Clay mineralogical differences between eight soils in South Island, New Zealand are related to differences in climate, vegetation, and formation time. Alteration of chlorite follows the sequence chlorite to interlayered hydrous mica to chlorite-swelling chlorite to chlorite vermiculite, with increased weathering leading to the destruction of the chloritic layers. Mica (muscovite) follows two alteration sequences depending on the overlying vegetation. Under tussock grassland the sequence is mica to mica-vermiculite to mica-beidellite to beidellite. This sequence is also followed under beech forest, together with the the sequence of mica to vermiculite to beidellite, with pedogenic chlorite forming when the pH is greater than 4.5. All but one of the interstratified products (the interlayered hydrous mica) appear to be based on a 1:1 regular interlaying of the two components