OBSERVATION AND ORIGIN OF SELF-ORGANIZED TEXTURES IN AGATES

One of the most impressive manifestations of spontaneous pattern gener ation 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 altern ating layers of fine-grained, highly defective chalcedony and coarse-g rained low-defect quartz. This oscillatory zonation in defect concentr ation may be ascribed to Ostwald-Liesegang crystallization cycles from silica-rich fluids that are variably polymeric and monomeric. Periodi c changes in defect concentration and grain size also are observed wit h wavelengths of hundreds of micrometers and of centimeters, so that a gates reveal textural self-similarity over three length scales.