Tektites vs Terrestrial Volcanic Obsidian
Firstly I would like to credit the two following pages from The Tektite Source - 'Is it a tektite?' and The Meteorite Times - 'Jim's Fragments, January 2003 - Tektite Testing'. These pages have excellent information on how to differentiate tektites from terrestrial volcanic obsidian and other glassy materials.
Our specific problem was that Desmond Leong had been supplied a couple of pieces of Moung Nong-type Philippinite from Davao City, which is in the south of Mindanao - the southern-most area of the Philippines. This area boasts a famous and very beautiful active volcano - Mount Apo (undoubtedly a ready source for obsidian). The supplier reportedly had 100 kilos of material, which in itself is suspicious. Desmond, obviously concerned about the authenticity of any specimens he adds to his personal collection or sells on his site, www.tektiteinc.com, asked me for my opinion. Instantly my feelings were that this material was terrestrial volcanic obsidian. It simply did not look correct and was not identical to Moung Nong-type tektites that I have from Thailand. It also had a hole running all the way through the specimen and was clearly related to flow and appeared more reminiscent of obsidian. Moung Nong-type tektites have been reported from the Philippines, but personally I have never seen any. I would question the existence of Moung Nong-type tektites in the Philippines, perhaps suggesting that they have been muddled up with tektites that show elongate bubbles rather than true layering. If Moung Nong-type tektites were to exist in the Philippines I would anticipate them to occur in the northern area of the Philippines, in closer proximity to the source crater, located somewhere on or close to the Indochinese Peninsula. One would not expect this tektite type in more distal regions of the strewnfield.
ABOVE: The material believed to be obsidian, not a Moung Nong-type tektite, that we intended to test. From near Davao City, Mindanao, Philippines. In the bottom photo, note the hole that goes all the way through the specimen in the middle-top left hand side.
ABOVE: A second piece of the material we believed to be obsidian, not a Moung Nong-type tektite. From near Davao City, Mindanao, Philippines.
So, to be certain whether this material was obsidian or Moung Nong-type tektites we decided to carry out some destructive tests (afterall there was 100 kg of material) !
First we took an oxy-acetylene torch to a nice biscuit-type Philippinite. It was spectacular to watch! The photos are shown in strict order.
With the fun over, we now allowed the tektite to cool at the outside temperature (around 30 degrees Centigrade).
The tektite behaved exactly as anticipated. The lack of volatile elements meant that no frothing/foaming was observed, the glass simply became molten and flowed. The most interesting part of this experiment was how quickly the glass cooled afterwards. I am not familiar with glass making and this surprised me. Within 5 minutes the glass had gone from white hot, to orange, red and then back to black. Once black the tektite appeared to become brittle. After about 20 minutes, whilst still warm, the tektite was fine to handle.
In Commercial glass making Schuler, F. and Schuler, L., 1970, give the following colour-temperature scale:
Colour Temperature (Degrees Centigrade)
Brilliant White 1515-1270
Cherry Red 975-850
Dull Red 850-600
(Rigid and Brittle glass) Under 450
It is worth noting that, due to the speed the tektite glass cooled, one split and one crack formed in the heated area of the specimen. In the last photo a chip of the glass is seen to break off the specimen in a very similar way to 'bald' areas noted in Indochinites. In commercial glass making, in order to avoid this cracking/breakage, which is caused by thermal expansion/contraction, an annealing process is followed. This basically slows the cooling rate - an example given is holding the glass at 500 deg C for 20 minutes: this is the annealing stage, which relieves stresses introduced by heating. Next, two cooling steps are carried out, dropping the temperature by 75 deg C to 425 deg C over 67 minutes, then to room temperature over a further 45 minutes. The cooling steps must be slow enough so that stress can be partially relieved and not become too great.
Assuming an inherited cosmic velocity of around 10 km/sec the tektites would have reached the Philippines around 3-4 minutes after impact. Whilst the size and surface area of the body and exterior temperature would have resulted in variable cooling rates, I think it is fair to say that by re-entry time the exterior of the tektites had cooled sufficietly to become brittle.
Next we attacked the obsidian or possible Moung Nong-type tektite from Davao City, Mindanao, Philippines. If this was obsidian (as we expected) it should have bubbled and foamed due to the volatile content. It didn't - it exploded within seconds of heat being applied. We did the same with a known piece of Obsidian and that also exploded. This highlights the importance of wearing proper eye protection. At this point the experiment was terminated as it was unsafe to have glass shards flying everywhere! On the foaming front, this experiment was inconclusive.
ABOVE: When heat was applied to obsidian (the possible Moung Nong-type tektite) it exploded!
Despite it not being possible to create the frothing and foaming, we were able to utilise the glass shards. By holding the shards up to a light source it was possible to observe the colour. Australasian tektites exhibit a coffee brown colour in transmitted light. (Other tektites may exhibit a green or brown-green/olive green colour in transmitted light). Obsidian, however, is commonly grey in transmitted light and this is what we observed in our possible Moung Nong-type tektite, thus confirming that it was, in fact, terrestrial volcanic obsidian.
ABOVE: On the left a possible Moung Nong-type tektite from near Davao City, Philippines. The grey colouration indicated that it was in fact terrestrial volcanic obsidian. On the right is a shard of a large, confirmed Philippinite tektite. This exhibited the typical coffee brown colour of Australasian tektites.
Schuler, F. and Schuler, L. 1970. Glassforming. Chiltern Book Company, Philadelphia, U.S.A. and Thomas Nelson and Sons, Ltd, Onatario, Canada.