Australite Shapes

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Australites are different to all other tektites as they have undergone secondary melting of the original primary body to form aerodynamically shaped forms. This must have been a very rare event as no similar forms are found anywhere else in the world.

Typical original primary bodies are spheres, ovals, dumbbells and teardrops - these shapes are typical of any flying, rotating drop of liquid and tektite examples can be found in Asian countries to the north of Australia (Indochinites) but these are generally much bigger than australites and are also called 'splash forms'.   The primary bodies that formed into australites are assumed to have all developed from these basic types but, unlike the Asian tektites, these had a  higher initial velocity and smaller mass, and were ejected right out of the earths atmosphere.

The re-entry through the atmosphere being the reason for the secondary melting and formation of the unique australite shapes.

          However, there are some shapes found in australites which do not correspond to these typical primary body shapes and which are rarely mentioned.  I will be discussing these forms later.

Formation of the Primary Forms.

increasing speed of rotation >>

sphere                 oval                   boat                         dumbbell                                     teardrops

The primary bodies that developed into australites may have formed in zero or near zero gravity conditions with the blobs of molten glass being shaped by surface tension and speed of rotation. Spheres developed from non rotating molten blobs, ovals from slow to moderate rates of rotation, dumbbells from rapid rates of rotation with possible separation into 'teardrops' at the most rapid speeds. Although  teardrop forms are commom in Asia, they are rare in Australia. Spheres are by far the most common of the primary bodies that developed into australites. It is possible that spheres could also have condensed out of   vapour formed during impact and ejected out into space. Here the zero gravity and lack of rotation would have formed perfect spheres.

The Shaw Collection and some numbers.

Throughout the descriptions I will be quoting data from Charles Fenner's article on 'The classification of the H.C. Shaw collection'  published in the 'Transactions of The Royal Society of South Australia 1934'.
Although this research was done so long ago there has never been a comparable study done on such a huge collection of tektites from one area  which includes every type of tektite found. Many modern collections are biased toward bigger and more perfect specimens.

The Shaw collection contained 3920 separate pieces and out of these 1993 were regarded as 'whole' (groupA) and out of the 1927 broken specimens, many were readily identifiable (groupB). The collection came from about 30,000 square miles of the semi arid region of the southern Nullarbor Plain. The collection may be taken as being representative of the average numbers, sizes and types of australites as they lay widespread over the surface of the Nullarbor Plain - with only a few larger tektites missing from the collection at the time (given away to friends!).

Below is the classification of groupA.  I have grouped the round forms together in this table to show the dominance of this form  all having  been derived from spheres.

                                                                             Whole Specimens (Shaw Collection)

Broken Tektites  (Shaw Collection)

Round forms                                                962              61.46%
Elongate forms (ovals, canoes, boats)        500              31.94%
Dumbbells                                                      79              5.08%
Teardrop                                                         24              1.53%

Total                                                           1565

It is obvious that  elongate forms would break more often than the round forms so a combination of whole and broken numbers should give the most accurate figures.

Combined Figures (Shaw Collection)

Round forms                                              2,331                           65.51%
Elongate forms (ovals, canoes, boats)      920                             25.85%
Dumbbells                                                    149                              4.18%
Teardrops                                                     158                              4.44%

Total                                                             3558

In comparison, Ken McNamara and Alex Bevan in their book 'Tektites'   quoted the following figures:

Spheres                           60-70%
Ovals, boats, canoes      21-25%
Dumbbells                          8%
Teardrops                           3%

The only difference seems to be in the estimates for teardrops and dumbbells with the Shaw collection being very poor in size and numbers of whole dumbbells. The story is slightly different in groupB (the recognisable fragments). It should be mentioned that one fragment was from a dumbbell and weighed 24.85 grams and probably represented an original specimen of at least 80grams! I found a similar fragment of a very large dumbbell (21 grams) see group7a.
Obviously dumbbells can break into two fragments but Fenner was adamant that each fragment represented an individual tektite.

In my own collections from the Eastern Goldfields of West Australia my numbers of dumbbells and teardrops were more similar in percentage to the Shaw Collection. Every area of Australia seems to differ in numbers, types and sizes of tektites though, but I do consider the Shaw Collection still to be the most comprehensive classification of forms found in one area.

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There are three parts of an australite used in descriptions:

1. The posterior surface which escaped melting sometimes called the 'top'  often showing pits and cracks and it represents the remains of the primary form.
2. The anterior surface  which underwent secondary melting, also called the 'front' or 'bottom'. characterised by spallation or in the case of flanged forms, of flow ridges which may be concentric, spiral or irregular.
3. The outer 'rim' in the case of cores, also called an equatorial ridge, or the 'flange' in the case of flanged forms. Fenner regarded all the rims as flanges but today we distinguish between them.

Two major types of australites are recognised :

• Cores - formed from larger primary bodies.
• Flanged Buttons -  formed from smaller bodies.

There is a certain size (usually less than 6 gms) which leads to the development of the flanged forms.
Also, possibly a longer travelling time through the atmosphere ie. a longer time in the molten state of the smaller forms has led to more flanged forms being formed especially towards the far southern area of Australia. It certainly seems that the further you go from the presumed location of the impact crater, the more likely you are to find the flanged buttons.

Cores

A 'core' is simply a primary body which has undergone secondary melting and it should show a distinct 'rim' or 'equatorial ridge' - but only if it has maintained a steady orientation in flight!
If it has not maintained a steady orientation ie. it has wobbled or tumbled during the secondary melting phase of re-entry, it may show irregular ablation in the form of flat areas on different sides. Australites not showing some evidence of a rim are uncommon. Many australites have weathered though, making the rim less distinct, but the difference in curvature between the original primary body (posterior surface) and the ablated anterior surface, is usually still apparent. This difference in curvature can be used to estimate the original size of the primary body before secondary melting, the posterior surface having escaped secondary melting.

At first re-entering the atmosphere the higher velocity causes  high temperature, rapid melting and fast stripping or peeling away of the surface layers, this is called ablation.  As the tektite slows there is more of a 'flaking' away of the surface creating a sharper rim. This is called  spallation   and must have been caused by  a build up of stresses caused by sudden cooling or alternate heating and cooling. This stressed surface layer is sometimes called the  aerothermal stress shell    and in a few australites, fragments of it still attached, have been found  (see below).

Continued >>