Abstract
Etched sections of agates examined by scanning electron microscopy show three types of compositional zonation: (1) two-unit oscillatory sequences in which the successive zones are of the same width, on a micron scale, but show a uniform difference in the content of (OH) in structural sites; (2) three-unit oscillatory sequences of various palindromic types in which successive zones are of the same width, on a micron scale, but show a uniform difference at three levels of (OH) content; and (3) disordered sequences in which the zone width is variable, on a micron to millimeter scale, and the (OH) content varies in a high-low but non-uniform way. All three types range in length up to thousands of successive zones, and often occur intermixed. These types of zonation constitute the whole of the fibrous parts of agates in general.
The oscillatory zonation is the result of a cyclic interplay between growth rate and diffusion rates at a crystal/solution interface. In agates the process operates at the interface between the quartz fibers growing on the cavity wall and the inner solution and is imposed on all fibers simultaneously. It is identical in crystallochemical mechanism with the coarse random zonation seen in large individual crystals of Type B quartz.
