Mineralogical Characteristics and Transformations of a Vertical Rock‐Saprolite‐Soil Sequence in the North Carolina Piedmont: II. Feldspar Alteration Products‐Their Transformations Through the Profile

Feldspar minerals near the rock‐saprolite interface weathered directly to gibbsite, tubular halloysite, and amorphous aluminosilicate minerals. The gibbsite precipitated as aggregates of tiny plates deep in the profile. This mineral then resilicated into a tabular halloysite pseudomorphic after the gibbsite. There was very little evidence of amorphous materials throughout the profile but apparently amorphous spheres formed on the surface of feldspars and quartz in the weathering rock. These amorphous spheres seemed to radially crystallize into tube‐shaped minerals, presumably halloysite. Halloysite in the deep saprolite later recrystallized, via a randomly‐interstratified phase, to plate‐shaped kaolin minerals. Morphology of the 1:1 layer aluminosilicates did not provide evidence for degree of crystallinity as determined by X‐ray diffraction; poorly crystalline plate‐shaped and more highly crystalline tube‐shaped kaolin minerals were found in the same profile.