XRD ANALYSIS OF QUARTZ CRYSTALLINITY: A PETRIFIED WOOD UPDATE

During a study of petrified wood the crystallinity of the quartz was observed to vary. The transition was initially tracked using the XRD height of the hkl [212] peak (Murata and Norman, 1976). There are three difficulties with the [212] peak; it is in the back reflection region making a separate scan necessary, in poorly crystallized quartz it merges with the adjacent peaks making peak size estimation ambiguous, and it is low intensity and does not resolve where quartz content is minor. Is there a better way? The maximum intensity peak of quartz, the hkl [101] peak, has three advantages over the [212] peak, it is in the 4^o to 40^o 2Q range commonly used to estimate bulk composition, it is isolated from other quartz peaks, and it stands above background even when quartz is a minor constituent of the sample. This study compares crystallinity estimates based on the height of the [212] peak with three different methods for estimating crystallinity based on the size of the [101] peak, height, FWHM, and area. A reference XRD standard prepared from a limpid quartz crystal ground to pass a 0.062 mm screen was repacked and scanned 10 times; the error obtained was applied to analyzed samples. Study samples were crushed and ground to pass a 0.125 mm screen and reanalyzed two to five times. Twenty-five samples of petrified wood from the Pliocene Upland Complex gravels in the Northern Mississippi embayment were analyzed. Estimates of quartz crystallinity using the hkl [101] peak, whether height, FWHM, or net area, give the same crystallinity index as the hkl [212] peak; height is the best correlated (R = 0.97): bottom line - use height of the [101] peak when evaluating the evolution of quartz crystallinity within the same study.