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

Abstract

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.

The effect of kauri (Agathis australis) on grain size distribution and clay mineralogy of andesitic soils in the Waitakere Ranges, New Zealand

“…The decrease of the halloysite to kaolinite ratio toward the soil surface is also described in other studies (Calvert et al, 1980;Churman, 1990) and is ascribed to dehydration (Churchman and Gilkes, 1989). The results show no relation between halloysite-kaolinite PHR and the vegetation ( Table 6).…”

Basalt weathering and pedogenesis across an environmental gradient in the southern Cascade Range, California, USA

“…During late winter and spring, the high elevation soils experience intense leaching by cold, Sipoor waters that may foster microsites of low Si activity and gibbsite formation. On the other hand, Calvert et al (1980) argued that gibbsite, formed by alteration of feldspars in a gneissic saprolite, was essentially re-silicated to halloysite or kaolinite in upper soil horizons via dissolution of quartz. Given the dacitic to rhyolitic composition of the more recent Lassen ashes that most strongly influence the WF-3 and RF-4 pedons, it seems reasonable that gibbsite produced near the rock-soil interface is unstable closer to the surface due to silica released during weathering of the Si-rich ash.…”

Relevance of Different Intercalation Tests for Distinguishing Halloysite from Kaolinite in Soils

“…Several studies have found a decrease in the abundance ofhalloysite relative to that of kaolinite with an increase in degree of weathering of softs or with increasing proximity to the surface of soil profiles (Shimuzu, 1972;Eswaran and Wong, 1978;Calvert et al, 1980;Hughes, 1980;Hewitt and Churchman, 1982), Hence, it is tempting to infer that halloysite transforms to kaolinite with weathering. The apparently continuous series of X-ray powder diffraction (XRD) patterns of Ross and Kerr (1934) and Brindley and Robinson (1946) showing increasing order from halloysite to highly crystalline kaolinite also suggest a natural progression between halloysite and kaolinite.…”