Crystal properties of goethite and hematite from three residual weathering profiles of the Georgia Piedmont

Analysis of secondary phase crystal properties in genetically related materials has the potential to distinguish between populations formed under variable geochemical conditions. This hypothesis was examined for three residual weathering profiles developed on granitic rocks of the Georgia Piedmont. X-ray diffraction (XRD), differential scanning calorimetry (DSC), X-ray fluorescence spectroscopy (XRF), and transmission electron microscopy (TEM) were utilized to characterize the regolith materials.; The authigenic mineral assemblages from all profiles are similar at the species level. Goethite, hematite, kaolinite, gibbsite and hydroxy-interlayered vermiculite (HIV) were identified in all three profiles. Aluminum substitution and crystallinity of the iron phases show systematic differences between the soils and saprolites.; More aluminum is incorporated into the iron phases, particularly goethite, from near surface environments. The presence of other highly aluminous phases such as gibbsite and/or HIV and low H₄SiO₄ concentrations near the soil surface suggest that this trend reflects increased aluminum activity in the soil zone. Although all three profiles show the same relative trend in aluminum substitution, the absolute amount of aluminum incorporation varies between the profiles. This likely reflects differences in the parent lithologies and/or age of the regolith materials.; The crystallinity of hematite and goethite vary with depth in the weathering profiles. In two profiles, goethite crystallinity increased moderately toward the surface while in the third maximum values were observed in the upper saprolite. Hematite crystallinity shows considerable variation with depth both within a single profile and between the three profiles investigated.; These variations in the crystal properties of hematites and goethites permit their segregation into two distinct populations characteristic of the soil and saprolite. The highly aluminous soil assemblage could not have been inherited from underlying materials in a system of regolith genesis facilitated by a downward-propagating weathering front. This observation in conjunction with the general trend of decreasing abundance of ferruginous primary phases towards the land surface suggests that in the upper soil zone, iron oxides formed from the last vestiges of Fe-containing primary phases and/or through recycling of earlier formed Fe oxides.