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Eye of the Tiger

Discovery about gem's structure overturns old theory

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9:51am, April 22, 2003
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In London in the mid-1870s, 25 shillings–about $85 in today's terms–went a long way. You could buy 7 grams of gold, 40 liters of rum, or about a half kilogram of opium. Where you couldn't get a bargain, however, was the jewelry store. That same amount of money bought just 1 carat, or 0.2 gram, of a gem called tiger's-eye. When rich sources of that precious stone were found in western South Africa in the 1880s, prices plummeted. By 1900, tiger's-eye was considered merely semiprecious. Today, a savvy shopper can purchase the gem for about $1.50 per carat.

The passage of time has transformed more than the gem's price. Recent research has upended a 130-year-old theory about how tiger's-eye forms. As a result, scientists soon will be scrambling to update everything from mineralogy textbooks to museum displays.

Shining bright

In its natural state, tiger's-eye is an unremarkable rock with a dull sheen. When polished and illuminated, however, the stone reflects a narrow band of light that changes position as the gem is turned back and forth. This effect, called chatoyancy, gets its name from the French phrase for "cat's eye" because of its resemblance to a feline's slitted pupil. Chatoyancy occurs when light reflects from minute, parallel ridges, fibers, or tubes within a transparent material.

Early in the 1800s, mineralogists recognized that tiger's-eye was a fibrous variety of quartz, or silicon dioxide. In 1873, the German mineralogist Ferdinand Wibel learned more. While studying the chemistry of hawk's-eye, a blue form of tiger's-eye, he found that the gem was almost entirely quartz but that it also contained fibers of crocidolite, an often bluish, iron-bearing form of asbestos. Wibel proposed that hawk's-eye forms in Earth's crust when quartz dissolved in hot water infiltrates spaces between crocidolite fibers and then slowly replaces the asbestos' molecules. Brown tiger's-eye, Wibel said, comes after yet another step.

It results when chemical reactions transform some of the iron in the bluish crocidolite into brownish iron oxide.

The idea that tiger's-eye is a pseudomorph–a mineral in which crystals of one material take on the form of another, which it replaces atom by atom–held sway for more than 125 years. In fact, tiger's-eye is cited in many textbooks as a classic example of a pseudomorph, says Peter J. Heaney, a mineralogist at Pennsylvania State University in University Park. During his own efforts to understand the processes underlying pseudomorphism, Heaney examined thin samples of tiger's-eye under a microscope and realized that Wibel was wrong.

Heaney expected to find that the quartz in tiger's-eye is chalcedony, a form that typically consists of fibrous, defect-riddled crystals less than 1 micrometer in diameter. Instead, Heaney was surprised to discover relatively fault-free, column-shaped quartz crystals that measured more than 100 micrometers across and up to 10 millimeters in length. Pseudomorphism doesn't produce such a uniform crystal form.

Heaney and his Penn State colleague Donald M. Fisher suggest that the crystal structure of tiger's-eye forms via a so-called crack-seal mechanism. In such a process, quartz and crocidolite crystals simultaneously condense from hot, mineral-rich fluids coursing through a tiny crack in a rock and grow to fill it. Repeated episodes of fracturing lead to more cycles of simultaneous, crack-filling growth of the two crystals.

In the tiger's-eye samples that Heaney studied, crocidolite fibers often ran parallel to the quartz columns. In some cases, however, the angle between the crocidolite and quartz was as much as 30. Because, in those instances, the reflected cat's-eye bands of light were perpendicular to the crocidolite fibers, the scientists conclude that in tiger's-eye the chatoyancy arises from the crocidolite fibers, not the quartz. The researchers report their findings in the April Geology.

Long time coming

So, why did it take 130 years for scientists to replace Wibel's tiger's-eye theory? After all, the techniques that Heaney used–optical and electron microscopy and X-ray diffraction–aren't new. The short answer, says Heaney, is that nobody had bothered to look. "Scientists merely accepted the old explanation, as I had," he explains. Also, because tiger's-eye is only a semiprecious stone, it hadn't attracted enough attention to merit a detailed investigation, he notes.

"It tickles me how [this finding] counters the longstanding assumption about how tiger's-eye forms," says Jeffrey E. Post, curator of gems and minerals at the Smithsonian's National Museum of Natural History in Washington, D.C. Says Post, who supports the new interpretation: "Sometimes an explanation is so pat that no one thinks to challenge it."

Tickled or not, Post joins the legion of curators in museums worldwide who will need to revise their mineralogy displays. But that's okay, he quips, because it's going to be even tougher for all those textbook editors.

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