Halloysitic Underclay and Amorphous Inorganic Matter in Hawaii

Abstract

AbstractPlastic clay deposits underlie peat beds in several swamps on the island of Kauai, Hawaii. These underclays are appreciably different in mineralogical and chemical composition from the associated well-drained weathered rocks (saprolite). The underclay is much higher in silica and lower in iron oxide than the saprolite. The underclay is composed chiefly of halloysite minerals, with minor amounts of gibbsite, anatase, quartz, goethite, and illite. The saprolite is predominately gibbsite, goethite, and hematite, with minor amounts of titaniferous magnetite, anatase, and halloysite minerals. Apparently both the clay and saprolite have formed by weathering from the same type of parent rock and have undergone similar geologic histories; therefore, differences in drainage and the presence or absence of an overlying peat layer appear to be the principal genetic variables.The underclay and saprolite are believed to contain amorphous gels and allophane because X-ray intensities for many samples are low, and estimated amounts of identifiable minerals are not adequate to account for the whole samples. Attempts to isolate the amorphous materials in samples from Kauai have not been successful, but both an alumina-silica gel and allophane were found in saprolite at one locality on Maui, Hawaii. These materials were collected from the face of a freshly bull-dozed roadcut at a depth of 2 to 6 ft. They occur as veins and as fillings and coatings on vesicles, vugs, and cracks in a dark-brown saprolite closely associated with hard basalt. The allophane is white, and most of it is covered by a clear or light-colored gel. Some of the gel occurs in clear irregular masses ranging in size from microscopic specks to about one-fourth inch in diameter and is similar to a mass of frog eggs or thick gelatin. Some occurs as light-grayish brown, paper-thin laminae that extend along cracks for several inches.Both the allophane and dried gel are similar in composition, containing approximately 50 per cent alumina and 22 to 26 per cent silica. The close association and similar composition indicate that allophane forms from the alumina-silica gel. Gibbsite and halloysite minerals occur in a manner very similar to that of the allophane and gel, suggesting that these amorphous materials are intermediate phases in the formation of gibbsite and halloysite. This is supported by the small amount of gibbsite present in the dried gel.