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17 April 2014 12:48 pm ,
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Officials last week revealed that the U.S. contribution to ITER could cost $3.9 billion by 2034—roughly four times the...
An experimental hepatitis B drug that looked safe in animal trials tragically killed five of 15 patients in 1993. Now,...
Using the two high-quality genomes that exist for Neandertals and Denisovans, researchers find clues to gene activity...
A new report from the Intergovernmental Panel on Climate Change (IPCC) concludes that humanity has done little to slow...
Astronomers have discovered an Earth-sized planet in the habitable zone of a red dwarf—a star cooler than the sun—500...
Three years ago, Jennifer Francis of Rutgers University proposed that a warming Arctic was altering the behavior of the...
- 17 April 2014 12:48 pm , Vol. 344 , #6181
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Poking at Tiger's Eye
4 April 2003 (All day)
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.