Geotimes - June 2003 - A New Fire in Tiger's Eye

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News Notes

Mineralogy
A new fire in tiger's eye

For more than a century, mineralogists have accepted that the lustrous bands in the tiger's eye gemstone occur because of a process called pseudomorphism, where one mineral replaces another but retains the original shape. However, when Peter Heaney took a look at the gemstone through an optical microscope, he found that this common assumption was in dire need of an update.

When light reflects off the parallel mineral bands of tiger's eye, the gem shimmers. The German mineralogist Ferdinand Wibel in 1873 first interpreted tiger's eye as a pseudomorph. That explanation stuck. Until now, mineralogists have assumed that the mineral bands are fibrous quartz that grew in place of pre-existing crocidolite asbestos.

This feline figurine made of tiger's eye has a new explanation for its sparkle. Photo by Chip Clark, Smithsonian Institution's National Museum of Natural History.

In the April issue of Geology, Heaney and Donald Fisher, both of Pennsylvania State University, reported a new origin for the mineral bands responsible for the shimmer, or chatoyancy, in tiger's eye gems. Intense tectonic pressures cause fractures within the gem that allow the quartz and crocidolite minerals to grow toward each other from either side of the fracture, with the quartz dominating as they both fill in the open vein. As the pressure continues, the gem breaks again — this time splitting the fracture on the side where the crocidolite had formed. "In order to make one centimeter of tiger's eye, it has to undergo twenty to a hundred of these cracking episodes," Heaney says. Each quartz-filled vein with its inclusions of crocidolite provides a tectonic history for the gem. "Since the chatoyancy is produced by the crocidolite and not the quartz crystals, the zig-zag cat's-eye pattern reflects the movement of the rock."

Christina Reed

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