The relationships between kaolinite crystal properties and the origin of materials for a Brazilian kaolin deposit

The clay particles in a kaolin deposit from Brazil were investigated by X-r ay diffraction (XRD), differential thermal analysis (DTA), analytical trans mission electron microscopy (ATEM), and electron paramagnetic resonance (EP R) to examine the relationships between morphological and chemical properti es of the crystals and to relate these properties to formation conditions. The XRD patterns show the dominant presence of kaolinite with minor amounts of gibbsite, illite, quartz, goethite, hematite, and anatase. ATEM observa tions show two discontinuities in the deposit as indicated by changes in mo rphology and size of the kaolinite crystals. At the base of the deposit, he xagonal platy and lath-shaped particles (mean area of 001 face = 0.26 mum(2 )) maintain the original fabric of the parent rock which characterizes an i n situ evolution. In the middle of the deposit a bimodal population of larg e (mean area of 001 face > 0.05 mum(2)) and small (mean area of 001 face < 0.05 <mu>m(2)) sub-hexagonal platy kaolinite crystals occurs. This zone def ines the boundary between the saprolitic kaolinite and the pedogenic kaolin ite. Near the top of the profile, laths and irregular plates of kaolinite, together with sub-hexagonal particles, define two different depositional so urces in the history of formation of the deposit. Crystal thickness as deri ved from the width of basal reflections and the Hinckley index are compatib le with the morphological results, but show only one discontinuity. At the base of the deposit, kaolinite has a low-defect density whereas in the midd le and at the top of the profile, kaolinite has a high-defect density. Like wise, EPR spectroscopy shows typical spectra of low-defect kaolinite for th e bottom of the deposit and typical spectra of high-defect kaolinite for th e other portions of the deposit. Despite the morphological changes observed through the profile, the elemental composition of individual kaolinite cry stals did not show systematic variations. These results are consistent with the deposit consisting of a transported pedogenic kaolinite over saprolite consisting of in situ kaolinized phyllite.