Wednesday, February 15, 2012

Microstructure matters

kw: observations, materials science

While preparing a talk on the semiprecious varieties of chalcedony, I read in a few articles about the way that these materials were used as tools, primarily in precolonial times. A significant dichotomy stood out: "hitting" tools such as hammers and clubs tend to be made of chert or light colored flint, and cutting tools such as arrow and spear points tend to be made of dark colored flint, or agate or jasper. These depend on material availability, but good material was often traded and transported long distances.

Cryptocrystalline quartz has a range of microstructures, from blocky to fibrous, that affect how the material can be used. These materials are called cryptocrystalline because the crystals cannot be seen under an optical microscope; these are nanomaterials. Seen with an electron microscope, chert and yellow flint have blocky structure, like sand grains compressed to eliminate the porosity, but the grains are very small, smaller than the wavelength of light. Agate, jasper and black or gray flint have a fibrous texture, as though felt had been petrified, again, with the fibrous nanocrystals smaller than a wavelength of light. That is why the broken surface of any of these materials looks very smooth, like broken glass. Yet they are crystalline on a very small scale.

The fibrous cryptocrystalline materials take a sharper edge and hold it better under wear. They are thus valued for cutting tools. Arrow points from the American southwest, for example, are usually agate or petrified wood, even where chert is the more abundant material. And grinding pestles and hammer stones are usually made of chunks of chert. It helps that chert often comes in larger pieces. It does take hammering well, while agate, for example, splinters more easily. But those shards are sharp!

As a lapidary hobbyist, I find jasper, including petrified wood, which is usually jasper, a favorite material for beautiful cabochons. But jasper is particularly tough on grinding wheels. Wheels of silicon carbide grit wear quickly, and diamond-charged wheels lose their diamonds faster—they get plucked out of the matrix—than with any other quartzose material. It comes with the territory. Chert can be beautiful also, but this is rather rare. Most chert is gray or off-white and dull. It takes a good polish, of course, and it is not quite as hard on the equipment. But the fibrous materials are nearly always prettier. Love 'em!

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