Poking at Tiger's Eye

Tiger, tiger burning bright. Two geologists say they've debunked a 130-year-old explanation of how the gem got its reflective stripes.

Geologists have thought for over a century that tiger's eye, the banded gold and brown rock commonly used for inexpensive jewelry, was formed in much the same fashion as a piece of petrified wood: one mineral replaced another while retaining the original structure of the rock. But now a pair of geologists says that's all wrong.

Peter Heaney, a mineralogist at Pennsylvania State University, University Park, traced the misconception to a German mineralogist, one Herr Dr. F. Wibel. Writing in 1873, Wibel concluded that tiger's eye forms when crocidolite (a blue form of asbestos) in a rock is replaced with fibrous crystals of chalcedony, a form of quartz. Wibel thought it was these fibrous crystals that gave the gem its shimmer.

But when Heaney took a closer look using various types of microscopes, he found no chalcedony in tiger's eye at all--only nonfibrous quartz. The pattern of the crystals suggested to Heaney and co-author Don Fisher, a structural geologist, that the original rock was made of solid columns of quartz speckled with crocidolite. Tiny cracks might have crazed the rock, providing a foothold for fibers of quartz and crocidolite. As these fibers grew, then cracked again and grew more, they would produce the gem's technicolor bands. This cracking could have happened, Fisher suggests, as high pressure deep inside Earth's crust squeezed fluid in the rocks. In this scenario, it's the crocidolite not chalcedony that dazzles the eye, the researchers report in the April issue of Geology.

Similar mechanisms have been proposed to explain the same kind of textures in slate and other rocks deformed by high pressures, but Dave Wiltschko, a structural geologist at Texas A&M University in College Station, says he doesn't think cracking should happen at all. If cracking happens in tiger's eye, Wiltschko says he'd expect to see damage in the quartz from deformation and tiny pockets made by fluids that the authors did not report.

Related sites
Heaney's site
Fisher's work on crack-seal deformation in slate in Alaska

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