Here's one of my favorite Australite specimens (from our private collection). I call it the "Rosetta Core" because it so plainly deciphers the story of core formation. In views 1, 2, and 3, you can see a core with two pie-shaped wedges still attached. Clearly 6 or 8 others have exploded away from a piece that began as a flanged button. Note the ring waves on the remnant skin wedges in #1 and 2, and the very obvious flange in #3. In the 4th view, I have included a core and an assortment of the flakes and spalls that were once attached to a similar core. We find both in approximately the correct proportions.
Here's the translation: During flight, atmospheric friction heats the frontal surface to incandescence. Evaporation in the trailing edge vacuum literally refrigerates that area. The heated area expands. The refrigerated area contracts. In the few seconds it takes to traverse the atmosphere, conduction rates are insufficient to establish thermal equilibrium, so fractures form between the expanding hot frontal surface and the cold contracting interior, ultimately popping flakes away.
In this manner, "cores" develop, with fluted anterior (front) sides and more smoothly curved posterior surfaces (which are approximate remnants of the primary spherical surface). The flaking on the frontal face is one form of "thermal ablation", a characteristic mostly restricted to Australites. I am aware of a few Bediasites that show ablation features, and one reported Ivory Coast, but that's it.
While we're on this subject, it's worth commenting on flanged buttons. The reason they form in a rather narrow range of sizes (and none are ever much over about an inch in diameter) is that larger pieces inevitably explode into cores due to thermal disequilibrium. There is some maximum size where rates of heat conduction allow enough equilibration that flakes don't explode away. In such instances, flanged buttons can survive. If someone ever offers you a monster 2"+ flanged button, BEWARE! By the laws of thermodynamics, it is almost assuredly a fake. I have seen such a swindle.
This page offers a variety of Australite thermal ablation cores. If you've read this far, you now know more than most tektite fans.
Pop Quiz! Which end of the core faced earth during re-entry? The smoothly curved heat-shield-shaped side or the faceted pointy end? (If you don't know, re-read the story above and study the photos!)
Aussie Cores for sale:
Here is an assortment of relatively big ones. These are from South Australia,
and in our experience, are of a size found in proportions less than 1 in a hundred. These are moderately abraded,
with subtle flake scars, but all are certainly ablation cores with a nice curved hind surface and a more pyramidal
front. Most still have residual brick-red soils in pores that could be further cleaned with a bit of effort.
This group ranges from 7.8 to 14 gms. $25 each (specify "biggest
remaining" or "best
features"(my call), first come, first served).
This group is similar in all respect to those above except that the sizes range
from 2.8 to 7.3 gms. $15 each (specify "biggest
remaining" or "best
features"(my call), first come, first served, if you come in close
to last, I'll offer a price break).
This is just a start. Lots more Australite pages are coming (lenses, patellates, irregulars, specials). We also can try to meet individual requests. Tell us what you are after and we'll try to get some images sent off to you for your consideration. It's going to take a while for me to do justice to our Aussie inventory. We have heaps, but I have a real mental block about selling them----
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