X-Ray Analysis

Following the first set of x-rays I returned to get more data. I mainly concentrated on larger Philippinites (breadcrusts, grooved spheres and smooth spheres). I also x-rayed more Indochinite dumbells. Here are my x-rays for the larger philippinites from visit 1 and 2.

ABOVE: A slide show of x-rayed larger tektites. Note that images are not to scale.

ABOVE: A table of x-rayed philippinites and interpreted number of bubbles and bubble dimensions.

Method:

I used an older x-ray machine with a photographic plate onto which the specimens were placed. It was necessary to image the specimens at different powers/factors in order to pick up the bubbles. If the power/factor was too high or too low in relation to the thickness of the tektite glass then the bubble was not observed. (Too high and the plate is black, too low and the whole of the specimen is white). The bubble dimensions obtained may be slightly larger than the actual bubble size (maybe up to 5% as a very rough estimate) because the x-rays project onto a photographic plate below from a smaller area above - so it is like seeing a shadow from a single light source above.

I used 28 large philippinites, predominantly from Paracale, Bicol, Philippines, 1 from Masbate and 2 from unknown localities (likely Bicol). Aside from the two spheres with known large bubbles (1,194.80g and PB1111747) the specimens were picked randomly, trying to get a representative variety of sizes. The smallest, more typical, philippinites were not analysed. Specimens were taken from breadcrust spheres, grooved spheres and smooth spheres.

Results:

Firstly the reader  should be aware that the photographic plates against a light table (you can also use a brightly lit blank computer monitor) are of a much higher resolution than what I was able to reproduce by scanning and photographing these plates. The presented images do, however, give a good impression of what I have seen. Within the limited database some clear trends were identified.

Specimens over 500g typically contained one 'large' bubble around 10-15 mm diameter, near to slightly off spherical. Typically this bubble was in the centre, or close to the centre, of the specimen (note that on the image the white part is the thickest/central part of the sphere and the margin/border is in the black area and can often only be picked up on a light table). Specimens in this size range are typically smooth spheres.

Roughly from 500 to 320g there was a trend of one 'medium' sized bubble and a couple of smaller bubbles, often 3 bubbles in total. The largest bubble was typically 7-10 mm in diameter and spherical or close to spherical. The bubbles were located randomly in the central portion of the tektite. Specimens in this size range are typically grooved spheres.

Below 320g the tektites were typified by bubble complexes, or numerous smaller bubbles, often 2 to 5 mm in diameter and spherical or close to spherical. Some larger bubbles are observed. The boundary between the 500-320 g and below 320g sub-sets is somewhat blurred and is more of a trend, but there does appear to be a valid subdivision. The bubble complexes were located in the central portion of the tektite. Specimens below 320g are typically breadcrust spheres.

ABOVE: A summary page of x-rayed larger philippinites. Please view the powerpoint presentation above for more details.

Discussion:

It is considered that, for the most part, the majority of bubbles were identified. The larger specimen PM1115009 (693.09g) exhibits both a large and a small bubble, which gives confidence that had smaller bubbles existed in other large specimens then they should also have been identified. The absence of small bubbles in the larger spheres is not considered a problem related to the power/factor and is likely genuine. 

It is probable that in larger spheres the stability of the glass sphere is increased by the presence of a bubble and therefore there may be a bias in nature, with spheres that contain bubbles surviving until today whilst spheres containing no/smaller bubbles may have fragmented. This may be true to some extent (particularly when a large bubble is present), but the consistency of bubbles would suggest this is a genuine primary trend.

Bubbles are fairly centrally placed as the tektite cooled from the outside-in.

The presence of a single large bubble in the large smooth spheres as oppose to numerous smaller bubbles in the smaller breadcrust spheres may be a reflection of cooling times. The smaller spheres cooled quicker, trapping the smaller bubbles in place before they reached their optimum configuration (1 bubble). The larger spheres would have cooled slower, probably allowing sufficient time for the smaller bubbles to merge into a single large bubble.

In these spherical philippinites the bubbles are typically spherical to very slightly ellipsoidal. The fact that the bubbles are close to spherical, even in the largest specimen, adds weight that when these tektites re-entered the atmosphere they had a hard and brittle exterior which resisted plastic deformation, which would have in turn distorted the bubbles. Philippinites re-entered the atmosphere with a solid and brittle exterior.

Contrary to popular myth, tektite glass is not bubble free. In fact in these larger specimens bubbles are the 'norm' and it would be unusual to find a 100% solid sphere.

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Brief update on 19 September 2011. I have now obtained and x-rayed a couple more large specimens (1281.89g and 995.10g), alongside 3 previously x-rayed specimens. The measurements on the previous 3 specimens differ very slightly in these images. Previous conclusions are supported by these two additional specimens.

ABOVE LEFT: The 1,194.80g specimen        ABOVE RIGHT: The 1,281.89g specimen

 ABOVE LEFT: 966.85g         ABOVE MIDDLE: 995.10g         ABOVE RIGHT: 1018.40g

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Study of Dumbbells:

As an aside, I wanted to test whether asymmetrical dumbbells typically contained a bubble in the bulbous end. Whilst I worked on a very limited database (8 additional indochinite specimens on top of 4 indochinite and 1 philippinite dumbbells previously x-rayed), I could find no evidence that this was the case. The asymmetrical bulbous ends mainly comprises glass and not bubbles, so I do not believe bubbles significantly influence the asymmetry of dumbbells.

ABOVE: A selection of Indochinites x-rayed at different powers/factors. No large bubbles were revealed in these specimens.

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