Halloysite in late pleistocene rhyolitic tephra beds near Opotiki, coastal Bay of Plenty, North Island, New Zealand

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Abstract
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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