Abstract
The mineralogy of the hydrothermally altered basalts of the Kailua Volcanic Series from Keolu Hills, Oahu, Hawaii, was examined by X-ray diffraction analysis. Vertical and horizontal clay-mineral alteration zones were found. In the vertical zones, mixed-layer chlo-rite-vermiculite occurs more commonly than chlorite, whereas in the horizontal zones, mixed-layer chlorite-montmorillonite and chlorite are both more common. The occurrence of the two clay minerals kaolinite and mixed-layer chlorite-montmorillonite in the soil is not related to the parent rock but to differences in drainage. The zeolites are calcium zeolites which commonly coexist with quartz; laumontite is the dominant zeolite in the study area. The widespread distribution of laumontite in the Kailua Volcanic Series suggests that low temperatures (approximately 300°C) prevailed during the formation of secondary minerals and indicates that as much as 3,500 feet of weathering has occurred since caldera formation.
