Sand‐sized Kaolinized Feldspar Pseudomorphs in a California Humult

Deeply weathered soils forming on andesitic mudflows are common in the foothills of the Sierra Nevada of California. Many of these soils (Humults) have oxidic mineralogy based on extractable Fe and gibbsite detected by x‐ray diffraction and differential thermal analysis. Thermal analysis and differential thermal analysis of whole soil samples reveal kaolin and gibbsite percentages greater than clay percentages obtained by particle‐size analysis. X‐ray diffractometry of the fine (0.1–0.25 mm) and coarse (0.5–1.0 mm) sands shows fractions dominated by kaolin throughout the profile. Halloysite predominates in the saprolite; kaolinite increases in abundance toward the soil surface. Gibbsite is present in the surface and upper argillic horizons. Observation of the fine sand fraction with a polarizing light microscope reveals highly weathered feldspar grains with very low birefringence. Scanning electron micrographs of coarse and fine sands show pseudomorphic replacement of feldspar by halloysite resulting in discrete sand‐sized grains wherein simple albite twins are preserved. Energy dispersive analysis by x‐ray of unweathered feldspars produces Si/Al count ratios of 6:1 and indicates the presence of K and Ca in the feldspar structure. Analysis of the surfaces and interiors of kaolinized feldspar grains shows Si/Al count ratios of 2:1 and an absence of K and Ca, demonstrating that the feldspars are completely altered to kaolin by desilication and loss of hydrolyzable cations. The alteration appears to be crystallographically controlled because the original feldspar crystal morphology is preserved.