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55 Common Lane, Auckley, Doncaster, DN9 3HX, England e-mail: tmoxon{at}auck.freeserve.co.uk
The degree of crystallinity in wall lining agates from host rocks of varying ages has been measured using X-ray powder diffraction. A plot of agate crystallite size vs log age of host rock shows a general increase in crystallite size with increasing age of the host rock. A regression line: size = 106 log host rock age + 211 was obtained. The SEM confirms that ageing produces a growth in the surface globulites and the development of plate edge-like structures within the white bands. This dynamic growth of the crystallites results in an increase in density.
A high correlation between agate crystallite size and age of host rock shows that the formation of wall-banded agates is penecontemporaneous with the formation of the host rock. The determination of crystallite size in wall-lining agate allows either an estimation of the approximate age of the host rock or an identification of a known agate region.
Key-words: agate, dating, silica, X-ray, genesis, density..
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