Quantification of minor phases in growth kinetics experiments with powder X-ray diffraction

Minor amounts of clay minerals precipitated from aqueous solution can be ra pidly identified and quantified in a mineral mixture with powder X-ray diff raction using a rotating-anode source and a position-sensitive detector. Fo r the case of gibbsite precipitated on a kaolinite powder substrate we demo nstrate a simple method having a minimum detection limit of 0.1 wt%, using pure gibbsite as the intensity reference in mechanical mixtures of gibbsite and kaolinite. The amount of gibbsite precipitated onto kaolinite at 80 de grees C, pH 3 is higher when determined from solution chemistry than from t he X-ray method, and the difference in amounts increases with increasing Al concentration in solution. This discrepancy can be explained by assuming t hat a fraction of the precipitated material is effectively invisible to the X-ray diffraction technique, either due to a small diffracting domain size along the gibbsite [001] direction or formation of an Al-phase that is amo rphous to X-rays. This method should be generally useful for a range of min eral mixtures where at least one intense reflection for the phase of intere st is not obscured. The ability to identify, characterize, and quantify tra ce phases by X-ray diffraction, especially when combined with surface analy sis by electron or atomic force imaging, is an important complement to the conventional approach of monitoring solution composition in growth kinetics experiments.