The < 1'0 am fractions of three rhyolitic tephras are shown to contain multifaceted squat cylinders of halloysite at various stages of development. The < 1-0 ,um fraction of one of the tephras, the Pahoia Tuff, is composed mainly of large well-ordered squat cylinders, together with amorphous silica flakes, and novel disk-shaped particles of halloysite. The disks always occur sandwiched between silica flakes. A spiral structure is proposed for the disks. The squat cylinders are interpreted as having formed by a similar spiral mechanism to that of the disks, but without the constraining influence of silica flakes.

Investigation of the clay mineralogy of 14 rhyolitic tephra beds with a suggested age of 125000 to 220000 years revealed that the squat cylindrical form of halloysite is the dominant crystalline mineral. This mineral has probably crystallized from allophane over a long time period, excess silica being precipitated as a weakly hydrated phase. The squat cylindrical crystals perhaps characterize halloysite formation in rhyolitic tephras.

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Grain size parameters and clay mineralogical analyses were used to characterize and compare Okareka Ash and post-Okareka tephric loess sampled at eight sites on a transect in Rotorua district, North Island, New Zealand. Grain size distribution analyses show consistently lower mean size and better sorting of the tephric loess compared with the Okareka Ash. The mean grain size of the loess is strongly influenced by the mean grain size of the tephra. Trends in the distribution of biotite and halloysite support the grain size distribution analyses. Taken with field observations, the analytical evidence allows differentiation between Okareka Ash and overlying associated tephric loess.

The clay fractions of 13 paleosols formed in rhyolitic tephras erupted between 900 and 20 000 years ago in the Central North Island, New Zealand, have been examined by X-ray diffraction, infrared spectroscopic, and electron optical techniques. The clay minerals present in the paleosols are similar to those of the parent tephras. This indicates that during the comparatively short periods (generally less than 2000 years) between tephra deposition and subsequent burial by later ash or loess deposits the paleosols were only weakly weathered. Halloysite occurring in the : three oldest paieosols has probably formed by mechanisms similar to those operating for the parent tephras, rather than as a result of soil forming processes.