How tiny crystals decorate iris agates
Formation of quartz stone
Agate, a type of quartz whose iridescent patterns sparkle with color, has long been valued as a semiprecious stone. Now, scientists can explain how its elegant swirls form.
Peter J. Heaney, a geologist at Princeton University, and Andrew M. Davis, a geological chemist at the University of Chicago, show that concentric shells of fine and coarse crystals alternate to create agate’s light-diffracting "iris" bands.
Agate, formed when mineral-rich water flows through volcanic rock, consists of millions of micrometer-sized crystals. Those crystals, the researchers observe in the Sept. 15 Science, come in different sizes and contain varying degrees of impurities, caused by changes in the water’s mineral concentrations.
Observing agate slices with transmission electron microscopy and ion mass spectroscopy, the two scientists found that the size of the tiny crystals and the degree of impurities change cyclically, forming the iris band’s crystal pattern. When scrutinized, agate slices reveal a self-similar pattern, which repeats itself at various levels of magnification: on the micrometer scale (a), on the millimeter scale (b), and on the centimeter scale (c).
"Agates show us one way that nature makes repetitive patterns," Heaney says. "Self-similarity is fascinating because it’s largely unexplained.
"Understanding this process may shed light on how materials scientists can mimic those textures in new materials."
COPYRIGHT 1995 Science Service, Inc.
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Richard Lipkin "How tiny crystals decorate iris agates – formation of quartz stone – Brief Article". Science News. Sept 16, 1995. FindArticles.com. 31 May. 2008. http://findarticles.com/p/articles/mi_m1200/is_n12_v148/ai_17495067
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