Mineralogy of the Kauroa Ash Formation of south-west and west Waikato, North Island, New Zealand

Abstract

The clay and sand mineralogy of old (possibly 750 000 years) rhyolitic tephras of two sections located in south-west and west Waikato, New Zealand, has been investigated by X-ray diffraction, differential thermal analysis, infrared spectroscopy, transmission electron microscopy, and the polarising microscope. The tephras are highly weathered and contain a srnall proportion of sand grains comprised mainly of quartz and magnetite, and are dominated by halloysite which occurs in several morphological forms. Allophane is present in all clay fractions together with gibbsite (in three beds) and lepidocrocite (in two beds). The mineralogy of the tephras is compared with that of the rhyolitic Pahoia Tuffs which are younger.