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Australite Indicator Core

Upon first introduction to an Australite ablation core it is a common error to confuse the anterior and posterior surfaces. The always present smoothly rounded surface is very suggestive of the frontal shield on an oriented chondrite, but this impression is incorrect. As Australites made their final plunge through the atmosphere, the frontal surface heated to the point where thin layers of melt formed and streamed back, exactly analogous to an oriented meteorite. In the case of the classic flanged button morphology, a nice roll-over lip formed and survived, but this seems only to have happened in a rather narrow size range. If the tektite was a little larger in diameter than typical flanged buttons, a thermal disequilibrium developed between the white-hot frontal surface and the vacuum-refrigerated rear. This expansion/contraction couplet caused patellate flakes to burst free of the frontal surface creating the irregular fluted face of an ablation core. The absolute clincher for this story is found in the “indicator cores” like that featured this month. These are ablation cores arrested in the very act of exfoliating skin wedges—flanged buttons that were just a tiny bit too big— and pie-shaped wedges began to burst off. But the crowning glory of an indicator core (and the reason for its name) is that the process didn’t go to full completion. A few wedges of the frontal skin remain attached to the core. In this example, the remnants display the spectacular and utterly uncontestable orientation features of a flanged Aussie button— frontal vortex ring-waves and well-developed roll-over flanges. The part of the core that has lost flakes is a perfect fluted ablation core. Together, these features tell the whole story in graphic detail. During a career devoted to trying to read rocks, I hugely appreciate one that tells a good story with exceptional clarity. Behold, the fabulous Australite Indicator Core! (from our collection).

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