Alteration of Basalt in Soils of Western Oregon

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Abstract
Basalt alteration from different pedo‐microenvironments in western Oregon was studied using petrographic, electron microscope, X‐ray diffraction, and differential thermal techniques. Initial alteration was similar in each environment and consisted primarily of mineral dissolution associated with the formation of etch pits and hairline cracks and the development of isotropic domains within feldspar phenocrysts. The stage of alteration characterized by secondary mineral formation was distinctly microenvironment dependent. Smectite formed as the initial crystalline weathering product of basalts in the Oregon Coast Range, but was metastable with respect to hydrated halloysite along fracture zones in the weathering crust where more leaching micro‐environments occureed. Smectite altered to chloritic intergrade and halloysite in cyclic wet and dry pedoenvironments. The occurrence of gibbsite was related to the alteration of glassy basalt lithologies under acid leaching conditions. Goethite formed form the alteration of an initial non‐crystalline iron oxide phase.