4)      All proximal tektites are oriented, so orientation is not a significant factor in classification. With teardrops that deformed during atmospheric climb the orientation of the body during ejection was important. The same body might form a ‘Hershey’s Kiss’ or a long arciform ellipsoidal teardrop, for instance, dependent on where the tail was pointing. If the deformation had solely taken place during freefall re-entry this would not be an issue.

In the proximal setting a very simple classification scheme can be established. Again, 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. This is basically a reflection of the degree of plastic deformation. The secondary shapes are catagorised as biconcave, concavo-convex, biconvex, ellipsoidal and spheroidal. These morphologies are demonstrated in Figure 12.3.

Proximal Tektites

e.g. Indochinites, Georgiaites, Ivory Coast Tektites, Moldavites (particularly Bohemian)

Proximal tektites are morphologically dominated by interaction with the atmosphere, which has flattened spheres towards discs. Many suffer very minor spallation, known as 'bald spots'.
 
Biconcave

Concavo-convex

Biconvex
W = 1
H = <0.5

Ellipsoidal
W = 1
H = 0.5 - <1

Spheroidal
W = 1
H = 1

ABOVE: The secondary classification of proximal tektites based on sectional profile. It is necessary to numerically define biconvex, ellipsoidal and spheroidal forms. Width (of secondary/final shape) vs. Height ratio is given.
The degree of plastic deformation is partly affected by size, but also distance from impact, melt viscosity/temperature and height of ejection are important factors. At a set distance from the source crater larger forms, which retain heat longer, will move further down the deformation sequence. Smaller forms, which cool quicker, will remain more spherical. Size can be removed from the proximal classification scheme as it is not important in order to classify the shape. If one were to use macro-tektite morphology to locate the crater, however, one must take into account both the morphology and the size/weight of the tektite.

Proximal tektites always have an orientation as they interact with the atmosphere whilst still molten. Deformation takes place immediately after the droplet forms. In some cases, such as the so-called ‘Hershey Kisses’ (see Figure 12.4), it is clear that the tektite was plastically deformed whilst it still had inherited cosmic velocity and was likely climbing atmospherically. These specimens are characterised by deformation that occurred in an orientation inconsistent with the natural free-fall position.

Other concavo-convex teardrops; the so-called onion-forms (see Figure 12.5) deformed in fairly natural free-fall orientations. Arciform tektites (see Figure 12.6), for the most part, appear to have deformed in natural free-fall positions.

Special names for certain unique pieces, as a sub-category, are acceptable. To lump onion-forms and ‘Hershey's Kisses’ in together is somewhat unsatisfactory.

How are they flattened?

Proximal forms are flattened through interaction with the atmosphere, in the same way as a rain drop is flattened (except with tektites a non-equilibrium form is locked-in by cooling and solidification). Some researchers claim that the flattening is due to rotational forces. If so, then why don't we see all the same forms, including discs, in the medial and distal settings? And, please explain teardrop forms, which simply don't fit into this pattern.
 
ABOVE: A concavo-convex teardrops known as a ‘Hershey’s Kiss’ and which deformed in an unstable orientation suggestive it still had inherited cosmic velocity. After Whymark (2009).
ABOVE TOP: A concavo-convex teardrop known as an ‘onion’-form which deformed in what appears to be a stable free-fall orientation. After Whymark (2009).
ABOVE BOTTOM: An example of an Arciform tektite.
ABOVE: The classification of proximal tektites as determined by this work.

Biconcave:

Biconcave tektites are identified purely by their shape, although inevitably their height will always be under half of the width. Very elongate biconcave specimens typically do not occur due to the generally small width diameter, which results in more rapid cooling. One would theoretically expect smaller sized biconcave tektites with proximity to the impact, whereas at distance one would anticipate that only the largest of the tektites remained sufficiently molten to form a biconcave morphology. The anterior concavity is termed a 'punt' and the posterior, usually less pronounced, concavity is termed a 'sump'.

Concavo-convex:

Concavo-convex tektites are identified purely by their shape, although inevitably their height will always be under half of the width. Normally, if not always, the concave side represents the anterior of the tektite and is termed a 'punt'. Very elongate concavo-convex specimens typically do not occur due to the generally small width diameter, which results in more rapid cooling.

Biconvex:

Biconvex tektites are defined by their biconvex shape and their height being under half of the width. Even very elongate tektites can be biconvex. If the teardrop or, on rare occasions, the dumbbell is plastically distorted and bent into a curved shape it may be described as ‘arciform’.

Ellipsoidal:

Ellipsoidal tektites are defined by their globular biconvex shape and their height being over half of, but less than, the width. Even very elongate tektites can be biconvex. Again, ‘arciform’ distorted tektites may be common-place amongst elongate forms.

Spheroidal:

Spheroidal tektites are effectively identical to the primary morphology with width being equal to height. Very minor spallation, resulting in bald spots may have occurred, but these slithers of lost glass do not detract significantly from the original shape. Elongate spheroidal tektites cannot be arciform as the exterior must have been rigid and brittle prior to significant atmospheric interaction. Spheroidal forms represent the smaller sized tektites in more proximal localities. At greater distances even the largest of tektites remains spheroidal as there has been sufficient time to cool the exterior surface prior to re-entry.