Tuesday, 15 September 2009

Collecting glasses

Tektites are lumps of natural glass formed from terrestrial rock on the impact of a meteorite. The instantly molten rock is thrown high into the air, cooling and solidifying on the way down, so is often distributed some distance away from the impact crater. Not all meteorites have created tektites -in fact so far there are only a handful of tektite sources.

The first tektites were found in 1787 in the Vltava or Moldau River, in what's now the Czech Republic. These are now called Moldavite, and are perhaps most appealing example. They vary slightly in colour, but are generally a deep bottle green. They're prized for jewellery now and are often cut and faceted for setting. Moldavite is linked to a huge meteorite impact in Germany, the Nordlinger Ries crater, which occurred around 14 million years ago.

Elsewhere, the Chesapeake Bay impact in the US has produced tektites found in some Southern states, a grouping of sites around Western Africa are thought to link to an impact in Ghana and tektites found in Australia, China and South East Asia may come from one huge impact. In this case, though, the crater has yet to be identified and there is a good chance they may be from more than one event.

While the impact theory is almost universally accepted, there are a few other suggestions for tektite formation. The most interesting is that they are little lumps of lava from volcanoes on the moon. As wild as this sounds, there is at least some evidence to lend it some weight. Tektites are very dry. Drier than bone. Drier than a towel in a tumble drier in the desert. Moon lava is also very dry. And so... Well - there's more to it than that. I'm being glib.

Another stone I should bring up here is the controversial Libyan desert glass. Although it was first found a long time ago - tools were made with it in the Pleistocene - it still isn't entirely clear exactly what it is. Prevailing opinion now puts it as a tektite, though the shapes it takes are not the usual tektite forms, it is far lighter in colour and it has a far higher silica content than normal. I can't see how this cannot at least be partly attributed to the terrestrial rock in the area of impact, though. If the meteorite hit a sandstone, might the resulting tektites not be silica-rich? Still lots of research being done on this interesting rock, though.

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