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Science 15 September 1995:
Vol. 269. no. 5230, pp. 1562 - 1565
DOI: 10.1126/science.269.5230.1562
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Articles

Observation and Origin of Self-Organized Textures in Agates

Peter J. Heaney 1 and Andrew M. Davis 2

1 Department of Geological and Geophysical Sciences and Princeton Materials Institute, Princeton University, Princeton, NJ 08544, USA.
2 Enrico Fermi Institute, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA.

One of the most impressive manifestations of spontaneous pattern generation in natural materials is iris agate, which contains submicrometer concentric striations that may cycle several thousand times within an individual specimen. Analysis by secondary ion mass spectroscopy and transmission electron microscopy identified the iris texture as alternating layers of fine-grained, highly defective chalcedony and coarse-grained low-defect quartz. This oscillatory zonation in defect concentration may be ascribed to Ostwald-Liesegang crystallization cycles from silica-rich fluids that are variably polymeric and monomeric. Periodic changes in defect concentration and grain size also are observed with wavelengths of hundreds of micrometers and of centimeters, so that agates reveal textural self-similarity over three length scales.

Submitted on February 21, 1995
Accepted on June 19, 1995


THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
Characteristics and origin of agates in sedimentary rocks from the Dryhead area, Montana, USA.
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Coexistence of twisted and untwisted crystals: An impurity/structural order model with implications for agate patterns.
J. Comer and P. Ortoleva (2007)
American Mineralogist 92, 1952-1957
   Abstract »    Full Text »    PDF »
Aluminium in quartz as an indicator of the temperature of formation of agate.
J. Gotze, M. Plotze, M. Tichomirowa, H. Fuchs, and J. Pilot (2001)
Mineralogical Magazine 65, 407-413
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